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.svs-v3-feedback{min-height:1.7em;font-size:14px;color:#214b3d;margin:10px 0}\n#sv-astronomy-page .svs-v3-next{padding:23px;border:1px solid #b9d5cf;background:#f1faf6;border-radius:18px;margin-top:24px}\n@media(max-width:1000px){#sv-astronomy-page .svs-v3-grid{grid-template-columns:1fr}}\n@media(max-width:780px){#sv-astronomy-page .svs-v3-routes,#sv-astronomy-page .svs-v3-eht{grid-template-columns:1fr}#sv-astronomy-page .svs-v3-routes{gap:12px}#sv-astronomy-page .svs-v3-project-grid{grid-template-columns:1fr}#sv-astronomy-page .svs-v3-project-grid>article:last-child{grid-column:auto}}\n@media(max-width:520px){#sv-astronomy-page .svs-v3-feature,#sv-astronomy-page .svs-v3-sharebox,#sv-astronomy-page .svs-v3-lab{padding:23px 17px;border-radius:23px}#sv-astronomy-page .svs-v3-questions,#sv-astronomy-page .svs-v3-quiz,#sv-astronomy-page .svs-v3-metrics{grid-template-columns:1fr}#sv-astronomy-page .svs-v3-questions summary{min-height:auto}#sv-astronomy-page .svs-v3-card{padding:22px}#sv-astronomy-page .svs-v3-control{padding:18px}#sv-astronomy-page .svs-v3-socials a,#sv-astronomy-page .svs-v3-socials button{flex:1 1 120px}}\n@media print{#sv-astronomy-page .svs-actions,#sv-astronomy-page .svs-index,#sv-astronomy-page .svs-v3-socials,#sv-astronomy-page .svs-v3-share-one,#sv-astronomy-page .svs-v3-control{display:none}#sv-astronomy-page{font-size:11pt;padding:0}#sv-astronomy-page .svs-section{break-inside:auto}#sv-astronomy-page .svs-v3-card{break-inside:avoid}}\n<\/style>\n<div class=\"svs-wrap\">\n  <nav id=\"as-top\" class=\"svs-breadcrumb\" aria-label=\"Breadcrumb\">\n    <a href=\"\/#sv-astronomy\">Stadia Home<\/a><span aria-hidden=\"true\">\/<\/span><span aria-current=\"page\">Astronomy &amp; Science<\/span>\n  <\/nav>\n\n  <section class=\"svs-hero\" aria-labelledby=\"as-title\">\n    <div>\n      <span class=\"svs-eyebrow\">Stadia Science &middot; Free exploration guide<\/span>\n      <h1 id=\"as-title\">Astronomy<br><span>&amp; Science<\/span><\/h1>\n      <p class=\"svs-hero-lead\">Look up. Ask why.<br>Follow the evidence.<\/p>\n      <p class=\"svs-hero-description\">Start with a surprising question. Explore the Moon, exoplanets and black-hole images; try a small experiment; then discover the instruments and people behind the evidence. Choose your path: curious reader, student or amateur observer.<\/p>\n      <div class=\"svs-actions\">\n        <a class=\"svs-button\" href=\"#as-curiosity\">Start with a surprising question &darr;<\/a>\n        <a class=\"svs-button svs-button-outline\" href=\"#as-observe\">Try a night-sky activity<\/a><a class=\"svs-button svs-button-outline\" href=\"#as-chapter-1\">Read the HARPS chapter<\/a>\n      <\/div>\n      <div class=\"svs-meta\"><span>Free learning resource<\/span><span>Public sources<\/span><span>Exercises with solutions<\/span><\/div>\n    <\/div>\n    <figure class=\"svs-hero-photo\">\n      <img fetchpriority=\"high\" src=\"https:\/\/cdn.eso.org\/images\/screen\/beletsky_lasilla_02.jpg\" width=\"1000\" height=\"666\" alt=\"The ESO 3.6-metre telescope at La Silla beneath the Milky Way.\" loading=\"eager\" decoding=\"async\" referrerpolicy=\"no-referrer\">\n      <figcaption>The ESO 3.6-metre telescope at La Silla, Chile, beneath the Milky Way. A real observatory photograph, not a view of an exoplanet.\n        <span class=\"svs-credit\">Credit: Y. Beletsky (LCO)\/ESO &middot; <a href=\"https:\/\/www.eso.org\/public\/images\/beletsky_lasilla_02\/\" target=\"_blank\" rel=\"noopener noreferrer\">Original image<\/a> &middot; <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\" target=\"_blank\" rel=\"noopener noreferrer\">CC BY 4.0<\/a><\/span>\n      <\/figcaption>\n    <\/figure>\n  <\/section>\n\n  <nav class=\"svs-index\" aria-label=\"On this page\">\n    <a href=\"#as-curiosity\">Curious questions<\/a><a href=\"#as-observe\">Observe<\/a><a href=\"#as-transit-lab\">Transit lab<\/a><a href=\"#as-participate\">Citizen science<\/a><a href=\"#as-chapter-1\">Chapter 1<\/a><a href=\"#as-instrument\">Inside HARPS<\/a><a href=\"#as-after-harps\">Beyond HARPS<\/a><a href=\"#as-quiz\">Quick quiz<\/a><a href=\"#as-share\">Share<\/a><a href=\"#as-performance\">Optical performance<\/a><a href=\"#as-practice\">Try the exercises<\/a><a href=\"#as-students\">Teaching notes<\/a><a href=\"#as-sources\">Sources &amp; credits<\/a>\n  <\/nav>\n\n\n<section id=\"as-start\" class=\"svs-v3-routes\" aria-label=\"Choose your reading path\">\n <a class=\"svs-v3-route\" href=\"#as-curiosity\"><small class=\"svs-v3-tag\">Curious readers<\/small><strong>One question to start.<\/strong><span>No equations required. Choose a mystery, reveal the explanation and follow its source.<\/span><\/a>\n <a class=\"svs-v3-route\" href=\"#as-transit-lab\"><small class=\"svs-v3-tag\">Students &amp; educators<\/small><strong>Predict. Calculate. Check.<\/strong><span>Try a simple transit model, the existing Doppler exercises and a discussion with solutions.<\/span><\/a>\n <a class=\"svs-v3-route\" href=\"#as-observe\"><small class=\"svs-v3-tag\">Amateur astronomers<\/small><strong>From looking to recording.<\/strong><span>Keep an observation note and explore routes into real citizen-science projects.<\/span><\/a>\n<\/section>\n<section id=\"as-curiosity\" class=\"svs-section svs-v3-feature\" aria-labelledby=\"as-curiosity-title\">\n <span class=\"svs-eyebrow\">A small question can open a large universe<\/span>\n <h2 id=\"as-curiosity-title\">Six questions worth passing on.<\/h2>\n <p>Make a prediction before opening an answer. Then ask what evidence supports it. Each question has its own link, so you can send the part that interests a friend or a class.<\/p>\n <div class=\"svs-v3-questions\"><details id=\"as-light-time\"><summary>Are we seeing the stars as they are now?<\/summary><div class=\"svs-answer\"><p>No: light needs time to reach us. Sunlight takes about 8.3 minutes to travel to Earth. A light-year is a distance, not an age. For a nearby star described as 100 light-years away, we receive light that travelled for roughly 100 years. <a class=\"svs-reference\" href=\"#as-source-10\" aria-label=\"Source 10\">10<\/a><\/p><p><strong>Your turn:<\/strong> Try explaining the difference between the age of a star and the travel time of the light you see.<\/p><a class=\"svs-v3-card-link\" href=\"#as-question\">Follow this question &rarr;<\/a><div><button class=\"svs-v3-share-one\" type=\"button\" data-svs-copy-fragment=\"as-light-time\" hidden>Copy this question&#8217;s link<\/button><\/div><\/div><\/details><details id=\"as-moon-side\"><summary>Is the far side of the Moon always dark?<\/summary><div class=\"svs-answer\"><p>No. The near side and far side both experience sunlight and darkness as the Moon rotates. Our changing view of its illuminated half produces the familiar phases; Earth&#x27;s shadow is involved in a lunar eclipse, not in the ordinary monthly sequence. <a class=\"svs-reference\" href=\"#as-source-11\" aria-label=\"Source 11\">11<\/a><\/p><p><strong>Your turn:<\/strong> Draw a Moon illuminated from one side. Change the position of the observer, not the light source.<\/p><a class=\"svs-v3-card-link\" href=\"#as-observe\">Follow this question &rarr;<\/a><div><button class=\"svs-v3-share-one\" type=\"button\" data-svs-copy-fragment=\"as-moon-side\" hidden>Copy this question&#8217;s link<\/button><\/div><\/div><\/details><details id=\"as-invisible-planet\"><summary>Can a planet be detected without a separate photograph?<\/summary><div class=\"svs-answer\"><p>Yes. A star&#x27;s changing radial velocity can reveal a companion. A transit offers another clue: a small dip when a planet crosses the star as seen by us. Neither a single dip nor a single shift is enough on its own to establish a planet. <a class=\"svs-reference\" href=\"#as-source-1\" aria-label=\"Source 1\">1<\/a><\/p><p><strong>Your turn:<\/strong> Compare the motion measurement in the chapter with the brightness calculation in the transit lab.<\/p><a class=\"svs-v3-card-link\" href=\"#as-transit-lab\">Follow this question &rarr;<\/a><div><button class=\"svs-v3-share-one\" type=\"button\" data-svs-copy-fragment=\"as-invisible-planet\" hidden>Copy this question&#8217;s link<\/button><\/div><\/div><\/details><details id=\"as-webb-colour\"><summary>Are the colours in Webb images real or invented?