Accuracy and sources

Solar System Simulator is built to be accurate enough to learn from and to check against the real sky. This page explains what it computes, how we test it, and what its limits are.

How positions are computed

All positions are computed in your browser for the exact moment on screen. Nothing is pre-recorded, and nothing about the scene is sent to a server.

• Planets and Pluto come from Astronomy Engine by Don Cross (MIT license), which implements the VSOP87 planetary theory and a dedicated model for Pluto.
• The Moon comes from Astronomy Engine's lunar theory.
• Jupiter's Galilean moons (Io, Europa, Ganymede and Callisto) come from Astronomy Engine's implementation of the L1.2 theory.
• The other moons (Phobos and Deimos; Mimas, Enceladus, Tethys, Dione, Rhea, Titan and Iapetus; Miranda, Ariel, Umbriel, Titania and Oberon; Triton; and Charon) use JPL's published mean orbital elements with the precession of their orbits. Mean elements are not a precise ephemeris, so we fitted each moon's position along its orbit to JPL Horizons over 2016 to 2036.
• Orientation of the Sun, Moon, planets and Pluto (the tilt of each axis and how far each has turned) follows the IAU Working Group on Cartographic Coordinates and Rotational Elements (2015). Moons without a published model are drawn facing their planet, which is true for all the large moons shown.

Positions are in the J2000 ecliptic frame: the plane of Earth's orbit, with the vernal equinox of the year 2000 as the reference direction. They are geometric, so they show where a body is at that instant rather than where it appears once its light has crossed the distance to us.

How we test it

We compare the simulator's positions with JPL Horizons, NASA's reference system for solar system positions. The comparison runs automatically every time the site is built, so a change that made the simulator less accurate would stop the release.

For scale, 30 arcseconds is about one sixtieth of the width of the full Moon in the sky: far too small to notice by eye, so every planet sits in the right place among the stars on the right night.

We also check events against known dates. The simulator's eclipse search finds the total solar eclipses of August 12, 2026 and August 2, 2027, and its list of events always comes back in date order.

The sky and the look of each world

• Stars: 8,920 stars down to magnitude 6.5, the limit of the naked eye at a dark site, from the HYG Database version 4.1 (CC BY-SA 4.0). Their colours come from each star's measured B−V colour index.
• The Milky Way is the Solar System Scope panorama, placed by converting every direction on screen to galactic coordinates. We confirmed the alignment against the positions of the 50 brightest stars.
• Surfaces: textures from Solar System Scope (CC BY 4.0), which are based on NASA imagery. Earth has separate day, city-light and cloud maps; the clouds drift slowly as a visual cue and are not a weather forecast.
• Physical data (diameters, masses, gravity, day lengths, tilts and temperatures) come from the NASA Planetary Fact Sheets, and moon sizes from JPL's satellite physical parameters.
• Shadows are computed for every point on each body: how much of the Sun's disc is hidden by each nearby sphere. That produces partial and total eclipses with soft edges, and the red glow of the Moon in Earth's shadow.

Known limits

• The date range is January 2, 1800 to December 30, 2099, the span we tested.
• Moons other than Earth's Moon and the Galilean moons are approximate: right orbit, right period, position within a few degrees.
• Surface textures of Venus, the gas giants and some regions of other worlds are artistic reconstructions of real imagery, not current photographs.
• The simulator is an educational tool. It is not meant for spacecraft navigation, precise astrometry or eclipse timing to the second.

If you find something that looks wrong, write to hello@solarsystemsimulator.com with the link from your address bar, which records the body and the moment you were looking at.