Caoimhín's Content

Today we are going to quickly review what some of the planets are doing in April, now that we’re getting a little bit closer to their conjunctions, and we’re also going to discuss a phenomenon known as Earthshine.

We’ll begin with the Sun setting into the west for the 18th. We’re just past the New Moon, the New Moon was on the 17th so on the 18th the Moon is just after coming around the Sun, it’s going to be an incredibly narrow crescent. We’re only seeing a tiny amount of the Moon illuminated by the Sun. The rest of the Moon that we are seeing is illuminated by us, it’s illuminated by the Earth. The atmosphere spreading out that orange glow of sunset will cause it to appear brighter, but really only a small part is being lit directly by the Sun. The only way that we can see the other parts of the Moon while the Moon is in this position is because of light reflecting off of the Earth. That is going to be a little bit tricky to see on the 18th, unless you have a very clear, flat horizon. In Stellarium, we can get rid of our realistic horizon with trees and hills, instead using a zero horizon, perfectly flat and level. With that adjustment the Moon has loads of space to continue setting and of course the sky gets much darker as well.

Earthshine lets us see the light side of the Moon, the near side of the Moon, when it is in nighttime. The opposite side, the dark side, of the Moon is being illuminated by the Sun, but we can still see our part of the Moon, often with enough detail to see the Sea of Tranquility, or the football player on the Moon. I did mention a hare or a rabbit on the Moon in a previous piece, it would look upside down as the Moon sets, but we can see those details thanks to the Moon being illuminated by the Earth. The same way that we see light coming from the Moon that’s really reflected sunlight, light from the Earth reflects off the Earth and illuminates the Moon. Moving ahead a couple of days, the Moon will pass the Pleiades. There will be an occultation or a conjunction of the Moon and the Pleiades, but it’s not visible from here in Ireland. As we move back in time, from darkness to just before sunset, the Moon and the Pleiades get closer together, but the Pleiades disappear because the Sun is too bright before the Moon lines up with them. This would be visible from other locations.

We’ll take a closer look at the Moon, now 7.9% illuminated, and we’re still getting a pretty good view of the whole nearside of the Moon. Only a small part of the Moon is illuminated by the Sun, so the little bit of reflected Earth light that is falling on the Moon is visible. Of course, it’s not as bright as direct sunlight, but it’s not really being outshone by the illuminated side as it will be when there is more of it. As we move ahead to the Half Moon, so much light gets reflected off the Moon that the darker part of the Moon, which is still the near side, that is in nighttime is now completely invisible, the Earthshine isn’t enough for us to see it. The light reflected off the Moon from the Sun is too bright, it’s overwhelming the reflected Earthshine. As we move back to smaller phases where less of the Moon is illuminated by the Sun, we can see a lot more of the Earth’s reflected sunlight. Only 2.7% of the near side of the Moon is illuminated by the Sun on the 18th, so the vast majority of the illumination we’re seeing is actually light reflected off the Earth. People sometimes ask how we can see the Moon so bright in space if it’s not producing its own light. If the only light we see coming from the Moon is light from the Sun, how does it look so bright? What about the Earth? Where is the Earth’s reflected light going? Well, it’s shining out into space, it very rarely illuminates anything, except for the Moon with this phenomenon called Earthshine. That’s one way that we can see, not only is the Moon reflecting light from the Sun, but we are also reflecting light from the Sun.

As we move on from the 18th, the Moon gets pretty close to Venus and Uranus before moving on to the Pleiades. As we move a little into the future, we can see Venus and Uranus making a very close conjunction on the 23rd. A lot of the close conjunctions that we’re going to be seeing at this time of the year are visible in the morning time and only from lower latitudes, but not this evening one. There is another, if we move a little back to the 22nd, the Moon and Jupiter will be very close together, the Moon passes just over Jupiter. Unfortunately we can’t really use this as a way of showing the Moon’s motion across the sky, it’s not going to move that much in the time that it’s up. This conjunction is not as useful as it has been in the past for showing the motion of the Moon, when the Moon, Pollux and Jupiter aligned closer to the Full Moon. This is closer to the Half moon, with just 37.4% illuminated it’s not quite the Half Moon, but almost a Half Moon quite close to Jupiter in the sky.