<\/summary><div class=\"svs-answer\"><p>Webb observes infrared wavelengths our eyes cannot see. Image specialists map those measurements to visible colours. This translation lets us examine real data; it does not mean our eyes would see the same scene in those colours. Read the filter information and caption before interpreting a colour. <a class=\"svs-reference\" href=\"#as-source-12\" aria-label=\"Source 12\">12<\/a><\/p><p><strong>Your turn:<\/strong> An image can be based on observations without representing natural colour as seen by a human observer.<\/p><a class=\"svs-v3-card-link\" href=\"#as-image-reading\">Follow this question &rarr;<\/a><div><button class=\"svs-v3-share-one\" type=\"button\" data-svs-copy-fragment=\"as-webb-colour\" hidden>Copy this question&#8217;s link<\/button><\/div><\/div><\/details><details id=\"as-black-hole\"><summary>How can we image something from which light cannot escape?<\/summary><div class=\"svs-answer\"><p>The EHT image of M87*, released in 2019, shows emission surrounding a dark shadow. It is reconstructed from radio observations, not a visible-light snapshot of the interior of a black hole. What is measured is light from the surrounding region; the black hole shapes the pattern. <a class=\"svs-reference\" href=\"#as-source-13\" aria-label=\"Source 13\">13<\/a><\/p><p><strong>Your turn:<\/strong> Look at the image below. Which part is bright, which part is dark, and what does the caption actually claim?<\/p><a class=\"svs-v3-card-link\" href=\"#as-image-reading\">Follow this question &rarr;<\/a><div><button class=\"svs-v3-share-one\" type=\"button\" data-svs-copy-fragment=\"as-black-hole\" hidden>Copy this question&#8217;s link<\/button><\/div><\/div><\/details><details id=\"as-habitable\"><summary>Does a planet in the habitable zone have life?<\/summary><div class=\"svs-answer\"><p>That conclusion does not follow. The term concerns conditions under which surface liquid water might be possible; atmosphere and other planetary properties still matter. A planet&#x27;s location alone establishes neither a habitable surface nor the presence of life. <a class=\"svs-reference\" href=\"#as-source-14\" aria-label=\"Source 14\">14<\/a><\/p><p><strong>Your turn:<\/strong> Keep three claims separate: in the habitable zone, actually habitable, and inhabited.<\/p><a class=\"svs-v3-card-link\" href=\"#as-evidence\">Follow this question &rarr;<\/a><div><button class=\"svs-v3-share-one\" type=\"button\" data-svs-copy-fragment=\"as-habitable\" hidden>Copy this question&#8217;s link<\/button><\/div><\/div><\/details><\/div>\n <div id=\"as-image-reading\" class=\"svs-v3-eht\">\n  <figure class=\"svs-figure\"><img loading=\"lazy\" src=\"https:\/\/cdn.eso.org\/images\/screen\/eso1907a.jpg\" width=\"1000\" height=\"583\" alt=\"The EHT image of M87*: an asymmetric bright ring around a dark central shadow.\" loading=\"lazy\" decoding=\"async\" referrerpolicy=\"no-referrer\"><figcaption><strong>Read an image, not just its headline.<\/strong> M87* image released in 2019, reconstructed from radio observations. The dark centre is a shadow, not a photograph taken inside the event horizon.<span class=\"svs-credit\">Credit: EHT Collaboration &middot; <a href=\"https:\/\/www.eso.org\/public\/images\/eso1907a\/\" target=\"_blank\" rel=\"noopener noreferrer\">Original image and explanation<\/a> &middot; <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\" target=\"_blank\" rel=\"noopener noreferrer\">CC BY 4.0<\/a><\/span><\/figcaption><\/figure>\n  <div><span class=\"svs-eyebrow\">Three useful labels<\/span><h3>Observation, reconstruction or illustration?<\/h3><p><strong>Observation-based image:<\/strong> ask which instrument and wavelengths produced the measurements.<\/p><p><strong>Reconstruction or processed composite:<\/strong> ask how measurements were combined and how colour was assigned. Processing is not, by itself, fabrication. <a class=\"svs-reference\" href=\"#as-source-12\" aria-label=\"Source 12\">12<\/a> <a class=\"svs-reference\" href=\"#as-source-13\" aria-label=\"Source 13\">13<\/a><\/p><p><strong>Artist&#8217;s concept:<\/strong> useful for communicating an idea, but not evidence of the depicted surface or detail. NASA&#8217;s Eyes tutorial identifies its close views of exoplanets as artist concepts. <a class=\"svs-reference\" href=\"#as-source-20\" aria-label=\"Source 20\">20<\/a><\/p><\/div>\n <\/div>\n<\/section>\n\n<section id=\"as-observe\" class=\"svs-section\" aria-labelledby=\"as-observe-title\">\n <span class=\"svs-eyebrow\">A first observing activity &middot; No purchase required<\/span><h2 id=\"as-observe-title\">Turn one look at the Moon into a small investigation.<\/h2>\n <p class=\"svs-intro\">Choose a clear night when the Moon is visible. Begin with your eyes; use binoculars only if you already have them and know how to use them safely. NASA notes that lunar terrain can be easier to distinguish outside full Moon, especially near the boundary between light and dark. <a class=\"svs-reference\" href=\"#as-source-15\" aria-label=\"Source 15\">15<\/a><\/p>\n <div class=\"svs-v3-grid\">\n  <article class=\"svs-v3-card\"><span class=\"svs-v3-tag\">1 &middot; Notice<\/span><h3>Draw before naming.<\/h3><p>From a safe, accessible place, sketch the illuminated shape and two or three obvious features. Mark what you actually see, not what a reference picture suggests should be there.<\/p><\/article>\n  <article class=\"svs-v3-card\"><span class=\"svs-v3-tag\">2 &middot; Record<\/span><h3>Write down the conditions.<\/h3><p>Record date, local time and time zone, general observing area, equipment and cloud conditions. Keep private addresses out of anything you share publicly. Our template below is a learning aid, not a calibrated measurement system.<\/p><\/article>\n  <article class=\"svs-v3-card\"><span class=\"svs-v3-tag\">3 &middot; Return<\/span><h3>Compare another evening.<\/h3><p>Return when the Moon is visible again. Compare illumination and shadows, then distinguish what changed in the view from what you can claim changed on the surface. Use the official guide to interpret the difference. <a class=\"svs-reference\" href=\"#as-source-11\" aria-label=\"Source 11\">11<\/a> <a class=\"svs-reference\" href=\"#as-source-15\" aria-label=\"Source 15\">15<\/a><\/p><\/article>\n <\/div>\n <div class=\"svs-note svs-note-warning\"><strong>Night-sky activities only; not solar observing instructions.<\/strong> Do not point unfiltered binoculars, telescopes or camera optics at the Sun. Ordinary sunglasses are not solar protection, and eclipse glasses must not be used behind optical instruments. Read the official solar-safety guidance before any separate solar activity. Children should observe with a responsible adult. <a class=\"svs-reference\" href=\"#as-source-16\" aria-label=\"Source 16\">16<\/a><\/div>\n <details id=\"as-observation-template\"><summary>Open the observation-notebook template<\/summary><div class=\"svs-answer\"><p>Copy this into a notebook. Nothing is uploaded or stored by this page.<\/p><pre id=\"as-log-template\" class=\"svs-v3-notebook\">Target:\nDate \/ local time \/ time zone:\nGeneral observing area (no home address):\nEquipment, if any:\nSky and cloud conditions:\nWhat I directly observed:\nMy sketch or photograph:\nMy interpretation:\nOne alternative explanation:\nWhat I will compare next time:\nSource used for checking:<\/pre><button type=\"button\" class=\"svs-v3-share-one\" data-svs-copy-target=\"as-log-template\" hidden>Copy notebook template<\/button><\/div><\/details>\n <p>Official reference: <a href=\"https:\/\/science.nasa.gov\/moon\/viewing-tips\/\" target=\"_blank\" rel=\"noopener noreferrer\">NASA Moon viewing tips &nearr;<\/a>. The exercise sequence above was prepared for Stadia.<\/p>\n<\/section>\n\n<section id=\"as-transit-lab\" class=\"svs-section svs-v3-lab\" aria-labelledby=\"as-transit-title\">\n <span class=\"svs-eyebrow\">Small learning lab &middot; Geometry, not a discovery claim<\/span><h2 id=\"as-transit-title\">How much starlight could a planet hide?<\/h2>\n <p>A transit can produce a small dip in a star&#8217;s measured brightness. <a class=\"svs-reference\" href=\"#as-source-1\" aria-label=\"Source 1\">1<\/a> In this idealised model, an opaque circular planet is fully in front of a uniformly bright stellar disc. The blocked fraction is the ratio of their apparent areas:<\/p>\n <div class=\"svs-equation\">Depth = (R<sub>planet<\/sub> \/ R<sub>star<\/sub>)<sup>2<\/sup><\/div>\n <div class=\"svs-v3-control\"><label for=\"as-radius-ratio\">Planet radius as a percentage of star radius<\/label><input id=\"as-radius-ratio\" type=\"range\" min=\"1\" max=\"20\" step=\"1\" value=\"10\" disabled aria-describedby=\"as-lab-status as-lab-limits\"><p id=\"as-lab-status\" class=\"svs-small\">Static example: 10%. The table and explanation work without JavaScript.