About ObserveTheSky

A model of the universe you can fly through, a planetarium for tonight’s sky from your own town, and explanations of how anybody knows — in one place, with one database under all of it. Free, no ads, no account, and it works offline.

ObserveTheSky is for everybody who wants to know what is really up there, and how it was found out. Every number here carries its working: what was measured, how, and where the certainty runs out.

For children there is AstroKids: the same sky, made to be fun and to get them asking questions.

Who makes it

I’m Jan Mach, a software engineer. Astronomy is my hobby, and what I love most is visual observing: the thing itself, seen with my own eyes through my own eyepiece.

I made this site because I could not find three things in one place.

The first was a model of the solar system and the universe beyond it, joined to the real sky through the Planetarium. Look at Jupiter and its moons in the model, see what they look like through a telescope and where each moon is right now, then find them in the sky with the Planetarium.

The second was a place for everybody interested in the sky, with the questions they have and sometimes find hard to get answered: how far, how big, and above all, how do we know? The answer also shows why people once did not believe a thing, and why it is believed today. A guess you can test has a name — a hypothesis — and every idea in astronomy started as one. The good part is how somebody found out whether it was right, often centuries before anybody could go and look, and that history is told here too.

The third was something that sends you outside: to feel the Earth turning under you, to watch the Moon change night after night, to see Jupiter’s moons move from one evening to the next.

Put simply: look at Jupiter in the model, where you can see Io’s volcanoes too, and find out how anybody knows about them; go outside and find Jupiter in the sky; and some other time, read how somebody worked out how far away it is.

Not everything is here, but the interesting things an ordinary person could remember are.

I write “we” because every time the site is used, it gets better, so everybody who uses it has a share in its corrections and improvements. Secondary-school maths teachers, for one, use it as inspiration to enrich their lessons.

Jan Mach on LinkedIn →

Dedicated to our daughters Marie, Dora and Zoe — to inspire them.

What’s here, and why

Space — find a thing, and go to it

Search the list for anything the site knows: a planet, a moon, a comet, a spacecraft, a galaxy. Choose it and the view travels there. Nothing is a still picture — the planets go round the Sun and the moons go round their planets, at whatever speed you pick, from ten minutes a second to a year a second — fast enough for Jupiter to go once round the Sun in twelve seconds.

Zoom out and the solar system becomes the nearby stars, then the Milky Way, then other galaxies. Zoom in on Jupiter and the Great Red Spot is where it really is: the planet turns at its real speed, so the spot comes round when it should. The four big moons keep their own time too.

Why the whole universe? Because the questions do not stop at Neptune: how far away is the nearest star, what is the Milky Way, how big is a galaxy. And why so little of it? The Milky Way alone has a few hundred billion stars, and beyond it there are hundreds of billions of galaxies. ObserveTheSky shows 113 things: ones you can find in your own sky, ones with a story, and ones that show how big it all is.

The model is not only for looking at. Things happen in it, on their own dates, and several of them are things you can then go outside and see for real: you learn what to expect before you are standing in the cold looking for it.

See what there is to catch →

Planetarium — the same things, in your sky, tonight

Much of what is in Space is also something you might go outside and see. Give the Planetarium your town and a date and it draws the sky you would actually walk out into: the constellations where they will be, the Moon at the phase it will be, and how far under the horizon the Sun has gone.

Why a planetarium as well as a model? The model shows where things are; the Planetarium shows where they are from where you stand. Anybody who wants to see something for themselves needs the second, and both read the same database, so the two cannot disagree.

It answers two questions. What’s up — either across a whole evening, in the order things set, or at the one moment you have chosen. And Coming up — eclipses, meteor showers, conjunctions (two things side by side), oppositions (a planet opposite the Sun, and near its closest), a planet turning retrograde (seeming to go backwards) — each with its date, its time, and how high it will stand from where you live.

It will also tell you what a thing looks like through a small telescope, which is usually less than a photograph led you to hope. Saturn is the one that really does look like its pictures. Most things are a faint smudge, and the site says so.

Sky — a page for every object

Every object has two pages of its own. One says what it is: its size, its distance, how it measures up against things you already know, and — where published values disagree — which one is used. The other says where to find it in the sky, and what you will really see with your eyes, with binoculars or through a small telescope.

Explanations — how big, how far, and how anybody knows

It starts by letting you feel how big the Earth is. Then the solar system, with a first look at the size of the Milky Way and the universe. On top of that sits the whole cosmic ladder of sizes and distances — and every rung of it carries how somebody found that number out.

Why explanations? Because a number on its own teaches nothing. That the Earth is 40,000 kilometres round is a fact; how somebody worked it out more than two thousand years ago, with two sticks and their shadows, is something you understand. Each explanation starts from a question, lets you make your own guess, and then shows how people found out.

