Asterarium
The sky of any place, at any moment.
This page introduces what Asterarium shows, together with how the sky itself works.
By the end, the sky on your screen should look a little different.
What is Asterarium?
Asterarium is a planetarium that runs in your browser. Choose a place on Earth and a date and time, and the sky of that moment appears.
What you see is the real sky. The arrangement of the stars, the shape of the Moon, the positions of the planets — all follow genuine astronomical calculations.
No telescope, no installation. The sky on your screen matches the one you would see if you stepped outside and looked up.
A sky made of real data
About 40,000 stars appear on screen, drawn from a real star catalog. Not one of them is invented.
A star's brightness is measured in 'magnitude'. Smaller numbers mean brighter stars, and a difference of five magnitudes is exactly a hundredfold difference in brightness.
Under a dark sky, the naked eye reaches about magnitude 6.5. Asterarium goes deeper, down to magnitude 8.
Stars have colors, and color comes from surface temperature. Hot stars burn blue-white; cool stars glow red.
In the winter sky, Betelgeuse shines red while Rigel, close by, is blue-white. You can see that contrast on screen, just as it is.
The sky turns
Stars rise in the east and set in the west. It is not the stars that move — it is the Earth beneath us.
The Earth turns once a day, so the whole sky appears to wheel slowly around us. This is called diurnal motion.
In the northern sky, the stars trace circles around Polaris. Polaris barely moves because it happens to sit almost exactly on the extension of the Earth’s axis.
Try speeding up time. Compress an hour into a second, and this rotation — invisible to the naked eye — becomes unmistakable.
Between day and night
The daytime sky is blue because sunlight is scattered by the atmosphere. Blue light scatters most, and reaches us from every direction of the sky.
Toward evening, sunlight travels a longer path through the air. The blue is scattered away en route, and the red that remains stains the horizon.
Even after sunset, the sky does not darken at once. This time of transition is called 'twilight'.
Twilight has three stages: civil twilight, when streetlights come on; nautical twilight, when the sea and sky can still be told apart; and astronomical twilight, when the faintest stars begin to appear.
When astronomical twilight ends, true night begins. The sky in Asterarium fills with stars in exactly this order.
The Moon
The Moon has no light of its own. Only the half lit by the Sun shines.
Its phases are set by the geometry of Sun, Moon, and Earth. When the Moon lies toward the Sun, it is new; when it lies opposite, it is full.
On the night of a thin crescent, the shadowed part sometimes glows faintly. That is 'earthshine' — sunlight reflected off the Earth, lighting the Moon in return.
Asterarium's Moon wears the correct phase for its date and time. Run time at one day per second, and a whole month of phases passes in thirty seconds.
The planets
Night after night, the planets wander slowly among the constellations. The word 'planet' comes from the Greek for 'wanderer'.
Venus circles close to the Sun, so it never appears at midnight. It shows itself only in the western sky at dusk or the eastern sky at dawn — the evening star and the morning star.
Venus and Jupiter outshine every true star. If a single point of light stands alone in the twilight, it is usually a planet.
Tap a planet on screen to see its name, its brightness, and its distance right now.
The Milky Way
The Milky Way is our own galaxy, seen from the inside. The light of hundreds of billions of stars overlaps into a pale band.
On summer nights it appears richest in the southern sky — because that is the direction of the galactic center.
From a city, the Milky Way is all but invisible. In Asterarium, you can give any place a perfectly dark sky.
The constellations
The night sky is divided into 88 constellations — a single worldwide standard set by the International Astronomical Union.
Constellation lines are only one way of connecting the stars. Still, they make fine guideposts for anyone looking up for the first time.
The brightest stars carry proper names: Vega, Altair, Deneb — the three corners of the great triangle that rides high on summer evenings.
Lines and names can be switched off at any time. A bare, unmarked sky has a beauty of its own.
Change the place, change the sky
There is no Polaris in the Sydney sky. The southern hemisphere shows an entirely different cast of constellations.
Polaris stands exactly as high as your latitude. It climbs as you travel north, and at the equator it rests on the horizon.
Pick a place on the globe, and the clock switches to local time. Every time-zone calculation is automatic.
Preview the sky of your next trip, or visit skies you may never stand under. There are as many skies as there are places.
Light pollution
City skies glow because urban light is scattered by the atmosphere and shines back down. This is light pollution.
As it grows stronger, the faintest stars vanish first. In a city center, only the brightest stars and the planets survive.
The 'light pollution' slider in the display settings reproduces this change. Try moving between a pitch-dark mountain sky and a downtown one.
How accurate is it?
Asterarium computes celestial positions to roughly one arcminute — about one-thirtieth of the full Moon's apparent diameter.
On the day of an eclipse, the Sun and Moon truly meet; on the night of a full moon, a truly round moon rises.
The sky rewinds and fast-forwards accurately, into the past and the future alike. Check tonight’s sky on screen, then step outside to see it — it is precise enough for that.
Ways to enjoy it
Screening mode fills the screen with nothing but sky. Leave the controls alone for a moment, and the interface quietly fades away.
Share the sky you are watching with a single URL. The place, the time, even the direction of your gaze are reproduced exactly.
Our favorite: start an hour before sunset and run time at 60×. Sunset, twilight, and the first stars unfold as one continuous scene.
Credits
Asterarium is built on openly published observational data and free software.
Our deep thanks go to everyone who created and shared them.
- AT-HYG / HYG Star CatalogCC BY-SA 4.0
Star catalog (v3.3), compiled by David Nash / astronexus
Ephemeris, rise/set, and sidereal-time computations, by Don Cross
- NASA CGI Moon KitNASA media (attribution)
Lunar color albedo map, NASA/Goddard SVS
- NASA SDO/HMINASA media (attribution)
White-light solar photosphere imagery (sunspots). Courtesy of NASA/SDO and the AIA, EVE, and HMI science teams
- ESO / S. BrunierCC BY 4.0
Milky Way panorama photograph, ESO/S. Brunier
- d3-celestialBSD-3-Clause
Constellation line figures and names, by Olaf Frohn
- GeoNamesCC BY 4.0
City locations, populations, and timezone data
- Natural EarthPublic Domain
Coastline data drawn on the location globe
- OpenNGCCC BY-SA 4.0
Catalog of nebulae and clusters (Messier objects)
Atmospheric light scattering (the colors of day, twilight, and night)
3D rendering engine and its React bindings
Created by
Support the project
Asterarium is a non-commercial personal project and will remain free to use.
If you enjoy it, an optional donation helps keep it free and motivates further development.