<\/p><\/div>\n <div class=\"svs-v3-metrics\" role=\"group\" aria-label=\"Idealised transit calculation\"><div><strong id=\"as-radius-out\">10%<\/strong><span>Radius ratio<\/span><\/div><div><strong id=\"as-depth-out\">1.000%<\/strong><span>Light blocked<\/span><\/div><div><strong id=\"as-light-out\">99.000%<\/strong><span>Light remaining<\/span><\/div><\/div>\n <p id=\"as-lab-limits\" class=\"svs-small\">Teaching model only: no limb darkening, partial overlap, blending, atmosphere, noise or stellar variability. It does not calculate an orbit, timing, mass or habitability and it does not fit real transit data.<\/p>\n <details><summary>Check the numbers without the interactive control<\/summary><div class=\"svs-answer\"><div class=\"svs-table-wrap\"><table><caption>Idealised area calculation. Not telescope data.<\/caption><thead><tr><th scope=\"col\">Radius ratio<\/th><th scope=\"col\">Light blocked<\/th><th scope=\"col\">Light remaining<\/th><\/tr><\/thead><tbody><tr><th scope=\"row\">1%<\/th><td>0.01%<\/td><td>99.99%<\/td><\/tr><tr><th scope=\"row\">5%<\/th><td>0.25%<\/td><td>99.75%<\/td><\/tr><tr><th scope=\"row\">10%<\/th><td>1%<\/td><td>99%<\/td><\/tr><tr><th scope=\"row\">20%<\/th><td>4%<\/td><td>96%<\/td><\/tr><\/tbody><\/table><\/div><p><strong>Try it:<\/strong> double the radius ratio from 5% to 10%. The ideal blocked fraction becomes four times larger, not twice as large: area depends on radius squared.<\/p><\/div><\/details>\n <p><strong>The important next question:<\/strong> could a different cause produce a brightness dip? Compare this simple model with <a href=\"#as-evidence\">the evidence checklist<\/a> before treating a pattern as a planet.<\/p>\n<\/section>\n\n<section id=\"as-participate\" class=\"svs-section\" aria-labelledby=\"as-participate-title\">\n <span class=\"svs-eyebrow\">Go beyond reading &middot; Independent external projects<\/span><h2 id=\"as-participate-title\">No telescope? There are still ways to take part.<\/h2>\n <p class=\"svs-intro\">Choose one route, follow the project&#8217;s tutorial and requirements, and check the current availability of tasks. These links leave Stadia. Follow each project&#8217;s account and age requirements; younger learners should work with a teacher or parent. A contribution is not a guaranteed discovery, publication, certificate or payment.<\/p>\n <div class=\"svs-v3-grid\">\n <article class=\"svs-v3-card\"><span class=\"svs-v3-tag\">Observe or analyse<\/span><h3>Exoplanet Watch<\/h3><p>NASA&#8217;s project supports transit observations and light-curve analysis. Participants without a telescope can request an existing observation to work on. Follow the project&#8217;s contribution workflow, which may involve observer registration and external software\/accounts. <a class=\"svs-reference\" href=\"#as-source-17\" aria-label=\"Source 17\">17<\/a> <a class=\"svs-reference\" href=\"#as-source-18\" aria-label=\"Source 18\">18<\/a><\/p><a class=\"svs-button\" href=\"https:\/\/science.nasa.gov\/citizen-science\/exoplanet-watch\/\" target=\"_blank\" rel=\"noopener noreferrer\">Explore Exoplanet Watch &nearr;<\/a><\/article>\n <article class=\"svs-v3-card\"><span class=\"svs-v3-tag\">Inspect real light curves<\/span><h3>Planet Hunters TESS<\/h3><p>Learn to recognise patterns in brightness measurements from the TESS mission. Volunteers inspect light curves and flag possible features for investigation. A flagged transit-like feature still needs follow-up; it is not a confirmed planet. <a class=\"svs-reference\" href=\"#as-source-19\" aria-label=\"Source 19\">19<\/a><\/p><a class=\"svs-button\" href=\"https:\/\/science.nasa.gov\/citizen-science\/planet-hunters-tess\/\" target=\"_blank\" rel=\"noopener noreferrer\">Read the project guide &nearr;<\/a><\/article>\n <article class=\"svs-v3-card\"><span class=\"svs-v3-tag\">Explore and compare<\/span><h3>NASA&#8217;s Eyes<\/h3><p>Use the official tutorial to explore known planetary systems and the methods used to detect them. Read the data and captions: close-up planet surfaces in the viewer are artist concepts, not photographs from a probe visiting those worlds. This route is exploration, not data submission. <a class=\"svs-reference\" href=\"#as-source-20\" aria-label=\"Source 20\">20<\/a><\/p><a class=\"svs-button\" href=\"https:\/\/science.nasa.gov\/tutorials\/eyes-on-exoplanets-tutorial\/\" target=\"_blank\" rel=\"noopener noreferrer\">Open the Eyes tutorial &nearr;<\/a><\/article>\n <\/div>\n <div class=\"svs-v3-next\"><strong>A reason to return, without an artificial streak.<\/strong> Use four visits as your own plan: ask a question; record one observation; compare an instrument or calculation; then try an external project. Bring a friend a specific question, not a promise that either of you will discover a planet.<\/div>\n<\/section>\n\n  <section id=\"as-chapter-1\" aria-labelledby=\"as-chapter-title\">\n    <span class=\"svs-eyebrow\">Light, stars &amp; exoplanets<\/span>\n    <h2 id=\"as-chapter-title\" class=\"svs-chapter-title\">How an optical instrument makes an astronomical discovery possible<\/h2>\n    <p class=\"svs-intro\">A planet can reveal its presence through the motion of its star, even when we do not obtain a separate picture of the planet. This chapter follows the radial-velocity method: the question, the instrument, the measurement and the checks that make an interpretation credible. <a class=\"svs-reference\" href=\"#as-source-1\" aria-label=\"Source 1\">1<\/a><\/p>\n    <div class=\"svs-path\">\n      <a href=\"#as-question\"><span class=\"svs-num\">01<\/span><strong>Ask a question<\/strong><small>Could an unseen companion explain a star&#8217;s motion?<\/small><\/a>\n      <a href=\"#as-instrument\"><span class=\"svs-num\">02<\/span><strong>Build an instrument<\/strong><small>Separate and record light reproducibly.<\/small><\/a>\n      <a href=\"#as-measurement\"><span class=\"svs-num\">03<\/span><strong>Make a measurement<\/strong><small>Compare spectra using a calibrated wavelength scale.<\/small><\/a>\n      <a href=\"#as-evidence\"><span class=\"svs-num\">04<\/span><strong>Test the interpretation<\/strong><small>Ask what else could produce the signal.<\/small><\/a>\n    <\/div>\n  <\/section>\n\n  <section id=\"as-question\" class=\"svs-section\" aria-labelledby=\"as-question-title\">\n    <span class=\"svs-eyebrow\">01 &middot; The scientific question<\/span>\n    <h2 id=\"as-question-title\">Look for a changing spectrum, not a photograph of the planet.<\/h2>\n    <div class=\"svs-split\">\n      <div class=\"svs-copy\">\n        <p>A star and an orbiting planet both move around their common centre of mass. We can look for the part of the star&#8217;s motion directed toward or away from us. That line-of-sight component is its <strong>radial velocity<\/strong>. <a class=\"svs-reference\" href=\"#as-source-1\" aria-label=\"Source 1\">1<\/a><\/p>\n        <p>As the star approaches, spectral features move toward shorter wavelengths; as it recedes, they move toward longer wavelengths. This is the Doppler effect. The shifts being measured are not a dramatic visible change in the star&#8217;s colour. <a class=\"svs-reference\" href=\"#as-source-9\" aria-label=\"Source 9\">9<\/a><\/p>\n        <div class=\"svs-note\"><strong>The first distinction:<\/strong> a spectral shift is a measurement. A planet is an interpretation that must explain a consistent body of evidence.<\/div>\n      <\/div>\n      <aside class=\"svs-panel\" aria-labelledby=\"as-spectrum-title\">\n        <h3 id=\"as-spectrum-title\">Reading a shift<\/h3>\n        <p>Compare the position of a pattern, not just its brightness.<\/p>\n        <div class=\"svs-spectrum\" aria-label=\"Schematic of three spectral features shifted toward longer wavelengths. The offset is exaggerated and is not measured data.