Today we are going to take a look at some solar eclipses from other planets. Just before we do, we are going to review the most recent eclipse that we saw from here on Earth, because our eclipses are different. The eclipses that we get here on Earth are unique. Back on the 12th of August there was a total solar eclipse, and one of the best places to see it was Reykjavik.
The eclipses that we get here on Earth are unique because the Moon and the Sun appear to be a very similar size from here on Earth. They both occupy a similar angular diameter in the sky, which really just means that they appear to be the same size or close to it. This means that the Moon perfectly fits in front of the Sun, it perfectly blocks out the disk and just the disk. Depending on where we are in the Moon’s cycle, whether the Moon is closer to the Earth or further from the Earth, we can get a perfect total eclipse with just the Sun’s corona around the side, or we can get an annular eclipse with a little bit more of the Sun visible around the sides of the Moon. This only works because the difference in the distance between us, the Sun and the Moon, and the difference in their sizes, lines up. The Sun is, of course, much bigger than the Moon, but it is also much further away. So the Moon, even though it is much smaller than the Sun, it is close enough to us that it perfectly blocks the Sun out, as if they were the same size. This kind of eclipse, this perfect eclipse where the Sun’s corona is still visible around the Moon, this kind of thing is pretty much unique to the Earth.
If we want to take a look at the view from a few other planets, we can skip Mercury. Mercury, of course, doesn’t have any moons, so it never gets an eclipse. Venus doesn’t have any moons either. However, the planet Mercury is closer to the Sun than Venus, so Mercury can pass between Venus and the Sun. If you were actually on Venus, then you wouldn’t be able to see such a transit, but we can simulate it in Stellarium. Due to Mercury and Venus both being so close to the Sun, they orbit very quickly. Thanks to this, Mercury is going to pass in front of the Sun more often from Venus than it does from the Earth. However, it’s still not going to happen all of the time, it’s not going to happen every day. These transits do happen, but its still a fairly occasional phenomenon. Transits of Mercury from Venus are far more common than transits of Mercury from here on Earth, but they still don’t happen all of the time. Heading back to 2016, to the 16th of December, puts Mercury in front of the Sun from Venus. It would be really just a black dot in front of the Sun, moving across the Sun’s surface. Of course, the atmosphere on Venus is incredibly thick, so from the surface of Venus, these kinds of transits wouldn’t be visible, but it is very clearly a transit. Even though we’re much closer to Mercury than we would be seeing a transit from here on Earth, it’s still an incredibly small dot from the perspective of Venus. That’s the closest Venus will ever get to an eclipse, Venus doesn’t have any moons. Mars is in a much better situation because, of course, Mars has two moons.
However, Mars’ moons, Phobos and Deimos, are quite small. Their orbits are very quick so any transit or eclipse that they cause will be very brief, but there have been eclipses or transits (depending on how you’d like to define them) of the Martian satellites observed from Mars. Even though nobody has been to Mars, no humans have been to Mars, but there are plenty of robots on Mars. Various rovers such as Perseverance and Curiosity have caught eclipses on Mars, they have seen them happening. We’re heading to the 2nd of April 2022. Eclipses on Mars, similar to eclipses on the Earth, require you to be in the right location to see them. This is the right date, this is a date where a rover on Mars saw a eclipse occurring, but we need to be in the right location as well. This means moving through a few different areas to get Phobos to actually go in front of the Sun. Even from the right place, this is tough to catch as it happens very quickly. Even moving at real time, one second per second, it’s visible how quickly Phobos is moving in front of the Sun. Deimos is even smaller than Phobos, so the eclipses that it causes are even more brief and even more annular. This is a very much an annular eclipse or a transit, really. Where the line is between an annular eclipse or a transit probably depends on how large the object is. From here on Earth, the Moon definitely covers the vast majority of the Sun during an annular eclipse and the transits caused by Venus and Mercury are much smaller. From Mars on the other hand, Phobos is a big, dark object moving in front of the Sun, it’s very clearly visible, but at the same time, it’s not covering that much of the Sun. That’s the best that Mars can do. Deimos is smaller and further from Mars, so it is a smaller dot passing in front of the Sun. Again, no humans have seen that eclipse, but a couple of robots have, a couple of rovers on Mars have seen that eclipse. Eclipses on Jupiter are even more frequent.
