The Solar System

This is the solar system seen from above, with the Sun in the middle, where each body is today, running forward at a week a second. Pause it and set a date, drag across the picture to move a year either way, or play it backward. Tap a body for its distance from the Sun and its speed. Tap the Earth and an inset shows the Moon going round it, both lit on the side that faces the Sun, with the phase an observer on Earth sees.

Loading the orrery…

Nothing on this page is read from a table of positions, the Moon included. The page holds the state of every body on one day, 1 January 2000, and the rule that moves them: each body pulls on every other by Newton’s law, with one small correction from relativity in the Sun’s field. A worker in your browser steps that rule forward or back from the nearest of 401 saved states, one every ten years, to the date you asked for. Moving the date is integrating.

The default layout is a layout, not a map. At true distances Mercury, Venus, Earth and Mars would sit on top of the Sun, so by default the distance from the centre grows with the logarithm of the distance from the Sun, and the rings are labeled in au. The other choice, true distance, uses the same number of au for every pixel, and the inner planets collapse into the Sun’s disc. Discs are never to scale; they are only as big as it takes to see them. The thin curves are each body’s path over one orbit, worked out the same way.

The other view turns the picture into a wheel seen from the Earth. Each body sits where it appears from here on that date: the light-travel time, the Earth’s motion and the slow turning of the equinox are all applied, so these are the longitudes an almanac would give. The twelve 30 degree sectors carry the tropical sign names. Tap a body and three answers to “where is it” appear side by side: its tropical sign and degree, its sidereal position (Lahiri), and the constellation it stands in. They differ, and the page keeps them apart.

Dates before 15 October 1582 are in the Julian calendar, as historians write them. The line above the picture counts years the astronomers’ way, with 1 BC as year 0, so 44 BC reads -0043.

Sources: masses from the DE440 header published by JPL; the starting state of every body from JPL Horizons (barycentric vectors, ecliptic and equinox of J2000); the equinox of date from the Vondrák, Capitaine and Wallace 2011 precession model with the IAU 1980 nutation; and the engine’s own checks, which compare the integrated positions and the apparent places with Horizons at dated points and fail if any body is further off than the figure they declare.