\">\n          <div class=\"svs-spectrum-row\"><strong>Reference pattern<\/strong><div class=\"svs-band\" aria-hidden=\"true\"><span class=\"svs-lines\"><\/span><\/div><\/div>\n          <div class=\"svs-spectrum-row\"><strong>Receding source: pattern shifted right<\/strong><div class=\"svs-band\" aria-hidden=\"true\"><span class=\"svs-lines svs-lines-shifted\"><\/span><\/div><div class=\"svs-scale\"><span>Shorter wavelength<\/span><span>Longer wavelength &rarr;<\/span><\/div><\/div>\n        <\/div>\n        <p class=\"svs-small\" style=\"margin-top:17px\">Illustrative schematic by Stadia. The colours and displacement are exaggerated; there is no calibrated scale and these are not HARPS data.<\/p>\n      <\/aside>\n    <\/div>\n  <\/section>\n\n  <section id=\"as-instrument\" class=\"svs-section\" aria-labelledby=\"as-instrument-title\">\n    <span class=\"svs-eyebrow\">02 &middot; A documented instrument<\/span>\n    <h2 id=\"as-instrument-title\">HARPS: engineering for repeatable measurements.<\/h2>\n    <div class=\"svs-split\">\n      <div class=\"svs-copy\">\n        <p><strong>HARPS<\/strong> stands for High Accuracy Radial velocity Planet Searcher. It receives light through optical fibres from ESO&#8217;s 3.6-metre telescope at La Silla. The spectrograph separates that light for precise comparison of spectral features. <a class=\"svs-reference\" href=\"#as-source-9\" aria-label=\"Source 9\">9<\/a><\/p>\n        <p>The ESO instrument overview describes a design aimed at long-term radial-velocity accuracy of about 1 m\/s. Mechanical stability, thermal control and a vacuum enclosure help reduce instrumental changes that could move the recorded spectrum. <a class=\"svs-reference\" href=\"#as-source-2\" aria-label=\"Source 2\">2<\/a><\/p>\n        <div class=\"svs-stats\">\n          <div class=\"svs-stat\"><strong>3.6 m<\/strong><small>Diameter of the telescope feeding HARPS<\/small><\/div>\n          <div class=\"svs-stat\"><strong>115,000<\/strong><small>Spectral resolving power, &lambda;\/&delta;&lambda;<\/small><\/div>\n          <div class=\"svs-stat\"><strong>~1 m\/s<\/strong><small>Long-term design goal described by ESO<\/small><\/div>\n        <\/div>\n        <p class=\"svs-small\">Instrument figures from ESO&#8217;s overview, not a promise of accuracy for every star or exposure. Resolving power is not a reflectivity percentage. <a class=\"svs-reference\" href=\"#as-source-2\" aria-label=\"Source 2\">2<\/a><\/p>\n      <\/div>\n      <figure class=\"svs-figure\">\n        <img loading=\"lazy\" src=\"https:\/\/cdn.eso.org\/images\/large\/eso0308b.jpg\" width=\"1600\" height=\"1134\" alt=\"HARPS during laboratory testing, with its vacuum vessel open to show the instrument inside.\" loading=\"lazy\" decoding=\"async\" referrerpolicy=\"no-referrer\">\n        <figcaption><strong>The instrument behind the measurement.<\/strong> HARPS during laboratory tests, with its vacuum vessel open. Photograph released by ESO in 2003.\n          <span class=\"svs-credit\">Credit: ESO &middot; <a href=\"https:\/\/www.eso.org\/public\/images\/eso0308b\/\" target=\"_blank\" rel=\"noopener noreferrer\">Original photograph and caption<\/a> &middot; <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\" target=\"_blank\" rel=\"noopener noreferrer\">CC BY 4.0<\/a><\/span>\n        <\/figcaption>\n      <\/figure>\n    <\/div>\n    <div class=\"svs-note\">These public images describe HARPS as an instrument. They do not identify a particular component as a Stadia or VIRGO HTE delivery, and do not constitute an endorsement by ESO.<\/div>\n  <\/section>\n\n  <section id=\"as-measurement\" class=\"svs-section\" aria-labelledby=\"as-measurement-title\">\n    <span class=\"svs-eyebrow\">03 &middot; From light to a number<\/span>\n    <h2 id=\"as-measurement-title\">The detector records a spectrum. Analysis turns it into evidence.<\/h2>\n    <div class=\"svs-split\">\n      <aside class=\"svs-panel\" aria-labelledby=\"as-detector-title\">\n        <span class=\"svs-eyebrow\">Look at the original evidence<\/span>\n        <h3 id=\"as-detector-title\">What does a detector actually record?<\/h3>\n        <p>ESO publishes a raw HARPS detector image showing spectral orders and laser-comb calibration features. Open the original image and read its caption before answering these questions.<\/p>\n        <ol><li>Can you see a planet in this image?<\/li><li>Why are there several bands rather than a single rainbow?<\/li><li>Which features provide the wavelength reference?<\/li><\/ol>\n        <a class=\"svs-button svs-button-purple\" href=\"https:\/\/www.eso.org\/public\/images\/harps_cc\/\" target=\"_blank\" rel=\"noopener noreferrer\">View the detector image at ESO &nearr;<\/a>\n        <p class=\"svs-small\" style=\"margin-top:16px\">Opens the original ESO page in a new tab. Image credit: ESO. The caption describes the data; it is not a photograph of an exoplanet.<\/p>\n      <\/aside>\n      <div class=\"svs-copy\">\n        <p>The ESO detector example shows why a spectrograph is not an ordinary camera. A long spectrum is arranged into shorter orders on the detector. Calibration features provide a reference for locating wavelengths. <a class=\"svs-reference\" href=\"#as-source-3\" aria-label=\"Source 3\">3<\/a><\/p>\n        <p>Very small velocity signals can correspond to a tiny fraction of a detector pixel. NASA&#8217;s description of the NEID spectrometer illustrates why this demands stability throughout the instrument, rather than merely a sharp image. <a class=\"svs-reference\" href=\"#as-source-4\" aria-label=\"Source 4\">4<\/a><\/p>\n        <div class=\"svs-panel\">\n          <h3>Four questions to ask of a result<\/h3>\n          <ol>\n            <li>What quantity was actually measured?<\/li>\n            <li>How was its scale calibrated?<\/li>\n            <li>What uncertainty accompanies the value?<\/li>\n            <li>Which checks could expose an instrumental change?<\/li>\n          <\/ol>\n          <p class=\"svs-small\">A reading checklist for this chapter, not a substitute for an instrument&#8217;s reduction and calibration procedures.<\/p>\n        <\/div>\n      <\/div>\n    <\/div>\n  <\/section>\n\n  <section id=\"as-performance\" class=\"svs-section\" aria-labelledby=\"as-performance-title\">\n    <span class=\"svs-eyebrow\">Optical engineering &middot; Public performance, not manufacturing recipes<\/span>\n    <h2 id=\"as-performance-title\">Ask what must remain stable, and how it will be verified.<\/h2>\n    <p class=\"svs-intro\">Consider these as engineering questions when reading a specification. They are not advertised capabilities of HARPS, a coating supplier or a particular historical mirror.<\/p>\n    <div class=\"svs-cards\">\n      <div class=\"svs-card\"><h3>Reflectivity<\/h3><p>At which wavelengths, angle of incidence and polarisation is a reflectivity value specified? Does the figure refer to a minimum, an average or one measurement?<\/p><\/div>\n      <div class=\"svs-card\"><h3>Thermal behaviour<\/h3><p>Does a temperature range describe survival, continued operation or measured optical stability? What cycling, duration and allowable drift are included in the acceptance test?<\/p><\/div>\n      <div class=\"svs-card\"><h3>Durability &amp; substrate<\/h3><p>Which hardness or resistance test is agreed? How will the substrate, surface quality and coating be evaluated together, rather than through a single headline figure?<\/p><\/div>\n    <\/div>\n    <details>\n      <summary>Worked example: why several reflections matter<\/summary>\n      <div class=\"svs-answer\">\n        <p><strong>Synthetic optical budget.<\/strong> Assume ten identical reflections, each with reflectivity R = 99.5%, all at the same agreed wavelength, angle and polarisation. Ignore every other loss.<\/p>\n        <div class=\"svs-equation\">Remaining fraction = 0.995<sup>10<\/sup> &asymp; 0.9511<\/div>\n        <p>About <strong>95.1%<\/strong> of the incident light remains after those reflections. This multiplication is an idealised example, not a measured throughput or a specification for HARPS. A reflectivity figure alone cannot certify thermal stability or durability.<\/p>\n      <\/div>\n    <\/details>\n    <p class=\"svs-small\">This learning series discusses requirements, performance and verification. Proprietary coating compositions, layer sequences and fabrication processes are not disclosed.<\/p>\n  <\/section>\n\n  <section id=\"as-evidence\" class=\"svs-section\" aria-labelledby=\"as-evidence-title\">\n    <span class=\"svs-eyebrow\">04 &middot; Interpretation and uncertainty<\/span>\n    <h2 id=\"as-evidence-title\">A repeating signal is a reason to investigate, not the end of the argument.