Why do moons have a weird orbit?

It’s not a secret that space is, well, pretty weird. Black holes, water on Mars, comets, need we go on? All that goes on in our universe is, for a lot of people, one giant question mark. It can be hard to understand intense cosmic happenings without a ton of scientific background, but certain questions can be answered relatively simply. For instance, the orbits of planets are well understood and not too complicated; the orbits of different moons, however, are a little stranger. If you’ve ever pondered over why moons have such weird orbits (or just pondered over moons in general), ponder no further; let’s get scientific with it!

What is an orbit?

If you’re already lost, don’t feel bad. Let’s start with the basics. An orbit is essentially just a path that one celestial body takes around another body. The path is assumed to be a regular shape that repeats over and over. For instance, the earth orbits the sun once every 365 days in the shape of an ellipse. Objects that orbit other objects are called satellites, so the earth is a satellite of the sun, and our moon is a satellite of the earth.

What causes an orbit?

Another excellent question! An orbit is caused by an object’s forward motion being in balance with the pull of gravity from the larger object that it is orbiting. In one way, this is an example of perfect balance; in another way, you can think of it as a constant struggle between the two objects. In the case of the earth, the gravitational pull of the sun wants to suck it in; the earth, however, wants to keep moving, and thusly goes forward, preventing it from ever fully being pulled in. This creates the travel it has around the sun. The same goes for all other satellites.


Photo: Unsplash.com


So, what’s up with the moon?

Well, it’s a little complex. The moon is a satellite of the earth in much the same way the earth is a satellite of the sun, yet its orbital path is not quite as simple as the earth’s. All orbits are understood to be ellipses (or, in other words, ovals and not circles), yet most are very close to being circles. The moon’s orbit, on the other hand, is pretty darn far from a circle, in terms of orbits. It ranges from 7% less far to 6% more far from the earth, calculated using average orbit lengths. This is a pretty huge variance. What this tells us is the orbit of the moon is much more elliptical than normal.

Another odd thing about the moon’s orbit is its tilt. The plane it exists in relative to the earth’s orbit around the sun is tilted about 5.15°, which is pretty unusual. If it wasn’t tilted, we would experience a total eclipse about once a month; instead, they are much rarer. Additionally, it’s been observed that the moon’s general location in the universe changes month to month due to the actual orbit swiveling and changing. A lot of these qualities contribute to the changing size of the moon in the sky, something we’ve all recognized and look in awe at. Yes, supermoons are caused by this wonky orbital pattern!


There are a few reasons the moon has a weird orbit. The reason the moon’s path changes from month to month is due to the additional gravitational pull of the sun, which basically yanks it in a different direction than the earth. The oddly-elliptical orbit can be explained similarly, with the sun creating enough of a gravitational effect to make the actual shape of the orbit that much more exaggerated.

In terms of the odd tilt, we aren’t quite as sure. The question is so infamous as to have garnered the name of the lunar inclination problem. One theory involves large objects (like asteroids) traveling close to the moon and pulling on it with their own gravitational fields, eventually pulling it to a severe tilt over time. It’s hard to be perfectly sure, but many scientists agree that this seems like the best explanation we have so far.

Are other moons odd too?

We won’t get too far into it, but so far, it seems that they are. Neptune’s innermost moons, Thalassa and Naiad, zig-zag around each other, yet never touch. They also have completely different orbital times, making their paths near each other that much more complex. In short, the universe is wild!


Photo: Unsplash.com


So there you have it, a crash course in the eccentricities of moon orbits. Next time you look up at the sky, think about all the strange paths and places the moon has been! Oh, and remember to blame the sun for the fact that we don’t have an eclipse every month (which would be pretty cool).

More about Roi Kadosh

“A world-famous writer, if not for the fact that very few people actually know me. While I wait for the collective penny to drop, I write about all things pop culture.