Frequently asked questions

What is Celestial Atlas?

Celestial Atlas is a free, open-source, interactive 3D model of Earth, the solar system, the Milky Way and nearby galaxies. It shows where the planets and Moon really are for any date from 1000 to 3000 CE, simulates eclipses, and zooms out to Andromeda and the Laniakea supercluster. Open it or read how to use it.

Does the solar system move in a helix?

In the Galaxy's frame, yes: the Sun carries the planets around the Milky Way at about 246 km/s, so each planet's path is a very stretched corkscrew (about 52 AU long per turn for Earth, but only 1 AU wide). The planets do not trail behind in a wake. Details.

How fast is the solar system moving through space?

About 246 km/s (around 885,000 km/h) around the centre of the Milky Way, and roughly 560 km/s relative to the cosmic microwave background once the Galaxy's own motion is added. Details.

How far is the Sun from the centre of the Milky Way?

About 26,700 light-years (8.18 kiloparsecs), in the Orion Spur between the Sagittarius and Perseus arms. One lap around the Galaxy takes about 223 million years. Details.

When will the Milky Way and Andromeda collide?

Andromeda is approaching at about 110 km/s. Simulations put the first close pass about 4 billion years from now and the final merger at roughly 6–7 billion years; a newer analysis suggests the merger is only about 50% certain within 10 billion years. The atlas simulates the "if it happens" case. Details.

Will stars collide when galaxies merge?

Almost never. Stars are so far apart compared with their sizes that galaxies pass through each other, though gravity flings stars into long tails.

What will the Milky Way–Andromeda merger be called?

The nickname is Milkomeda (or Milkdromeda); it should be a giant elliptical galaxy.

How far away is the Andromeda galaxy?

About 2.5 million light-years. Details.

What is Laniakea?

The supercluster of about 100,000 galaxies, 520 million light-years across, that includes the Milky Way and flows toward the Great Attractor. Details.

What happens to Earth when the Sun dies?

Earth becomes uninhabitable in about a billion years as the Sun brightens, and is probably swallowed when the Sun becomes a red giant about 7.6 billion years from now; the Sun then ends as a white dwarf. Details.

When will the Sun become a red giant?

It leaves the main sequence in about 5.4 billion years and reaches its largest red-giant size, about 256 times its present radius, around 7.6 billion years from now.

How long does a total solar eclipse last?

No more than about 7½ minutes anywhere; typically 2–4 minutes. The 8 April 2024 eclipse lasted about 4 minutes in Texas, and 2 August 2027 will exceed 6 minutes near Luxor. Details.

When is the next total solar eclipse?

12 August 2026 (Greenland, Iceland, Spain) and 2 August 2027 (Spain, North Africa, Egypt, Saudi Arabia). Use the time-travel panel to find the next one visible from your location.

Why is the Moon red in a total lunar eclipse?

Earth's atmosphere refracts red sunlight into its shadow and scatters blue light away, so the Moon is lit by the combined sunrises and sunsets of the world.

Why isn't there an eclipse every month?

The Moon's orbit is tilted about 5° to Earth's, so New and Full Moons usually miss the Sun–Earth line. Eclipses occur only near the two points where the orbits cross. Details.

How far away is the Moon?

About 384,400 km on average, roughly 30 Earth diameters, with light taking 1.28 seconds to travel the distance. Details.

How accurate are the planet and Moon positions?

Within about 5 arcseconds of NASA/JPL Horizons for the planets, 4.4″ for the Moon and 1.2″ for the Sun (1969–2026); accuracy degrades gradually outside 1700–2200 CE. Details.

What dates can I explore?

Any moment from 1000 to 3000 CE, in your local time zone or UTC. Details.

What planets are visible tonight?

Open the Sky view or the “Up tonight” card in the atlas for your location; it lists planets above the horizon during the night and which are visible to the naked eye. Details.

Are the stars real?

In the Stars scale, yes: more than 140,000 real stars from ESA's Gaia mission at their measured 3D positions. In the Milky Way model and the other galaxies the stars are generated to match the structure. Details.

What is the Gould Belt?

A tilted ring of young stars and star-forming clouds about 2,300 light-years across around the Sun, drawn in the atlas from 5,321 real Gaia stars.

What is the Local Bubble?

A cavity about 100 parsecs wide of hot, low-density gas around the Sun, blown out by supernovae over the last ~14 million years.

How is the weather shown for past and future dates?

Forecast within about two weeks, real ERA5 reanalysis for dates since 1940, and a clearly labelled climatology (average) otherwise. Details.

Is it free and open source?

Yes, free to use and MIT-licensed; the source is at github.com/sitens/celestial-atlas.

Does it work on phones and in other languages?

Yes: it is designed for phones, and the guide, tooltips and tour captions are available in English, Spanish and Hindi.

Which parts are simulated rather than measured?

The galaxy collision, the star-formation proxy, the Sun-life temperatures and ocean model, and the generated stars inside galaxy models. Each is labelled as illustrative. Details.