<\/h2>\n    <div class=\"svs-split\">\n      <div class=\"svs-copy\">\n        <p>A companion is not the only explanation to test. Stellar activity can affect the measured spectrum: ESO highlights the diagnosis of starspots as a way to guard against false planet detections. <a class=\"svs-reference\" href=\"#as-source-5\" aria-label=\"Source 5\">5<\/a><\/p>\n        <p>An instrument also needs checks against drift. HARPS&#8217;s stable design and reference spectra address that part of the problem; they do not remove the need to study the star. <a class=\"svs-reference\" href=\"#as-source-2\" aria-label=\"Source 2\">2<\/a> <a class=\"svs-reference\" href=\"#as-source-5\" aria-label=\"Source 5\">5<\/a><\/p>\n        <p>Ask whether the proposed model explains the observations and their uncertainty, whether competing explanations have been examined, and what independent observation would strengthen or weaken the interpretation.<\/p>\n      <\/div>\n      <aside class=\"svs-panel\">\n        <h3>Do not confuse these three statements<\/h3>\n        <p><strong>Observation:<\/strong> a recorded spectrum or time series.<\/p>\n        <p><strong>Model:<\/strong> a mathematical description proposed to explain it.<\/p>\n        <p><strong>Conclusion:<\/strong> the claim justified by the evidence and by the alternatives that were tested.<\/p>\n        <p class=\"svs-small\">Being able to draw a smooth curve through a few points is not, on its own, a discovery.<\/p>\n      <\/aside>\n    <\/div>\n    <div class=\"svs-method\"><span class=\"svs-eyebrow\">The chapter in one sentence<\/span><h3>A discovery connects a question, a reliable measurement and a tested interpretation.<\/h3><p>That is why astronomy also needs optical engineering, calibration, careful analysis and transparent reporting.<\/p><\/div>\n  <\/section>\n\n<section id=\"as-after-harps\" class=\"svs-section svs-v3-feature\" aria-labelledby=\"as-after-title\"><span class=\"svs-eyebrow\">Beyond HARPS &middot; Selected projects<\/span><h2 id=\"as-after-title\">One scientific ambition. Several different paths.<\/h2><p>These projects are related by the pursuit of better measurements, not a single sequence of replacements. Historical milestones and future objectives are identified separately. Public sources checked 25 September 2026; this is not a live operational-status page.<\/p><div class=\"svs-v3-project-grid\"><article id=\"as-harps-n\" class=\"svs-v3-card\"><span class=\"svs-v3-status \">Northern counterpart<\/span><h3>HARPS-N<\/h3><p><strong>Telescopio Nazionale Galileo, La Palma<\/strong><\/p><p>A closely related precision radial-velocity instrument in the Northern Hemisphere, built to discover and characterise planets using Doppler measurements, including follow-up of transiting candidates. <a class=\"svs-reference\" href=\"#as-source-21\" aria-label=\"Source 21\">21<\/a><\/p><p><strong>Discuss:<\/strong> What can a new observing site add without changing the basic measurement method?<\/p><\/article><article id=\"as-espresso\" class=\"svs-v3-card\"><span class=\"svs-v3-status \">First light announced in 2017<\/span><h3>ESPRESSO<\/h3><p><strong>Very Large Telescope, Paranal<\/strong><\/p><p>ESO describes ESPRESSO as HARPS&#x27;s successor. It combines stable spectroscopy with the VLT and can receive light from one or all four Unit Telescopes. First light is a milestone, not a statement of the uncertainty attainable for every observation. <a class=\"svs-reference\" href=\"#as-source-22\" aria-label=\"Source 22\">22<\/a><\/p><p><strong>Discuss:<\/strong> How do light collection, spectral information and calibration address different limits?<\/p><\/article><article id=\"as-nirps\" class=\"svs-v3-card\"><span class=\"svs-v3-status \">Science operations from April 2023<\/span><h3>NIRPS<\/h3><p><strong>ESO 3.6-metre telescope, La Silla<\/strong><\/p><p>NIRPS observes in the near infrared alongside HARPS, with simultaneous operation on the same telescope. ESO gives 1 April 2023 as the start of operations. This is a complementary wavelength range, not simply a replacement for visible-light spectroscopy. <a class=\"svs-reference\" href=\"#as-source-23\" aria-label=\"Source 23\">23<\/a><\/p><p><strong>Discuss:<\/strong> Why compare two wavelength ranges when evaluating a possible planetary signal?<\/p><\/article><article id=\"as-harps3\" class=\"svs-v3-card\"><span class=\"svs-v3-status svs-v3-future\">Terra Hunting survey: start pending<\/span><h3>HARPS3<\/h3><p><strong>Isaac Newton Telescope, La Palma<\/strong><\/p><p>The Terra Hunting Experiment plans repeated observations over at least ten years with HARPS3 and a robotic telescope. Its current notice expects the survey to start in 2027 after a cryostat failure during commissioning in June 2026. This remains a forecast, not a guaranteed date or an already completed survey. <a class=\"svs-reference\" href=\"#as-source-24\" aria-label=\"Source 24\">24<\/a><\/p><p><strong>Discuss:<\/strong> Why do the spacing and duration of observations matter as well as precision?<\/p><\/article><article id=\"as-andes\" class=\"svs-v3-card\"><span class=\"svs-v3-status svs-v3-future\">In development, not yet observing<\/span><h3>ANDES \/ ELT<\/h3><p><strong>Extremely Large Telescope, Cerro Armazones<\/strong><\/p><p>Formerly ELT-HIRES, ANDES is planned for high-resolution studies including exoplanet atmospheres, early stars and fundamental physics. These are research goals. The consortium reported submission of the system Preliminary Design Review documents in June 2026, with the review meeting then expected in late October. <a class=\"svs-reference\" href=\"#as-source-25\" aria-label=\"Source 25\">25<\/a> <a class=\"svs-reference\" href=\"#as-source-26\" aria-label=\"Source 26\">26<\/a><\/p><p><strong>Discuss:<\/strong> How do a science goal, an engineering requirement and a verified performance result differ?<\/p><\/article><\/div><p class=\"svs-small\" style=\"margin-top:18px\">Project-team schedules can change; follow the linked notices before planning observations. These public summaries do not attribute components or contracts to an individual supplier. Personal engineering case studies will be documented separately. No institutional partnership or endorsement is implied.<\/p><\/section>\n  <section id=\"as-practice\" class=\"svs-section svs-workshop\" aria-labelledby=\"as-practice-title\">\n    <span class=\"svs-eyebrow\">Learning lab &middot; Work first, then reveal the explanation<\/span>\n    <h2 id=\"as-practice-title\">Try the reasoning yourself.<\/h2>\n    <p>These exercises are part of this page. Use paper or a calculator, then open each worked solution. They do not require an account, payment or an external app.<\/p>\n\n    <h3 id=\"as-doppler\">Exercise A &middot; How small is a Doppler shift?<\/h3>\n    <span class=\"svs-synthetic\">IDEALISED CALCULATION &middot; NOT AN OBSERVATION<\/span>\n    <p>For line-of-sight speeds much smaller than the speed of light, use the first-order Doppler relation below. In this exercise, positive velocity means motion away from the observer.<\/p>\n    <div class=\"svs-equation\">&Delta;&lambda; \/ &lambda;<sub>0<\/sub> &asymp; v<sub>r<\/sub> \/ c<\/div>\n    <p>Assume &lambda;<sub>0<\/sub> = <strong>500 nm<\/strong>, v<sub>r<\/sub> = <strong>+30 m\/s<\/strong> and c &asymp; <strong>3 &times; 10<sup>8<\/sup> m\/s<\/strong>. Calculate the wavelength shift. Is it toward red or blue?<\/p>\n    <details>\n      <summary>Show the calculation and its meaning<\/summary>\n      <div class=\"svs-answer\">\n        <p>&Delta;&lambda; &asymp; 500 &times; 30 \/ (3 &times; 10<sup>8<\/sup>) nm = <strong>0.000050 nm<\/strong> = <strong>0.05 pm<\/strong>.<\/p>\n        <p>The shift is positive: the wavelength becomes longer, toward red. The model wavelength is about 500.000050 nm. This is arithmetic within the stated approximation, not the precision claimed for a real measurement.<\/p>\n        <p>For comparison, at a resolving power of 115,000, &lambda;\/R<sub>spec<\/sub> at 500 nm is about 0.00435 nm, roughly 87 times this shift. The width of a resolution element and the precision of locating a spectral shift are different quantities. Real precision depends on the information in the spectrum and the measurement system; shifts may be much smaller than a pixel. <a class=\"svs-reference\" href=\"#as-source-2\" aria-label=\"Source 2\">2<\/a> <a class=\"svs-reference\" href=\"#as-source-4\" aria-label=\"Source 4\">4<\/a><\/p>\n      <\/div>\n    <\/details>\n\n    <h3 id=\"as-series\" style=\"margin-top:34px\">Exercise B &middot; A pattern is not yet a planet.