Caoimhín's Content

Today we are going to look ahead just a little, to the 20th of August. As usual, we begin at sunset, so we will begin on the 20th. For those of us here in Ireland, we will be waiting until the morning of the 21st to catch the occultation of Saturn by the Moon. The term occult means something that is hidden. This is the case as well when people talk about a occult magic or occult texts, it really means something that’s hidden, such as secret knowledge. The term didn’t always have an association with the esoteric or supernatural, it just meant that something was hidden away and in astronomy an occultation is where something hides something else in space, in this case it’s the Moon hiding Saturn.

The Moon is able to move in front of Saturn and block it from our view for the same reason that the Moon can block out the Sun. An occultation is very similar to an eclipse in the way that it occurs. The Moon of course goes around the Earth’s equator while Saturn goes around the Sun’s equator. Like all the planets, Saturn follows the ecliptic where as the Moon follows our equator, and those two lines don’t match up perfectly. Due to the Earth’s tilt the circle that the Moon makes around the Earth crosses the circle that the planets make around the Sun, but doesn’t follow it the whole way around. Also, these aren’t true circles of course, but rather ellipses, but that doesn’t matter too much here. Thanks to these two circles being out of line, the line that the Moon traces around the Earth crosses the line that planets and the Sun make across the sky in two places. The Moon travelling around the Earth will move across these crossing points twice every month, the Moon will ascend from below to above the ecliptic once a month and then it will descend from above to below the ecliptic once a month as well. Each of these crossings these will happen on roughly opposite sides, and this is why we often get a lunar eclipse and solar eclipses in tandem. If the Moon is directly behind us when it is full then there is a good chance it will cross directly in front of us again when it is new. There are slight deviations in the real world from this simplified explanation, and there is the fact that the Moon goes around the Earth every 29 and a half days not every 30. Just because you see a lunar eclipse does not mean you will see its associated solar eclipse. The same thing is happening here where the Moon is crossing the ecliptic where Saturn happens to be. The Moon just happens to be crossing the ecliptic when it’s not full, not directly behind the Earth, and it just so happens that it’s not full at exactly the right place to block out, or occult, Saturn.

Saturn and all of the planets are moving as well. We may be orbiting the Sun, but the Sun isn’t really moving, it will be inn the same place relative to the stars every year at the same time. The planets however will drift around from year to year, making occultations just a little bit more complicated to occultations. This is one of the reasons that occultations are rare, they are occasional occurrences. There is a nice quirk of occultations that is only possible because they can occur when the Moon is not full. It is just about visible with this occultation although it is definitely quite difficult to catch. This particular Moon is just past full, a Waning Gibbous Moon, which means we have a little portion of the Moon in shadow on the Western edge of the Moon. Titan is leading Saturn on this date, and so it is going to disappear just a little ahead of Saturn. Titan will then reappear from behind the Moon ahead of Saturn as well. Just as Titan emerges, it will illuminate or backlight the edge of the Moon that is in darkness, showing that there is more Moon there that we cannot see directly. Titan will appear from behind the Moon, but because that part of the Moon is in shadow it will almost look like there is a little bit of a delay, between the last part of the Moon we can see and the point where Titan reappears. It may look like there is this empty space and Titan won’t be visible until it pops out from behind that empty space, but of course it’s not empty space it is the Moon, just a portion of it that is dark. The opposite thing can happen with other occultations, if the other side of the Moon is in darkness. This is especially dramatic for example with a Waxing Half Moon, because half the Moon is going to appear dark. This makes it look like the object, whatever it is, simply vanishes into darkness, far ahead of the visible portion of the Moon. Of course it isn’t simply vanishing, it is going behind the Moon, just a part of the Moon that we can’t clearly see.

From here in Ireland, around 3 o’clock in the morning is a good time to aim for, as the Moon and Saturn will be very close together with Saturn still visible just next to the Moon. As time moves forward the two will slowly draw closer together. By 3:30 Saturn will have disappeared behind the Moon, and it will then pop out the other side at just about 4 o’clock in the morning. This gives us a good half an hour where the Moon is completely blocking out Saturn. With a sufficiently big telescope, several of Saturn’s smaller moons, such as Tethys should be visible around Saturn before and after the occultation. If you do have a telescope powerful enough, especially one that can slew or follow the planet across the sky, You should see Saturn just barely about to start hitting the Moon as it comes up to 3:30, and it will then slowly vanish as it gets blocked out by the Moon. However, you do need a pretty powerful telescope to see this in detail, otherwise it will look more like a bright dot fading and then vanishing. Occultations, just like eclipses, aren’t always visible from every part of the world, and in this case it only lasts for half an hour in Ireland. For anyone who the Moon isn’t up for while it’s 3:30 here in Ireland, won’t be able to see this happen. Here in Ireland, we’re also quite high on the planet. For some astronomical events, including some eclipses, you can be too high on the planet, essentially seeing over the Moon to the object behind it. We do certainly see Saturn getting blocked out from here in Ireland, but Saturn does seem to be crossing across the top of the Moon, kind of making a tangent across the top of the Moon. This shows that we are looking at the occultation almost from too high an angle, so there’s a good chance that this would last longer if we were closer to the equator. There is also good chance that we would be able to see this better if we were just slightly further to the West than we are here in Ireland. If we were seeing the Moon directly over South, rather than seeing it falling over towards the Southwest, then would at least be higher in the sky and easier for us to observe through a telescope.