If we come back to the Earth and move around to morning time, Jupiter will come into view and it won’t take us long to see what are sometimes called shadow transits. Jupiter has what are known as shadow transits and these are basically eclipses seen from the outside. We have discussed some earthly eclipses from the outside in previous pieces and it does appear like a shadow moving across the surface of the planet. If we move through a little bit of time while looking at Jupiter, we’ll get one. The Jovian system, which we’ve talked a lot about in recent pieces, we know that it’s full of moons. They’re always moving around, sometimes occulting each other and moving in front of Jupiter as well, transiting in front of Jupiter. When these moons transit in front of Jupiter, they cast a shadow and that shadow is essentially an eclipse. The smaller moons, like Adrastea, Thebe and Metis, those moons are too small to really cause the eclipses. We need a bigger moon like Io, Callisto or Ganymede to come in front of Jupiter for it to cast a shadow that we can see. Once Io moves into a position where its shadow falls on Jupiter, the shadow is visible as a dark spot, looking about as big as the moon itself.
Io is just off of Jupiter, and the shadow of Io is on the planet. Leaving it move across the clouds a little bit, it doesn’t get too close to the Great Red Spot. I wanted it to get closer to the Great Red Spot because I was looking for, I guess, a “gasmark” rather than a landmark, some way to make sure that when we try travel to Jupiter to take a look at this eclipse, that we’re actually in the right location. Jupiter does have some, I will say landmarks to make it easier, but they are not very clear. The shadow of Io seemed to pass along the pale region just above the Great Red Spot and to the east region. If we were on Jupiter, we would want to be up above the atmosphere, really high in the clouds in order to actually see the eclipse. Of course, Jupiter doesn’t really have a solid surface, it doesn’t have ground, and so we wouldn’t be able to observe this, any thing living on Jupiter is going to be shrouded in the atmosphere and wouldn’t see this occurring.
Even from a little off the correct observing location, you can get an idea of how this is a different eclipse from the kind that we get here on Earth. Io looks gigantic compared to the Sun. We will change our location just a little bit, I want to get Io as centered over the Sun as possible, so we will move pretty far. Even with all of these jumps and this object distinctly moving across the sky, we’re barely moving across the upper atmosphere of Jupiter because Jupiter is so large. Moving through a little bit of time, the Sun gets blocked out more than completely by Io. We don’t get to see the Sun’s corona because the Sun’s corona is also blocked. The shadow of Io passes across Jupiter for quite a while, several minutes. These moons are moving quite quickly around Jupiter, but thanks to the difference in their relative size, how big they appear compared to the Sun, they do take a while to pass over it because they’re covering up so so much of the Sun.
We would see similar things if we went to Saturn, Neptune or Uranus. With Saturn, there is more of an axial tilt, so these eclipses are actually rarer, similar to the way that they’re rare on Earth (we have an axial tilt as well) the shadows of the moon will often go over or under the planet rather than onto it. From Jupiter, we get an idea of what these massive moons look like when they block out the Sun and of course, from Saturn, the Sun would appear even smaller, so even smaller moons would appear much larger from that perspective. I hope you’ve got an idea now of how special our eclipses are, our eclipses are different to the eclipses visible on all of the other planets and our eclipses are pretty much unique, we’re lucky to have them. Of course, planets without moons don’t have eclipses at all, but Venus at least gets to see Mercury transit. Mercury gets nothing at all between it and the Sun.
I hope that you enjoyed seeing these things that, of course, we never really get to see, we’d have to travel to these other planets to see these things and even then, the atmosphere on Venus and Jupiter would cause these to be obscured. I hope you enjoyed the opportunity to hear about them here. If you did like this piece, then please do like it and if you enjoy this kind of content, please subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.
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