It’s not a particularly close conjunction, not as close as Venus and Uranus, but it is also visible to us here in the Northern hemisphere, visible from pretty much anywhere, unlike what we’re seeing in the morning. Moving around to sunrise, we’ll move forward until the Sun is above the horizon and we’ll get rid of the atmosphere. I did mention this conjunction in a previous piece, this is just to review it because there are so many objects involved. One object I didn’t mention before is the comet 141P-D/Machholz. We’re going to take a look at in a future piece because I didn’t mention it already, unlike the other comets like 2025 R3 PANSTARRS. With no atmosphere in the way, we’ve got several planets very close together and they’re moving past each other over the course of the month. From the 17th to the 18th they move close together, and then they slide past each other, really looking good on the 20th, and then continuing to move away from each other as we move later into the month. Mercury will be in retrograde, falling back towards the Sun, while Saturn and Neptune will continue their orbit around the Sun, moving in the direction that we’d expect. Mercury and Neptune get reasonably close together on the 16th as well.

Mars is also continuing its orbit around the Sun, but because Mars is almost on the far side of the Sun to us, it is still moving away from us. Mars is in its own orbit and it’s going to take a while for us to sort of catch up as we also move around the Sun in that direction. There’s also the planet Neptune out there, Mars and Neptune made a really close conjunction back on the 13th. That comet, 141P/Machholz, is also making a very close pass with Mars on the 14th. Stellarium isn’t generating a tail for this comet at the moment and it is still pretty far from the Sun, so that does make sense. The comet is going to move past Saturn on the 18th and continue in towards the Sun, we will check that out in another piece.

All of these things are going to be visible from closer to the equator, so if you are at a lower latitude than we are here in Ireland, you’re going to be in with a much better chance of seeing those planets sliding past each other. I often mention when things are not going to be visible from here in Ireland without checking where they are visible. We did take a look at this close conjunction in particular already, but we’re going to take another look at it here. I want to see just how close to the equator you need to be to actually see these things in the sky. Going down to about 32 degrees north, we’ve moved about 20 degrees of latitude closer to the equator and that seems to be enough. That gives us Mars, Saturn and Mercury at sunrise, and we can see them over a range of several days sliding past each other.

For the vast majority of people, I’m sure for a lot of people reading this, it is going to be visible. Way up here in Ireland at 52 degrees north, there’s not many people living up here. Of course Ireland and England, we’re fairly populated, the Nordic States have a fair bit of population as well. A lot of the bigger cities even Canada are lower latitude than us here in Ireland, along with all of the United States, all the rest of Europe, all of the most populated cities in the far east, in India, they’re all much closer to the equator than we are. For the vast majority of people, this is going to look great and so I think it’s even more important that I show it off. It’s not like this is something that’s only going to be visible from close to the equator, this is going to be visible for the vast majority of the world and it is a particularly nice conjunction to catch. Really it’s a set of conjunctions, a series of conjunctions with these planets, Mercury, Saturn, Mars. Neptune is out there as well, of course, very difficult to see with all of this light from the Sun in the sky at sunrise. There is a comet out there as well but we’ll talk about that one in the next piece.

Most importantly, I hope that you get to see the Earthshine. Coming back to before the New Moon, a few days in the past, the same phenomenon occurs. This will of course happen next month as well, but at sunrise the Moon is also a very narrow crescent illuminated by the Sun just before the New Moon. Only a sliver of the Moon is illuminated by the Sun, the rest of the Moon is visible because of Earthshine, and this is an even easier phenomenon to see when you are closer to the equator. For us here in Ireland, the Moon isn’t going to be visible in a good level of darkness when it’s this narrow, the Sun will already be starting to come up. This is a particularly easy phenomenon to catch if you’re closer to the equator but it is only visible when the Moon is a very narrow crescent. It needs to be a very narrow crescent for Earthshine to be visible, otherwise the other light will overwhelm the light reflected from the Earth.

That is a little bit about Earthshine, I hope that you get a chance to catch it, it’s coming up over the next few days, you just need to take a look at the sunset and look out for Moon. You should be able to see parts of the Moon illuminated by the Earth reflecting light rather than the Sun’s direct illumination. I hope that you get a chance to see the planets, they also are visible due to reflected light. Jupiter and the Moon’s conjunction is going to be visible from pretty much everywhere as well as Uranus and Venus at sunset. However, the other multi-conjunction, you’re going to need to be closer to the equator for that one. I hope that you enjoyed this piece, if you did then please do like it. If you like this kind of content, then 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.