LearnPlanetary scienceSome maths
Synodic period
The time between successive identical alignments of two bodies as seen from a third — for a planet, the gap between one opposition and the next.
Why it matters
It, not the orbital period, is the number that governs an observing calendar. Mars orbits in 687 days but only comes to opposition every 780.
The maths, in layers
1 · Intuition
Two runners on a circular track: how often the faster laps the slower depends on the difference in their rates, not on either rate alone.
2 · The equation
1 / T_syn = | 1/T_earth − 1/T_planet |
Rests on
3 concepts in the full chain back to first principles.