Caoimhín's Content

Today we are going to be discussing the shape of various objects in the sky. Unfortunately, it can be tricky to actually see these shapes, especially with stars, but we will at least be discussing them.

We’re taking a little bit of a break from the eclipses, although I will write another piece in the near future talking about the very long eclipse that’s coming up in the far future in the 2100s. We will be looking far into the future for that in a near future piece, hopefully quite soon. Also, there should be another piece this week, coming up in the next couple of days, based on a video on my third YouTube channel, Caoimhín’s Content as Gaeilge. It will be posted on this website with a written English piece to accompany it. There will also be a piece soon based on a constructed language, a type of post I have made here only once before.

For today, we will first off to make a quick comparison between Venus and Jupiter. These are planets and one of the defining characteristics of a planet is its shape. Dwarf planets and planets are the objects that can become round under their own gravity, or reach hydrostatic equilibrium. We’re looking back to a conjunction between Jupiter and Venus in August 2025. The planets aren’t quite as close as I’d like them to be, but close enough to easily hop back and forth to make this comparison. If we take a look at Venus, an incredibly slowly rotating planet, it looks pretty round and it is pretty much a perfect sphere. It rotates incredibly slowly, so its gravity is able to pull it into a sphere without too many forces combating it or pulling it in different directions. Jupiter, on the other hand, is very much an oblate spheroid and technically the Earth is an oblate spheroid as well.

Looking at Jupiter, it is definitely wider around the middle than it is from pole to pole. Jupiter, of course, rotates incredibly quickly, its whole day is just, about 10 hours, really 9 hours and 55 minutes. Thanks to this incredibly quick rotation, it’s bulging around the equator. The centrifugal force and centripetal force are part of the reason. The centripetal force here is gravity, keeping the momentum of the atmosphere angular, stopping it from flying into space. The centrifugal force is a pseudo-force or fictive force, it isn’t a real force but the result of angular momentum and its generated inertia, but we won’t get into the discussion of that nitty gritty part of physics for now. What I will say is that due to the very quick rotation of Jupiter, it bulges around the equator, material around its equator is essentially being spun away from it, but its gravity is still able to catch it and keep it in shape. This results in a bulge around the equator compared to its distance from North Pole to South Pole. Certainly in Stellarium you can tell that Jupiter is a little bit wider around the middle and if we hop back to Venus, it’s visibly a little bit closer to a perfect sphere. Even without all of Venus visible, it looks more even, it doesn’t quite stick out around the equator as much, and that’s down almost entirely to the speed of its rotation.

Now, of course, Venus is a rocky planet, Jupiter is a gas giant, and gas is a little bit more malleable than a solid. However, Venus does have a very thick atmosphere. So if Venus was rotating very quickly, we’d at least expect its atmosphere to bulge, even if its physical material doesn’t. The Earth also bulges a little bit around the equator. Even though Mount Everest is the tallest, when measured from ground level, there are mountains around the equator like Mount Chimborazo in Ecuador that, because of the bulge at the equator, are further from the Earth’s core. Even though Mount Everest is the tallest mountain when measured from the ground, if you’re measuring from the Earth’s core, thanks to the bulge that the Earth has, the mountain closer to the equator is further from the Earth’s core even though it’s shorter when measuring from the ground.

Taking a look at Saturn, I think the rings of Saturn make it even more obvious, they kind of highlight that it’s a little bit wider around the equator than it is from pole to pole. It’s rotational period is just 10 hours and 40 minutes, another very quick rotation that is also causing it to bulge around the side. Even solid rocky objects can do this, if they’re rotating quickly enough then solid rocky objects will appear to bulge. Of course, the Earth is a solid rocky object today but that wasn’t always the case. The Earth began as a liquid ball of rock, we began as a very soft lava or magma. Haumea is an extreme example, Haumea is dwarf planet with a very ellipsoidal shape. There are dwarf planets that have reached hydrostatic equilibrium, they’ve become round, they’re no longer lumpy and bumpy like a potato or an asteroid, but they are bulging because of their very quick rotation. This also happens to stars. We’re going to hop back a little bit closer to the present day, I just wanted to go back to the very close conjunction that Venus and Jupiter had late last year, just so they were close enough in the sky that we could quickly hop between them to make a comparison between their shapes because they are such different shapes, Jupiter is a lot more oblate than Venus.

