Kepler's third law
The square of a planet's orbital period is proportional to the cube of its average distance from the Sun.
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Analogy
Farther planets take much longer to go round.
Example
T^2 = r^3 (years, AU)T = 1.52^1.5T = 1.87 years
In practice
Mars is 1.52 times farther from the Sun than Earth, so its year is about 1.87 Earth years.
Questions people ask
Does Kepler's third law work for moons too?
Yes, for any set of objects orbiting the same central body. The ratio of period squared to distance cubed is constant for that body. Jupiter's moons share one constant, and planets orbiting the Sun share a different one, because the central masses differ.
What does Kepler's third law tell us about distance?
Outer planets are not just slower, they travel farther too. A planet twice as far from its star has a period about 2.8 times longer, since 2 to the power 1.5 is 2.83. Period squared grows in step with distance cubed.