There is more in there than the ladder: what an eclipse actually is, how a star is born and the ways it can end. And every page of the ladder ends the same way — the assumptions somebody had to make, the arithmetic, and something to go and try for yourself.

There are more than thirty of these, and they are not a course: each one stands on its own, so you can start wherever the question came from.

Games — a minute each, and the real answer afterwards

Short games about what is actually up there. Which is hotter, which is more massive, how long a world takes to turn. Nothing is scored and nothing is kept — each one tells you what is true once you have committed.

Why games? Because a guess you have made yourself is remembered better than an answer you were given.

Guides — the long way round, when you want it

Longer pages that cut across everything here: the solar system in one read, our galaxy in another, and lists of what is worth looking at.

Honest about what you will see

Most of the pictures are drawn, not photographed, and that is deliberate: a photograph promises something your telescope will not give you. This site promises what you can go outside and actually find — with a small telescope, with binoculars, or with nothing at all.

The distances are true; the sizes are not, because a world drawn to scale would be a dot too small to see — switch on True sizes and see for yourself. And the constellations, the Milky Way and the far galaxies are scenery: nice to look at, roughly placed, not to be measured.

Go out and see it yourself!

Everything here is offered as it is, for learning and for going outside. Use it at your own risk: we cannot be responsible for what happens when you follow an activity, build something from a printable, or put one of our sheets to use — including anything damaged or lost, or anyone hurt. A grown-up is responsible for the children doing these activities. If you put a printed poster or marker anywhere that is not your own, ask whoever owns it first, and take it down when you have finished.

How it is built

Everything here comes from one database, and its numbers are taken from space agencies, catalogues and published papers. The model, the pages and the games all read that one copy, so a correction made once is right everywhere.

The planets move along the orbits NASA JPL publishes, and the dates of eclipses and other events are worked out with JPL’s DE440 ephemeris, the tables professional astronomers use, rather than copied from a list.

Every page exists in every language the site offers, and the whole site works offline.

How it is checked

Before anything is published, every build runs more than a hundred automatic checks. Every link has to lead somewhere, every page has to be complete in every language, the planets’ sizes in the sky have to match published values, every comparison picture has to be drawn at its measured ratio, and a view through an eyepiece must never show more than that instrument really would. A separate test breaks many of them on purpose, to prove they still catch what they are there for.

Then the pages are read whole, in every language, round after round, and each mistake is traced back to where it came from and fixed there.

Written with AI

This site is written and built with the help of an AI assistant, Claude, made by Anthropic. I decide what goes on it, I read it and correct it, and its facts are checked against published sources. An AI can still be wrong, and so can I.

If you find a mistake, please tell us — the Contact us link is at the bottom of every page. Every correction is also a small measure of how often the AI got it right, and we would like to know.

What we count — and no cookies

To see which parts of this site are used, we read our own web server’s log. While you use a page, it now and then loads a tiny image from our server, and the address of each image says what happened — so from the log we see how long the page is on screen (after 30 seconds, 2, 5 and 15 minutes), which part of the site you open, which object you pick, how you use the moving parts (playing time, speed, zoom, date, the lists, how you set your place), how many results a search finds, and when you start or answer a game and, where the site has them, reach the end of a story or the welcome tour. When a search for an object or a page finds nothing, we see what was searched for, so that we can add it. Like any web server’s, our log also notes the address each request came from; we use it only to tell one visit from another. Nothing else: no cookies, nothing sent to anybody else, and not your location. What your browser remembers for you here — your language, the town you chose, your kind of sky — stays on your device. Offline, or if your browser asks sites not to track it, nothing is sent at all. Save a page while you have signal and it still works with none — on the train, in a field, up a hill.

Credits

ObserveTheSky is made with love for anyone who looks up.