Coming back a little bit closer to today, we’ll let the Sun go down which will bring out the Summer Triangle low in the west, and Altair close to the horizon. The blue star Altair is also rotating very quickly, but in Stellarium it does look quite round, even in close up. In the attached video, I mistakenly included Deneb as a fast rotating star, which it is not, an taking a closer look iot appears to be as round as Altair, but this is due to the limitations of the Stellarium software. Regardless, stars that rotate quickly, especially stars that rotate very quickly, can become almost ellipsoidal in shape or lenticular in shape. An ellipse is a 2D shape of course, and an ellipsoid is the 3D version of that. Lenticular, the shape of a lens, is sort of an even symmetrical shape and that’s the kind of shape that stars that rotate very, very quickly can be. An oblate spheroid is a less extreme version, a slightly squished, or eccentric, spheroid.

We generally think of these things, stars and planets, particularly planets, as being spheres, as being these big balls, but really they are often a little bit off. Venus is very close to a sphere, and many stars are quite close to spheres, there are stars that are rotating very slowly that would be close to perfect spheres, but the quicker these objects rotate, the more they distort around the equator. Jupiter and Saturn, for example, rotating very quickly are quite distorted, they’re very much an oblate spheroid, a ball that has been squished a little bit at the top and at the bottom. Even the Earth is an oblate spheroid to a lesser degree, but we rotate much, much faster than Venus. Venus’s slower rotation is a big part of why it’s closer to actually being spherical and the same thing does happen with stars. It’s unfortunate that we’re not able to see it with the level of magnification that Stellarium allows but stars that rotate very quickly, they take on that oblate shape, they take on that shape that is not exactly the shape of a sphere.

This is something you might be able to tell if you watched the past few videos, when we looked at the eclipses. If we take a closer look at the Sun (never look directly at the Sun in real life) it’s going to look pretty round. It is quite spherical and because the Sun is so large the little bit of a deviation, the slight widening around the equator compared to the north-south diameter, it’s very tough to see. The Sun doesn’t even rotate quite as quickly as Jupiter and Saturn, it rotates a little bit slower, and with different speeds of rotation at different locations. This is true for Jupiter and Saturn as well, Jupiter and Saturn, because they’re gassy planets, the atmosphere around the equator is moving a little bit faster than the atmosphere at the poles, but the Sun is just a gigantic ball of plasma. The gas giants such as Jupiter and Saturn, there is a good chance that they have some sort of core, whether it’s a very small rocky core or the more likely core of metallic hydrogen under a lot of pressure. The Sun doesn’t really have that, it’s pretty much a amorphous mass the whole way through. The plasma that’s near the center where the actual fusion happens, it is under a lot of pressure, and it may behave closer to a solid due to that pressure, but definitely around the outside, the Sun is pretty soft. It’s a giant ball of plasma, the phase of matter just beyond gas, making it even looser, a less tight association than the molecules in a gas would have, so the Sun also has that oblateness where it’s a little bit wider around the middle.

Of course, I think this is a great thing to bring up when people ask if the world is round. It is round, but it’s not a sphere, it’s an oblate spheroid, so it is just a little bit off what you might be expecting from the typical globe, but not quite as far off as those gassy planets, Jupiter and Saturn. I hope you enjoyed this piece, I hope you enjoyed this little break from the eclipses. We will be getting back to the eclipses, not just for future eclipses in the far future, but for the multiple eclipses we have coming up this year, which we will be reviewing soon. If you did enjoy this piece, then please do like it, if you enjoy this kind of content, then please subscribe to this website and my YouTube channel. Also, coming up soon will be pieces based on videos from my other channels, Caoimhín’s Other Content and Caoimhín’s Content as Gaeilge, which are going to be posted very soon as well. Thank you very much for reading and hopefully I’ll see you back here next time.