How to use the online planetarium
1. Set your location
Select Detect location, choose a built-in city, or enter decimal coordinates such as 40.7128, -74.0060. Check the UTC offset because it converts the entered local time to UTC.
2. Choose a moment
Use Now, enter a local date and time, step by an hour, or animate time. The supported planning range is 1800–2050; precision is best near the present.
3. Explore and identify
Drag the sky, pinch or zoom, toggle layers, and search by name. Select a plotted object by click, tap or keyboard to keep its details open.
How to read the sky map
The circular edge is the horizon; the centre is the zenith, the point directly overhead. N, E, S and W show compass direction. An object's altitude runs from 0° at the horizon to 90° at the zenith, while azimuth runs clockwise from north. Dragging rotates the compass orientation without changing the calculated sky.
Star size follows apparent magnitude: smaller or negative values mean a brighter object. The map includes stars to approximately magnitude 6, but haze and light pollution often make the real limit brighter. Constellation lines are learning aids connecting bright catalog members; they are not official boundaries.
How the calculation works
Catalog right ascension and declination are converted into local hour angle using the selected UTC moment, longitude and Greenwich mean sidereal time. Spherical trigonometry then converts hour angle and declination into local altitude and azimuth. Only objects above the mathematical horizon are drawn.
Sun and planet coordinates come from compact Keplerian orbital elements; the Moon uses a low-precision lunar orbit with the largest periodic corrections. Atmospheric refraction is applied close to the horizon when Atmosphere is enabled. The tool does not model precession of catalog stars, nutation, parallax, terrain, weather, light pollution or telescope-grade ephemerides.
Online planetarium FAQs
What stars and planets are visible tonight?
Set your location and choose Now. The map plots catalog stars, the Sun, Moon and planets that are above your horizon. Use search to check a specific object even when it is below the horizon.
Why does the sky change with location and time?
Earth rotates once per sidereal day and orbits the Sun yearly. Your latitude, longitude and moment therefore change an object's altitude and compass direction.
How do I find a constellation?
Turn on constellation lines and names, then search for its common or IAU name. The view rotates toward its reference position and the details panel says whether it is currently visible.
What does apparent magnitude mean?
It is a brightness scale as seen from Earth. Lower numbers are brighter: Sirius is about −1.46, while the unaided-eye limit under dark skies is roughly +6.
How accurate is this planetarium?
It is suitable for learning and approximate observing plans. Catalog stars are generally much more precise than the compact solar-system calculation. Planet positions are usually within about 1° and the Moon within about 1–2° in 1800–2050; the horizon ignores buildings and terrain.
Does location detection store my position?
No. If you grant browser permission, coordinates are used in memory by this page and are not transmitted by the planetarium. Your browser or device controls the permission.
Can I use it on a phone?
Yes. Use one finger to rotate, pinch to zoom, tap objects for details and collapse the options panel when you want a larger map.