• Planet facts & orbits based on public NASA/JPL data.
• Space agencies: NASA & ESA — public images and data.
• Fonts: Fredoka & Nunito (Google Fonts).
• Inspiration: NASA, ESA, and everyone who looks up at night.
• Earth map: NASA Earth Observatory, Blue Marble: Next Generation (Reto Stöckli, NASA GSFC), lightened for the screen so the sea reads as sea; nothing added, nothing removed.
• Earth at night: NASA Earth Observatory, Earth at Night (Black Marble) 2016, from Suomi NPP VIIRS Day-Night Band data.
• Moon map: NASA’s Scientific Visualization Studio, CGI Moon Kit, from Lunar Reconnaissance Orbiter data.
• Mars map: USGS Astrogeology Science Center, Mars Viking Global Color Mosaic, from NASA Viking Orbiter data.
• Mercury map: Applied Coherent Technology Corporation and the MESSENGER team (NASA, Johns Hopkins Applied Physics Laboratory, Arizona State University, Carnegie Institution of Washington), published by the USGS Astrogeology Science Center.
• Venus map: USGS Astrogeology Science Center, Venus Magellan Global C3-MDIR Synthetic Color Mosaic, from NASA Magellan radar data (PDS Geosciences Node). Radar, not a photograph: it shows the rock under the clouds, which no eye can see.
• Io map: USGS Astrogeology Science Center, Io Galileo SSI Global Color Merge Mosaic, from NASA Galileo data (Planetary Data System). False colour: the tones are stretched to bring out the volcanoes, and the real surface is paler and more muted than this.
• Europa map: USGS Astrogeology Science Center, Europa Voyager – Galileo SSI Global Mosaic, from NASA Voyager and Galileo data (Planetary Data System). The mosaic is greyscale — there is no colour version of it — so Europa looks grey here; the real surface is a pale, ochre-streaked white.
• Ganymede map: USGS Astrogeology Science Center, Ganymede Voyager – Galileo SSI Global Mosaic, from NASA Voyager and Galileo data (Planetary Data System). The mosaic is greyscale — there is no colour version of it — so Ganymede looks grey here; the real surface is a dull brownish grey with paler grooved bands.
• Callisto map: USGS Astrogeology Science Center, Callisto Voyager – Galileo SSI Global Mosaic, from NASA Voyager and Galileo data (Planetary Data System). The mosaic is greyscale — there is no colour version of it — so Callisto looks grey here; the real surface is a dark brownish grey, covered in craters, with brighter ice where impacts have dug it up.
• Enceladus map: USGS Astrogeology Science Center, Enceladus Cassini Global Mosaic 110m, from NASA Cassini data — credit: the Cassini Team, the Jet Propulsion Laboratory and the Space Science Institute. The mosaic is greyscale, and stretched to bring out the cracks and craters, so Enceladus looks mid-grey here; the real moon is brilliant white — almost pure water ice, and the most reflective thing in the solar system.
• Ceres map: USGS Astrogeology Science Center, Ceres Dawn FC Global Mosaic 140m, from NASA Dawn Framing Camera data — credit NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. The mosaic is greyscale and its tones are evened out to show detail, so Ceres looks lighter here than it is: the real surface is dark, about as dark as fresh asphalt. The bright spots in Occator crater are real, though — they are salt left behind by water.
• Pluto map: USGS Astrogeology Science Center, Pluto New Horizons LORRI/MVIC Global Mosaic 300m (July 2017), from NASA New Horizons data. Public domain. The mosaic is greyscale — the map was made in black and white, and Pluto’s real colour is a butterscotch tan. New Horizons flew past rather than going into orbit, so the side it passed is sharp and the far side coarse — and the far south, in the dark of winter, was never photographed at all: it is shown as a plain grey.
• Triton map: USGS Astrogeology Science Center, Triton Voyager 2 Global Color Mosaic 600m, by the Lunar and Planetary Institute from NASA/JPL Voyager 2 data — please cite the Lunar and Planetary Institute, NASA, the Jet Propulsion Laboratory and Paul Schenk. Public domain. Voyager 2 saw only the south and the middle of Triton; the north was dark in winter and has never been photographed, so it is shown as a plain surface in the average colour of the rest.
• Io’s volcanoes: positions from the USGS Gazetteer of Planetary Nomenclature (IAU); which vents are persistently active, and that they glow in eclipse, from Davies, Perry, Williams, Veeder & Nelson, The Planetary Science Journal (2024), CC BY 4.0.
• Sun map: courtesy of NASA/SDO and the AIA, EVE, and HMI science teams. HMI white-light images from one turn of the Sun, April and May 2024, laid into a map of its whole surface. Public domain. Shown in grey, because sunlight is white.
