Why do planets have elliptical orbits?
In: Physics
If the universe were perfect, then maybe all of the planets could orbit in perfect circles around their parent stars.
But the universe, as it turns out, is very complicated and rarely works out so nice. In our solar system, there are eight planets (plus or minus Pluto) and millions of other tiny moons, asteroids, comets, and other junk. Everything with *mass* (so… basically everything but light) has *gravity*, which means that everything in the solar system pulls on everything else.
Imagine a million ropes tied to you, and each of them is pulling in a different direction. Some pull harder than others, though, depending on the mass of the thing doing the pulling.
The Sun contains the vast majority of the mass in the solar system, so you can just pretend that there’s only one thing pulling on the Earth. But it’s not a perfect approximation, because the Moon is there, too, and it’s gravity affects the earth enough to create tides. Jupiter is the largest planet in the Solar System, and it does have a measurable pull on the Earth, even from tens of millions of miles away.
So, even if the Earth’s orbit started out as a perfect circle, it’s not going to stay that way. In a perfectly circular orbit, a planet would move at a constant speed during their orbit. But a little tug from Jupiter might be enough to speed the Earth up a little bit (maybe only a few meters per second). But now the Earth is moving faster, which means it now has more *energy*. Energy, and this other thing called *angular momentum* like to be conserved, though. That basically means that the little bit of energy that the Earth gained has to go somewhere. Specifically, it goes into the orbit—in order to compensate for the increase in energy, **the orbit actually has to get bigger.**
The current position of the Earth acts kind of like an “anchor” point for the orbit’s expansion. If Earth gets a little bit faster, the orbit will stretch out *away from its current position*. A stretched out circle becomes… an ellipse!
This also means that the Earth will be ever so slightly further away from the sun at the opposite end of its orbit, since we stretched the orbit out a bit.
So, after the 4.5 billion-year lifetime of the Earth, its been pulled and tugged every direction by all the different planets in the solar system, which are constantly causing the Earth’s orbit to stretch and squeeze in different directions. The difference is extremely small, but it’s important when we’re trying to send stuff to other planets, where the difference of a few feet might mean landing a probe on Mars and crashing into the surface.
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