<\/h3>\n    <span class=\"svs-synthetic\">SYNTHETIC VALUES &middot; NO REAL STAR &middot; NO MEASUREMENT ERRORS MODELLED<\/span>\n    <p>The following values were generated for this lesson from the idealised function v(t) = 12 sin(2&pi;t\/8), with t in days and v in m\/s. The zero point is arbitrary. No noise, activity or instrumental drift has been added.<\/p>\n    <div class=\"svs-table-wrap\">\n      <table>\n        <caption>Table 1. Synthetic relative radial velocities. These are not HARPS or ESO observations.<\/caption>\n        <thead><tr><th scope=\"col\">Time<br>(days)<\/th><th scope=\"col\">Relative velocity<br>(m\/s)<\/th><\/tr><\/thead>\n        <tbody>\n          <tr><th scope=\"row\">0<\/th><td>0<\/td><\/tr><tr><th scope=\"row\">2<\/th><td>+12<\/td><\/tr><tr><th scope=\"row\">4<\/th><td>0<\/td><\/tr><tr><th scope=\"row\">6<\/th><td>&minus;12<\/td><\/tr><tr><th scope=\"row\">8<\/th><td>0<\/td><\/tr><tr><th scope=\"row\">10<\/th><td>+12<\/td><\/tr><tr><th scope=\"row\">12<\/th><td>0<\/td><\/tr><tr><th scope=\"row\">14<\/th><td>&minus;12<\/td><\/tr><tr><th scope=\"row\">16<\/th><td>0<\/td><\/tr>\n        <\/tbody>\n      <\/table>\n    <\/div>\n    <ol class=\"svs-tasks\"><li>What are the period and semi-amplitude of the generating model?<\/li><li>What are the maximum, minimum and peak-to-peak velocity?<\/li><li>Could these points alone establish that a planet exists? What would you ask to see next?<\/li><\/ol>\n    <details>\n      <summary>Show the solution and the limits of this example<\/summary>\n      <div class=\"svs-answer\">\n        <p>The generating model has a period of <strong>8 days<\/strong> and a semi-amplitude of <strong>12 m\/s<\/strong>. The maximum is +12 m\/s, the minimum is &minus;12 m\/s and the peak-to-peak change is 24 m\/s.<\/p>\n        <p>Because we supplied the generating function, we know what this teaching model does. If we had only the sparsely sampled points, a unique model would not follow automatically: other time behaviour can pass through the same points.<\/p>\n        <p><strong>No planet has been detected in this exercise.<\/strong> For a real target, request uncertainties, observation times, calibration checks, observations at additional times and tests for stellar activity. Do not infer a planet&#8217;s size, habitability or an exact mass from this table.<\/p>\n      <\/div>\n    <\/details>\n    <details>\n      <summary>Discussion &middot; What changes if the instrument drifts?<\/summary>\n      <div class=\"svs-answer\"><p>Imagine adding an unknown wavelength offset to every observation. Before interpreting that change as motion of the star, ask how an independent reference would reveal the offset. Then ask the complementary question: which changes originate in the star and would not appear in the calibration source? This separates instrument testing from interpretation of the target.<\/p><\/div>\n    <\/details>\n  <\/section>\n\n<section id=\"as-quiz\" class=\"svs-section\" aria-labelledby=\"as-quiz-title\"><span class=\"svs-eyebrow\">A short challenge for a friend or a class<\/span><h2 id=\"as-quiz-title\">Six statements. What would you question?<\/h2><p>Decide true or false before revealing the answer. Explain your reasoning; there is no account, scoreboard or certificate.<\/p><div class=\"svs-v3-quiz\"><details><summary>1. A light-year measures time.<\/summary><div class=\"svs-answer\">False. It is a distance: the distance light travels in a year. <a class=\"svs-reference\" href=\"#as-source-10\" aria-label=\"Source 10\">10<\/a><\/div><\/details><details><summary>2. The far side of the Moon never receives sunlight.<\/summary><div class=\"svs-answer\">False. Far side and dark side are not the same thing; illumination changes during the lunar cycle. <a class=\"svs-reference\" href=\"#as-source-11\" aria-label=\"Source 11\">11<\/a><\/div><\/details><details><summary>3. One repeating dip proves the existence of a planet.<\/summary><div class=\"svs-answer\">False. A pattern needs calibration, uncertainty and competing explanations to be checked. <a class=\"svs-reference\" href=\"#as-source-1\" aria-label=\"Source 1\">1<\/a><\/div><\/details><details><summary>4. Webb&#x27;s visible colours can encode infrared measurements.<\/summary><div class=\"svs-answer\">True. Colour mapping allows information beyond human vision to be displayed. Read which filters and mapping were used. <a class=\"svs-reference\" href=\"#as-source-12\" aria-label=\"Source 12\">12<\/a><\/div><\/details><details><summary>5. Doubling the planet radius doubles the ideal transit depth.<\/summary><div class=\"svs-answer\">False for the uniform-disc model in this page: doubling the radius ratio multiplies the blocked area by four.<\/div><\/details><details><summary>6. You must own a telescope to contribute to Exoplanet Watch.<\/summary><div class=\"svs-answer\">False. The project provides a route for requesting existing observations to analyse. Follow its own instructions. <a class=\"svs-reference\" href=\"#as-source-17\" aria-label=\"Source 17\">17<\/a><\/div><\/details><\/div><\/section>\n  <section id=\"as-students\" class=\"svs-section\" aria-labelledby=\"as-students-title\">\n    <span class=\"svs-eyebrow\">For students, educators &amp; curious readers<\/span>\n    <h2 id=\"as-students-title\">One subject. Two ways to explore it.<\/h2>\n    <div class=\"svs-split\">\n      <div class=\"svs-panel\"><h3>For curious readers<\/h3><p>Read the four stages and inspect the photographs. You can skip the equations without losing the main idea: the measurement and the interpretation are not the same thing.<\/p><p>Try explaining why a spectrograph can help find a planet even when the detector image does not show the planet.<\/p><a href=\"#as-question\">Return to the explanation &uarr;<\/a><\/div>\n      <div class=\"svs-panel\"><h3>For a classroom discussion<\/h3><p><strong>Suggested level:<\/strong> upper-secondary or introductory undergraduate physics. Adapt to the group; this is not an accredited course.<\/p><p><strong>Prerequisites:<\/strong> wavelength, speed, ratios and scientific notation. Suggested session: 30&ndash;45 minutes, including discussion.<\/p><p><strong>Learning goals:<\/strong> calculate a small shift, distinguish semi-amplitude from peak-to-peak change, and explain why a model needs independent checks.<\/p><a href=\"#as-practice\">Open the exercises &uarr;<\/a><\/div>\n    <\/div>\n    <details>\n      <summary>Suggested teaching sequence and assessment prompts<\/summary>\n      <div class=\"svs-answer\">\n        <ol><li><strong>5 minutes:<\/strong> ask what could reveal a planet without a separate image.<\/li><li><strong>10 minutes:<\/strong> inspect the instrument photograph and open the linked detector image. Identify observation, reference and interpretation.<\/li><li><strong>15 minutes:<\/strong> complete Exercises A and B before opening their solutions.<\/li><li><strong>5&ndash;15 minutes:<\/strong> discuss an alternative explanation and propose one useful follow-up check.<\/li><\/ol>\n        <p>Ask each learner to write three separate sentences: <em>what was measured<\/em>, <em>what model was proposed<\/em> and <em>what remains untested<\/em>. Assess the distinction between these statements, not merely the numerical answer.<\/p>\n      <\/div>\n    <\/details>\n  <\/section>\n\n  <section id=\"as-glossary\" class=\"svs-section\" aria-labelledby=\"as-glossary-title\">\n    <span class=\"svs-eyebrow\">Keep the vocabulary clear<\/span>\n    <h2 id=\"as-glossary-title\">Four useful terms.