No position on this page is stored. Each body carries six Keplerian elements and their per-century rates, and the browser solves Kepler's equation — M = E − e·sin E, by Newton iteration — to place it at whatever date is on the dial. Drag to tilt the plane, scroll to zoom, drag the dial to travel. At the default speed five seconds is one year, so Earth laps in five and Neptune takes fourteen minutes.
Kepler's third law says the period is a3/2 in years when a is in astronomical units. Nobody enforces that here — the semi-major axis and the observed period are separate fields, entered from separate columns of the source table. That they agree is a check on the data, and a bad edit would show up in the last column first.
| body | a (au) | e | i (°) | period (yr) | a^1.5 | agree |
|---|
Elements from E. M. Standish, Keplerian Elements for Approximate Positions of the Major Planets (JPL Solar System Dynamics), Table 1, fitted to 1800–2050 — which is why the dial stops at both ends. Heliocentric longitude error over that window runs from about 10 arcsec for Neptune to 600 for Saturn: correct to the pixel here, not good enough to point a telescope. Earth's row is the Earth/Moon barycentre, as the source gives it. Pluto comes from Standish's earlier table, JPL having dropped it; its 3:2 resonance with Neptune makes any fixed ellipse a poor model over long spans.