Space Time
Free astronomy software in your browser
Most astronomy software asks you to download an app, pick a platform and sometimes pay. Space Time is free online astronomy software that runs in your browser, with no install and no account. It is a planetarium and star map of the night sky, an ephemeris and almanac, a tonight observing planner, an eclipse predictor and a real-time 3D solar system, all in one page. Use it for casual stargazing or serious observing, on desktop, tablet or phone.
🚀 Try it now, nothing to install Free · No account · No subscription · Any modern browser
Everything the term covers, in one tool
When people look for astronomy software they usually mean a handful of distinct jobs: a planetarium to see the sky, tables to plan by, an eclipse predictor, a deep-sky atlas. Space Time does each of them, and they share one consistent set of positions computed from the astronomy-engine library (JPL ephemerides), accurate to about an arcsecond.
🌌 Planetarium & star chart
The sky for your location and time, with 2.5 million real stars (HYG plus Tycho-2, Gaia DR3 distances), constellations and the Milky Way. Stand on Earth, the Moon or Mars and look up, with rise, transit and set times for everything in view.
📈 Ephemeris & almanac
Generate almanac tables for any planet or body: position, brightness and apparent size over whatever date range you choose, with brightness and distance graphs and CSV export. Oppositions and best nights are flagged automatically.
📋 Observing planner
A what's-up-tonight planner that ranks the Moon, the planets and the deep-sky catalog for your site and night, scored by altitude, magnitude and how much the Moon interferes. Build your own named observing lists and see every target circled in the live sky.
🌑 Eclipse prediction
Solar and lunar eclipse prediction for any date and place, then played out in 3D: totality with the Sun's corona, or Earth's shadow turning the Moon blood-red, viewed from the ground or from space.
🔭 Deep-sky catalog
Over 32,000 deep-sky objects: the full NGC/IC plus Sharpless, Arp, Abell, globular and open clusters, planetary nebulae and PGC galaxies, drawn as classic chart symbols color-coded by type. Click any object to identify it, with real survey photography on 200+ showpieces.
🛰️ Satellite & ISS passes
Visible-pass predictions for the ISS, Tiangong and Hubble, computed from live two-line element sets with the SGP4 propagator. Or turn on the full Celestrak catalog, about 9,000 satellites, and watch them orbit Earth in real time.
A real-time 3D solar system, not just a flat chart
Most astronomy software stops at the dome of the sky. Space Time keeps going. Every planet, moon, dwarf planet, asteroid and comet rides its true orbit in 3D, rendered with real surface maps. Fly out to the Milky Way and on to 43,500 real galaxies in the cosmic web, or replay the Apollo missions, Voyager and New Horizons on their historical trajectories. The chart and the spaceship are the same app.
Browser-based, versus installed desktop astronomy software
The big names in astronomy software (Stellarium, SkySafari, Starry Night, Celestia) are installed apps. They are mature and capable, and several are excellent. Space Time takes a different shape: free, browser-based, nothing to install, and identical on every device.
Nothing to install
No download, no setup, no platform choice. Open the page and it runs. Share a link and the other person is looking at the same view a second later, no install on their end either.
Same on every device
One build runs on Windows, Mac, Linux, Android and iOS. Installed software is usually bought and updated per platform; here there is nothing to repurchase when you switch machines.
Free, with no tiers
No paid version, no subscription, no premium add-on for the planner or the weather. Where desktop astronomy software charges per app or per upgrade, Space Time is free.
When installed software is the better choice
We would rather be straight about it. If you drive a motorized GoTo mount at the eyepiece, do astrophotography with camera control and plate solving, or need to observe fully offline under a dark sky, an installed app like Stellarium or SkySafari Pro is still the right tool. Space Time has a telescope and eyepiece field-of-view simulator and a push-to finder that guides you by hand, but it does not control a mount, and it needs a connection to load. Where it competes is everywhere else: the browser, the phone, the classroom, the price, and every "what is the sky doing tonight, and what is that thing" question that should not cost a subscription.
FAQ
What is the best free astronomy software?
It depends on the job. For an installed desktop planetarium, Stellarium is the standard. For free astronomy software with nothing to install, that also covers ephemerides, planning, eclipses, satellites and a 3D solar system, Space Time runs in any browser.
Is it really free, with nothing to install?
Yes. No account, no download, no paid tier and no subscription. It runs client-side in your browser, and the same build works on every platform.
How accurate are the positions?
Planet, Moon and Sun positions come from the astronomy-engine library (JPL ephemerides) and land within about an arcsecond. Satellites use real TLE data with SGP4, and small-body orbits come from JPL.
What does it do as astronomy software?
Planetarium and star chart, ephemeris and almanac tables, a tonight observing planner, eclipse prediction, a 32,000+ object deep-sky catalog, ISS and satellite pass forecasts, an FOV simulator, and a real-time 3D solar system with mission replays.
Can it control my telescope?
Not yet. It has an FOV simulator and a push-to finder that guides you by hand, but it does not drive a motorized GoTo mount.
Does it work offline?
Not yet. It needs a connection to load, then runs fully client-side.
Is it good astronomy software for beginners?
Yes. There is nothing to install or set up, so it is an easy first step into stargazing: open the page, set your location, and the night sky is drawn for you, with a what's-up-tonight planner that tells you what to look at. Amateur astronomers get the depth too, with ephemeris tables, deep-sky catalogs and positions accurate enough to plan a real session. It works as a light astronomy app or a serious program, in the same browser tab.