<\/h2>\n    <dl class=\"svs-glossary\">\n      <div><dt>Radial velocity<\/dt><dd>The component of relative velocity along the line of sight, not the full speed through space. <a class=\"svs-reference\" href=\"#as-source-1\" aria-label=\"Source 1\">1<\/a><\/dd><\/div>\n      <div><dt>Spectrum<\/dt><dd>A representation of light separated by wavelength. A spectrum is not the same as a picture of the sky. <a class=\"svs-reference\" href=\"#as-source-3\" aria-label=\"Source 3\">3<\/a><\/dd><\/div>\n      <div><dt>Resolving power<\/dt><dd>Here, R<sub>spec<\/sub> = &lambda;\/&delta;&lambda; describes spectral resolution. Do not confuse it with a mirror&#8217;s reflectivity R. <a class=\"svs-reference\" href=\"#as-source-2\" aria-label=\"Source 2\">2<\/a><\/dd><\/div>\n      <div><dt>Calibration reference<\/dt><dd>A reference used to establish or check the measurement scale. The comb features in the linked detector image are a wavelength-reference example. <a class=\"svs-reference\" href=\"#as-source-3\" aria-label=\"Source 3\">3<\/a><\/dd><\/div>\n    <\/dl>\n  <\/section>\n\n<section id=\"as-share\" class=\"svs-section svs-v3-sharebox\" aria-labelledby=\"as-share-title\"><span class=\"svs-eyebrow\">Pass on a question, not a sensational claim<\/span><h2 id=\"as-share-title\">Share one thing that made you curious.<\/h2><p>Invite a friend to try the quiz, give a teacher the exercise link, or send an astronomy club a topic for discussion. These buttons open a draft on the chosen service; you decide what to send and to whom. Nothing is posted automatically.<\/p><div class=\"svs-v3-socials\"><a href=\"https:\/\/wa.me\/?text=How+can+we+find+a+planet+without+photographing+it%3F+Explore+free+astronomy+questions%2C+a+transit+calculator%2C+HARPS+and+citizen-science+routes+on+Stadia.+https%3A%2F%2Fstadiaorg.com%2Fastronomy-science%2F\" target=\"_blank\" rel=\"noopener noreferrer\">WhatsApp<\/a><a href=\"https:\/\/t.me\/share\/url?url=https%3A%2F%2Fstadiaorg.com%2Fastronomy-science%2F&amp;text=How+can+we+find+a+planet+without+photographing+it%3F+Explore+free+astronomy+questions%2C+a+transit+calculator%2C+HARPS+and+citizen-science+routes+on+Stadia.\" target=\"_blank\" rel=\"noopener noreferrer\">Telegram<\/a><a href=\"https:\/\/www.linkedin.com\/sharing\/share-offsite\/?url=https%3A%2F%2Fstadiaorg.com%2Fastronomy-science%2F\" target=\"_blank\" rel=\"noopener noreferrer\">LinkedIn<\/a><a href=\"https:\/\/www.reddit.com\/submit?url=https%3A%2F%2Fstadiaorg.com%2Fastronomy-science%2F&amp;title=How+can+we+find+a+planet+without+photographing+it%3F+A+free+Stadia+astronomy+guide\" target=\"_blank\" rel=\"noopener noreferrer\">Reddit<\/a><a href=\"mailto:?subject=A%20free%20astronomy%20guide%20to%20explore%20together&amp;body=How%20can%20we%20find%20a%20planet%20without%20photographing%20it%3F%20Explore%20free%20astronomy%20questions%2C%20a%20transit%20calculator%2C%20HARPS%20and%20citizen-science%20routes%20on%20Stadia.%0A%0Ahttps%3A%2F%2Fstadiaorg.com%2Fastronomy-science%2F\">Email<\/a><button type=\"button\" data-svs-copy-page hidden>Copy page link<\/button><button type=\"button\" data-svs-native-share hidden>Share with an app<\/button><\/div><label for=\"as-public-link\"><strong>Public page link<\/strong><\/label><textarea id=\"as-public-link\" class=\"svs-v3-public-link\" rows=\"2\" readonly spellcheck=\"false\">https:\/\/stadiaorg.com\/astronomy-science\/<\/textarea><p id=\"as-share-status\" class=\"svs-v3-feedback\" role=\"status\" aria-live=\"polite\"><\/p><div id=\"as-manual-copy\" hidden><label for=\"as-copy-fallback\">Copy this text manually:<\/label><textarea id=\"as-copy-fallback\" class=\"svs-v3-copybox\" rows=\"5\" readonly spellcheck=\"false\"><\/textarea><\/div><p class=\"svs-small\">Respect each community&#8217;s rules and disclose your connection when sharing your own site. No mass messages, artificial engagement or promises of discovery. Third-party sharing services have their own terms and privacy settings.<\/p><details><summary>For a teacher or an astronomy club: a ready-to-use session<\/summary><div class=\"svs-answer\"><p><strong>Suggested session, 30&ndash;45 minutes:<\/strong> begin with one curiosity question, compare the transit and Doppler methods, work through one calculation, then discuss an alternative explanation and one useful follow-up observation.<\/p><p>Send the links <a href=\"#as-transit-lab\">Transit lab<\/a>, <a href=\"#as-practice\">Worked exercises<\/a> and <a href=\"#as-quiz\">Quick quiz<\/a> before the session. These are independent learning materials, not an accredited university course.<\/p><p>Share the original source links and image credits as well as the Stadia explanation.<\/p><\/div><\/details><\/section>\n  <section id=\"as-sources\" class=\"svs-section\" aria-labelledby=\"as-sources-title\">\n    <span class=\"svs-eyebrow\">Read the evidence &middot; Primary sources<\/span>\n    <h2 id=\"as-sources-title\">Scientific sources &amp; image credits.<\/h2>\n    <p class=\"svs-intro\">The original HARPS chapter is retained. New curiosity, observing and participation sections use the public sources linked below; instrument-family summaries use official project documentation. Activities, quizzes and numerical examples are teaching material prepared for Stadia, not newly acquired astronomical data. Source review: 25 September 2026.<\/p>\n    <ol class=\"svs-source-list\">\n      <li id=\"as-source-1\"><strong>1 &middot; NASA Science<\/strong><a href=\"https:\/\/science.nasa.gov\/exoplanets\/how-we-find-and-characterize\/\" target=\"_blank\" rel=\"noopener noreferrer\">How We Find and Characterize<\/a><small>Radial-velocity detection and the distinction from other ways of finding planets.<\/small><\/li>\n      <li id=\"as-source-2\"><strong>2 &middot; ESO instrument documentation<\/strong><a href=\"https:\/\/www.eso.org\/sci\/facilities\/lasilla\/instruments\/harps\/overview.html\" target=\"_blank\" rel=\"noopener noreferrer\">HARPS instrument overview<\/a><small>Instrument design, telescope, vacuum enclosure, reference spectra and resolving power. No claim here that HARPS is the most precise instrument currently available.<\/small><\/li>\n      <li id=\"as-source-3\"><strong>3 &middot; ESO observation image<\/strong><a href=\"https:\/\/www.eso.org\/public\/images\/harps_cc\/\" target=\"_blank\" rel=\"noopener noreferrer\">A raw spectrum, straight from HARPS<\/a><small>Public explanation of the detector image and the calibration features. Image credit: ESO.<\/small><\/li>\n      <li id=\"as-source-4\"><strong>4 &middot; NASA Science<\/strong><a href=\"https:\/\/science.nasa.gov\/science-research\/science-enabling-technology\/technology-highlights\/neid-a-new-ultra-precise-window-into-nearby-worlds\/\" target=\"_blank\" rel=\"noopener noreferrer\">NEID: A New, Ultra-Precise Window into Nearby Worlds<\/a><small>A separate instrument illustrating the need for stability when shifts correspond to fractions of a pixel. NEID is not HARPS.<\/small><\/li>\n      <li id=\"as-source-5\"><strong>5 &middot; ESO, 2010 announcement<\/strong><a href=\"https:\/\/www.eso.org\/public\/announcements\/ann1005\/\" target=\"_blank\" rel=\"noopener noreferrer\">HARPS polarimetry and stellar activity<\/a><small>Why investigating starspots matters when checking a candidate signal. Historical announcement; its superlatives are not presented here as current rankings.<\/small><\/li>\n      <li id=\"as-source-6\"><strong>6 &middot; Observatory photograph<\/strong><a href=\"https:\/\/www.eso.org\/public\/images\/beletsky_lasilla_02\/\" target=\"_blank\" rel=\"noopener noreferrer\">ESO 3.6-metre telescope and the Milky Way<\/a><small>Credit: Y. Beletsky (LCO)\/ESO. Displayed proportionally without intentional crop; published by ESO in 2014.<\/small><\/li>\n      <li id=\"as-source-7\"><strong>7 &middot; Instrument photograph<\/strong><a href=\"https:\/\/www.eso.org\/public\/images\/eso0308b\/\" target=\"_blank\" rel=\"noopener noreferrer\">The HARPS spectrograph during laboratory tests<\/a><small>Credit: ESO. Displayed proportionally without intentional crop; published by ESO in 2003.<\/small><\/li>\n      <li id=\"as-source-8\"><strong>8 &middot; Image-use conditions<\/strong><a href=\"https:\/\/www.eso.org\/public\/copyright\/\" target=\"_blank\" rel=\"noopener noreferrer\">ESO copyright notice<\/a><small>Images used under <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\" target=\"_blank\" rel=\"noopener noreferrer\">Creative Commons Attribution 4.0<\/a>, with credits visible beside each image. Attribution does not imply partnership or endorsement.<\/small><\/li>\n      <li id=\"as-source-9\"><strong>9 &middot; ESO public instrument guide<\/strong><a href=\"https:\/\/www.eso.org\/public\/teles-instr\/lasilla\/36\/harps\/\" target=\"_blank\" rel=\"noopener noreferrer\">HARPS and the radial-velocity method<\/a><small>Instrument identity, telescope and explanation of spectral shifts. Technical values used in the exercises follow the science overview in source 2.