To see how much better it might look, we will go a little bit closer to the equator and a little bit further to the West. I won’t go too close to the equator, but stay a little bit in the northern hemisphere. Just a bit to the North of the equator and west of Ireland puts us pretty much in the middle of the Atlantic ocean. This is pretty close to the Azores, which are difficult to see on the small map used in Stellarium. Other Atlantic Islands such as Gran Canarias and Cabo Verde would also be good options in this vicinity. Moving to this location, the Moon and Saturn will be a lot closer to the middle of the sky when the occultation is happening. From a little North of the equator Saturn won’t go straight behind the lunar equator, it will still be making a tangent, but the tangent is much closer to the center of the Moon rather than being closer to the top as it is in more Northern latitudes. Even though the atmosphere generally makes things harder to see, it’s important that the atmosphere is turned on here. The atmosphere refracts light, making things appear to be in slightly different locations. This bending means that the Sun appears above the horizon while it is really a little below. This bending could change the apparent time at which the occultation begins. A similar distortion is caused by the travel speed of light. If you were floating just in front of the Moon, you would see the occultation happen jus a little earlier, because you wouldn’t have to wait for the light to travel the roughly 375,000 kilometers down to the Earth. Having the atmosphere turned on also ensures that we’re seeing the event as you would actually see it, unless you are an astronaut in space. If any astronauts read my articles, that would be great, even if you aren’t currently in space.

The occultation seems to begin at about 11:40 local time from the middle of the Atlantic. Saturn will still be a little separate from the Moon, but unless you have a very powerful telescope they will pretty much appear like they’re touching. Of course you will have to contend with the bright light of the Moon, you may need something to block out some light of the Moon if you are using a telescope and want to see Saturn as it is in the attached video, it may look a little bit washed out otherwise. Taking our start time as 11:40, the occultation will continue all the way through to 12:45, a full 65 minutes, just over an hour. This is certainly significantly longer than for us here in Ireland. Getting to see the occultation for longer and higher in the sky will make it much easier to observe with a telescope, if you are very close to the equator and in the middle of the Atlantic. I’m sure most of you won’t be, people do tend to live in cities and on dry land, there is not much of either in the middle of the Atlantic. I do think if you are living in the Azores that should be one of the best locations to see this occultation, closely followed by the other Atlantic Island. There isn’t really any land right there, smack dab in the middle of the Atlantic on the Equator, although along the Northern coast of Brazil should also be a pretty good place and even up towards a Suriname and French Guiana. The Atlantic Coast of Africa and even a fair portion of the Caribbean should also get a reasonable view, though along the Atlantic Coast of North America, it may occur much earlier in the evening, closer to sunset than sunrise.

Occultations, are certainly not as big and impressive as a solar eclipse, watching the entire sky turn dark in the middle of the day is an incredible thing to catch. However they are pretty much of equal rarity, even though occultations as a whole happen more often. Anything around the ecliptic can be occulted, it could be Saturn, it could be Jupiter, it could be Mars, and of course stars can be occulted by the Moon as well, that happens regularly. Limiting it to specifically an occultation of Saturn, that isn’t likely to happen again for a while, they are quite rare especially for any given location on the Earth. There are plenty of locations around the world, most of North America, most of Southeast Asia and South Asia, Oceania and any where along the Pacific, that won’t be able to see this. If you’re on the wrong side of the planet to catch this event which, at most, will last just about an hour, you will need to wait for the next one, and it is unlikely that the next one will be visible from Ireland again.

Moving past the occultation and continuing onto sunrise. Saturn will definitely be out the other side of the Moon by the time the Sun rises, even a little before the sunrise, just as light begins to come into the sky, Saturn will be distinctly separate from the Moon in the sky. Mars and Jupiter will be getting very close together and of course they will have their conjunction around the 15th, a little bit earlier in the month than Saturn’s occultation. However, they are only having a conjunction, if we take a close look, we will see that although they get very close together in the sky, they don’t actually block each other out, so they will not be occulting each other, at least for us here in Ireland, maybe from other locations, but I do not think so,

That is a quick overview of the upcoming occultation of Saturn, coming up on the morning of the 21st, during the night beginning with sunset on the 20th. I hope you get a chance to see it, even if it does mean you have to get up very early, for those of us here in Ireland at least. If you are out in the middle of the Atlantic, then of course you’ll be able to see this happening for closer to an hour rather than our half an hour, and much earlier in the night as well. Regardless of whether or not you get to see it in real life I hope you enjoyed this little description of the phenomenon. If you did enjoy reading this and if you’d like to see more like this you can subscribe to this website and my YouTube channel, which will help keep me producing more and make sure you’re notified when I do. With all of that, I’ll hope you come back to see the next video.