• Sun’s corona and prominences: NASA/Keegan Barber, a photograph of the total solar eclipse of 8 April 2024, from Dallas, Texas.
• Picture of Sagittarius A*: EHT Collaboration, the Event Horizon Telescope’s 2022 image, via ESO (CC BY 4.0).
• Picture of M87*, the black hole at the heart of M87: EHT Collaboration, the Event Horizon Telescope’s 2019 image, via ESO (CC BY 4.0).
• Infrared picture of the stars round Sagittarius A*: ESO, VLT/NACO, 2002 (CC BY 4.0).
• Shapes of Phobos, Deimos, Vesta, Bennu, Didymos, Dimorphos, comet Wild 2 and Halley’s Comet: the missions’ shape models, all public domain — Vesta’s from NASA’s 3D models, the rest from NASA’s Planetary Data System: R. W. Gaskell (Phobos); P. C. Thomas (Deimos); NASA VTAD (Vesta); NASA/OSIRIS-REx, M. G. Daly, O. S. Barnouin, R. C. Espiritu, D. S. Lauretta (Bennu); NASA/Johns Hopkins APL (Didymos, Dimorphos); T. Farnham, M. Duxbury, J.-Y. Li (Wild 2); P. Stooke (Halley). Simplified to about 5,000 triangles each.
• Shape of Ryugu: ISAS/JAXA, from Watanabe et al. 2019. Simplified to about 5,000 triangles.
• Shape of comet 67P/Churyumov-Gerasimenko: ESA/Rosetta/NAVCAM (CC BY-SA 3.0 IGO). The simplified copy on this site is shared under the same licence.
• Spacecraft shapes: NASA 3D Resources’ models, public domain — the International Space Station and the Voyagers by NASA/Michael D. Carbajal, Hubble by NASA VTAD, Parker Solar Probe by NASA/Matthew J. Garcia, Cassini by NASA/JPL (Solar System Simulator, M. Oberle), the James Webb Space Telescope by NASA/Christopher R. Meaney. Simplified to a few thousand triangles each.
• Voyager’s Golden Record: photograph of the record’s cover, courtesy of NASA/JPL-Caltech.
• Galaxy shapes in Space: drawn from published measurements — the S4G survey (Salo et al. 2015, ApJS 219, 4), THINGS (de Blok et al. 2008), Courteau et al. 2011 (Andromeda), Kam et al. 2015 and Regan & Vogel 1994 (Triangulum), Davis et al. 2012 (spiral arms).
• The Large Magellanic Cloud’s shape in Space: from a photograph by Zdeněk Bardon/ESO (CC BY 4.0), laid into its measured tilted disc.
• The Andromeda Galaxy’s shape in Space: from the Local Group Survey’s mosaic, Local Group Survey Team and T.A. Rector (University of Alaska Anchorage), via NOIRLab (CC BY 4.0), laid into its measured tilted disc.
• The Triangulum Galaxy’s shape in Space: from the Local Group Survey’s mosaic, Local Group Survey Team and T.A. Rector (University of Alaska Anchorage), via NOIRLab (CC BY 4.0), laid into its measured tilted disc.
• Bode’s Galaxy’s shape in Space: from T.A. Rector (University of Alaska Anchorage) and NOIRLab/NSF/AURA’s image of M81 and M82 (CC BY 4.0), laid into its measured tilted disc.
• The Whirlpool Galaxy’s shape in Space: from K. Rhode, M. Young and WIYN/NOIRLab/NSF/AURA’s image of M51 (CC BY 4.0), laid into its measured tilted disc.
• The Pinwheel Galaxy’s shape in Space: from George Jacoby, Bruce Bohannan, Mark Hanna/NOIRLab/NSF/AURA’s image of M101 (CC BY 4.0), laid into its measured tilted disc.
• The Fireworks Galaxy’s shape in Space: from KPNO/NOIRLab/NSF/AURA/T.A. Rector (University of Alaska Anchorage)/H. Schweiker (WIYN)’s image of NGC 6946 (CC BY 4.0), laid into its measured tilted disc.
• The Sombrero Galaxy’s photograph in Space, seen from the Earth’s direction: ESO/IDA/Danish 1.5 m/R. Gendler and J.-E. Ovaldsen (CC BY 4.0).
• The Cigar Galaxy’s photograph in Space, seen from the Earth’s direction: from T.A. Rector (University of Alaska Anchorage) and NOIRLab/NSF/AURA’s image of M81 and M82 (CC BY 4.0).
• Star clusters in Space: where the Pleiades’ and the Beehive’s stars are on the sky, from UCAC4 (US Naval Observatory), public domain. How far each one lies in front of the middle or behind it is a model.
• Pillars of Creation in Space: NASA and STScI’s 3D model (Leah Hustak, Ralf Crawford, STScI), public domain under STScI’s content policy, simplified to about 7,500 triangles.
• Orbits of the stars S2, S29, S38 and S55 round Sagittarius A*: GRAVITY Collaboration 2022, A&A 657, L12, Table B.1 (CC BY 4.0).
• Young stars in the Pillars of Creation: positions from the SIMBAD database, operated at CDS, Strasbourg, France.
• Exoplanet systems in Space: This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Each planet’s numbers are credited to its own paper in the database.
• The Sagittarius Dwarf’s stream in Space: drawn from the simulation of Vasiliev, Belokurov & Erkal 2021 (MNRAS 501, 2279), its present-day snapshot on Zenodo (record 4038141), CC BY 4.0 — 8,000 of its 200,000 star particles.
• The Antennae Galaxies in Space: the shape is traced from a Legacy Surveys image (Legacy Surveys / D. Lang (Perimeter Institute), CC BY 4.0); the depth, which nobody can measure, is a model after Karl et al. 2010 (ApJL 715, L88).