<\/small><\/li>\n<li id=\"as-source-10\"><strong>10 &middot; NASA Space Place<\/strong><a href=\"https:\/\/spaceplace.nasa.gov\/light-year\/en\/\" target=\"_blank\" rel=\"noopener noreferrer\">What is a light-year?<\/a><small>Light travel time and looking into the past. Historical record claims in the original article are not repeated as current records.<\/small><\/li>\n<li id=\"as-source-11\"><strong>11 &middot; NASA Science<\/strong><a href=\"https:\/\/science.nasa.gov\/moon\/top-moon-questions\/\" target=\"_blank\" rel=\"noopener noreferrer\">Top Moon questions<\/a><small>Why the far side is not permanently dark; phases are not ordinary shadows cast by Earth.<\/small><\/li>\n<li id=\"as-source-12\"><strong>12 &middot; NASA \/ STScI<\/strong><a href=\"https:\/\/science.nasa.gov\/mission\/webb\/science-overview\/science-explainers\/how-are-webbs-full-color-images-made\/\" target=\"_blank\" rel=\"noopener noreferrer\">How are Webb full-colour images made?<\/a><small>Infrared observations mapped to visible colours. An assigned colour is not evidence that an image is fabricated.<\/small><\/li>\n<li id=\"as-source-13\"><strong>13 &middot; ESO \/ EHT Collaboration<\/strong><a href=\"https:\/\/www.eso.org\/public\/images\/eso1907a\/\" target=\"_blank\" rel=\"noopener noreferrer\">First image of a black hole<\/a><small>M87* image released on 10 April 2019; radio emission around the shadow. Image credit: EHT Collaboration. Displayed in its original proportions under the ESO image-use terms in source 8.<\/small><\/li>\n<li id=\"as-source-14\"><strong>14 &middot; NASA, 17 April 2014<\/strong><a href=\"https:\/\/www.nasa.gov\/news-release\/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone\/\" target=\"_blank\" rel=\"noopener noreferrer\">Habitable zone does not establish habitability<\/a><small>The historical Kepler-186f announcement explains why location alone does not determine surface conditions. We do not present being in this zone as evidence of life.<\/small><\/li>\n<li id=\"as-source-15\"><strong>15 &middot; NASA Science<\/strong><a href=\"https:\/\/science.nasa.gov\/moon\/viewing-tips\/\" target=\"_blank\" rel=\"noopener noreferrer\">Moon viewing tips<\/a><small>Naked-eye and binocular observations; lunar shadows and the terminator. The activity here is our suggested exercise, not a NASA-run course.<\/small><\/li>\n<li id=\"as-source-16\"><strong>16 &middot; NASA Science<\/strong><a href=\"https:\/\/science.nasa.gov\/eclipses\/safety\/\" target=\"_blank\" rel=\"noopener noreferrer\">Solar observing safety<\/a><small>Never use unfiltered optics on the Sun. Eclipse glasses and ordinary sunglasses are not a substitute for a proper front-mounted solar filter on an optical instrument.<\/small><\/li>\n<li id=\"as-source-17\"><strong>17 &middot; NASA Science<\/strong><a href=\"https:\/\/science.nasa.gov\/citizen-science\/exoplanet-watch\/\" target=\"_blank\" rel=\"noopener noreferrer\">Exoplanet Watch<\/a><small>Real transit observations and analysis. Data can also be requested without owning a telescope. Participation, availability and validation are controlled by the project.<\/small><\/li>\n<li id=\"as-source-18\"><strong>18 &middot; NASA Science<\/strong><a href=\"https:\/\/science.nasa.gov\/citizen-science\/exoplanet-watch\/how-to-contribute\/\" target=\"_blank\" rel=\"noopener noreferrer\">Exoplanet Watch: how to contribute<\/a><small>External steps, observer registration and EXOTIC workflow. These are not services hosted by Stadia.<\/small><\/li>\n<li id=\"as-source-19\"><strong>19 &middot; NASA Science<\/strong><a href=\"https:\/\/science.nasa.gov\/citizen-science\/planet-hunters-tess\/\" target=\"_blank\" rel=\"noopener noreferrer\">Planet Hunters TESS<\/a><small>Learn to inspect real light curves. A flagged feature is a candidate for follow-up, not a confirmed discovery or a promise of credit.<\/small><\/li>\n<li id=\"as-source-20\"><strong>20 &middot; NASA \/ JPL-Caltech<\/strong><a href=\"https:\/\/science.nasa.gov\/tutorials\/eyes-on-exoplanets-tutorial\/\" target=\"_blank\" rel=\"noopener noreferrer\">Eyes on Exoplanets tutorial<\/a><small>Explore systems and observing methods. Planet surfaces in the viewer are artist concepts, not close-up photographs of the planets.<\/small><\/li>\n<li id=\"as-source-21\"><strong>21 &middot; University of Geneva<\/strong><a href=\"https:\/\/plone.unige.ch\/HARPS-N\/overview\" target=\"_blank\" rel=\"noopener noreferrer\">HARPS-N project overview<\/a><small>Northern Hemisphere counterpart at the TNG. Public instrument history, not supplier attribution.<\/small><\/li>\n<li id=\"as-source-22\"><strong>22 &middot; ESO, 6 December 2017<\/strong><a href=\"https:\/\/www.eso.org\/public\/news\/eso1739\/\" target=\"_blank\" rel=\"noopener noreferrer\">ESPRESSO first light<\/a><small>ESPRESSO as the successor to HARPS at the VLT. The announcement date is distinguished from a statement about present observing availability.<\/small><\/li>\n<li id=\"as-source-23\"><strong>23 &middot; ESO, 4 April 2023<\/strong><a href=\"https:\/\/www.eso.org\/sci\/publications\/announcements\/sciann17568.html\" target=\"_blank\" rel=\"noopener noreferrer\">NIRPS starts operations<\/a><small>Science operations from 1 April 2023 and simultaneous near-infrared observations with HARPS.<\/small><\/li>\n<li id=\"as-source-24\"><strong>24 &middot; Terra Hunting Experiment<\/strong><a href=\"https:\/\/www.terrahunting.org\/\" target=\"_blank\" rel=\"noopener noreferrer\">HARPS3 \/ Terra Hunting project status<\/a><small>The team forecasts a 2027 survey start after the June 2026 cryostat failure. Checked on 25 September 2026; this is not a guaranteed date.<\/small><\/li>\n<li id=\"as-source-25\"><strong>25 &middot; ESO ELT<\/strong><a href=\"https:\/\/elt.eso.org\/instrument\/ANDES\/\" target=\"_blank\" rel=\"noopener noreferrer\">ANDES, formerly ELT-HIRES<\/a><small>A planned ELT spectrograph, not an instrument already producing the discoveries set out in its science goals.<\/small><\/li>\n<li id=\"as-source-26\"><strong>26 &middot; ANDES consortium \/ INAF<\/strong><a href=\"https:\/\/andes.inaf.it\/news\/\" target=\"_blank\" rel=\"noopener noreferrer\">ANDES development milestones<\/a><small>30 June 2026: system PDR documents submitted; review meeting expected in late October. No completed review or operational date is inferred.<\/small><\/li>\n    <\/ol>\n  <\/section>\n\n  <aside class=\"svs-closing\" aria-labelledby=\"as-next-title\">\n    <h3 id=\"as-next-title\">Begin with evidence. Keep asking questions.<\/h3>\n    <p>You do not have to read everything in one sitting. Choose one question, test one idea and return with an observation or a better question. Use the sources and external projects to go further; use the sharing tools to invite someone into the discussion.<\/p>\n    <div class=\"svs-actions\"><a class=\"svs-button svs-button-purple\" href=\"#as-practice\">Return to the learning lab &uarr;<\/a><a class=\"svs-button\" href=\"\/#sv-astronomy\">Back to Stadia Home &rarr;<\/a><\/div>\n  <\/aside>\n  <p class=\"svs-editorial\">Stadia Science &middot; Explore, observe &amp; share &middot; Version 3.0 &middot; 25 September 2026.<br>Independent educational material, not an accredited course. Cited institutions and citizen-science projects are external sources, not Stadia partners or endorsers. No payment, account or new data collection is required by this page. Translation depends on the site&#8217;s existing language system; only the supplied English content is included here.<\/p>\n<\/div>\n\n<script>\n(function () {\n  'use strict';\n  function initStadiaScience() {\n    var root = document.getElementById('sv-astronomy-page');\n    if (!root || root.dataset.svsReady === '1') return;\n    root.dataset.svsReady = '1';\n    var publicURL = root.getAttribute('data-public-url');\n    \/\/ Fixed public URL: do not copy preview parameters, account links or room credentials.\n    if (publicURL !== 'https:\/\/stadiaorg.com\/astronomy-science\/') return;\n    var feedback = root.querySelector('#as-share-status');\n    function say(message) { if (feedback) feedback.textContent = message; }\n    function manual(text) {\n      var box = root.querySelector('#as-manual-copy');\n      var field = root.querySelector('#as-copy-fallback');\n      if (!box || !field) return;\n      field.value = text; box.hidden = false;\n      field.focus(); field.select();\n      say('Automatic copying is unavailable. 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