If you could measure the average distance from the Earth to the Sun over the course of an entire year, you’d discover something unsettling. With each passing year that you made that measurement, you’d find the Earth was a little bit farther away from the Sun — about 1.5 centimeters (0.6 inches) more distant — than the year prior. For billions of years, Earth has been migrating outward in its orbit, a trend that should continue for billions of years to come.
But this is only a temporary situation. Eventually, the Earth will lose its orbital energy and spiral into the Sun, even in the event that the Sun doesn’t engulf the Earth in its red giant phase. A whole lot of factors will come into play in the Solar System’s far future, but in the end, Einstein himself will have the last say. Here’s how the Earth’s orbit will evolve, right up until the bitter end.
For most people, the idea that Earth would change its orbit over time is a bizarre and confusing one. After all, planetary motion has been very well understood since the time of Kepler, more than 400 years ago. His first law of planetary motion — that planets move in elliptical orbits with the Sun at one focus — is exactly true in Newtonian gravity.
This is even more impressive when you consider that Newton’s law of gravitation itself wasn’t even derived until more than 60 years after Kepler laid out his laws. And yet, both Kepler’s and Newton’s laws are only approximately true in reality, with six separate effects all potentially playing the “spoiler” role to what would otherwise be an exact, perfectly stable solution. Here’s a rundown of each one, along with the effects they induce.