Orbits: How the Planets Move

Every planet travels around the Sun along its own path. That path is called an orbit, and it is very nearly a circle.

The planets do not fly off into space because the Sun's gravity pulls on them constantly, holding each one on its curve. Gravity is the same force that pulls a dropped ball to the ground — the Sun's is simply vast.

One complete trip around the Sun is that planet's year. And here is the pattern worth understanding:

The further a planet is from the Sun, the longer its year.

There are two reasons stacked together. A distant planet has a far longer path to travel, and it moves more slowly along that path, because the Sun's pull weakens with distance.

Worked example — reading the pattern in a table.

Read down the last column and the rule announces itself: every step further out, the year gets longer — from under three months for Mercury to a century and a half for Neptune.

So if you were born on Neptune, you would not reach your first birthday. Nobody ever has.