For anything going round the Sun, the year squared equals the distance cubed — in Earth years and astronomical units, with no constant to look up. A planet four times further out takes eight times as long to get round.
Gravity falls off as the square of distance, and that single fact fixes the speed a body needs to hold any given orbit. Kepler found the relationship in Tycho Brahe’s observations decades before Newton explained why it had to be true. It works for moons, asteroids and exoplanets alike — change what they orbit and only the constant changes.
It assumes the orbiting thing is far lighter than what it orbits, which is safe in the solar system and not safe for binary stars, where both masses count. It also ignores the pull of everything else: real orbits perturb one another, which is how Neptune was found, and Mercury precesses by an amount only general relativity gets right.
Estimate with the rule, then check it against the calculator that models it properly.
Open Solar System →