Caoimhín's Content

Today I am going to hop through a couple of topics thanks to an idea inspired by a comment on the previous video on my YouTube channel. It is a bit of a journey, but we are going to begin by taking a look at the night sky for just a couple of nights ago, and we are going to take a dark sky perspective from the countryside immediately as well.

I’ve spoken a lot about the planets in the past couple of videos, simply because the planets are so prominent and interesting at the moment. We don’t have all of them in the sky, but we do technically have six planets in the sky right now, if we count Uranus next to the Pleiades and Neptune near Saturn Of course, those two aren’t usually visible, but four planets visible to the naked eye is still pretty good. We will technically have seven planets in the sky in the near future as well, when Mercury joins Saturn at sunset. We do have the Moon in the sky for most of the night around now as well, though it was not quite full a few nights ago. At just 58.8% illuminated it was a little bit past half, and already adding quite a lot of light to the sky. The Moon of course is the brightest thing we see at nighttime, followed reasonably closely by Venus. In fairness, Venus’s brightness is much more comparable to a star than the Moon, but it is still brighter than any star.

The Full Moon does provide a lot of light. I’ve spoken a lot about how much light the Full Moon creates and how it can obscure things like the Milky Way or the Andromeda galaxy, even if you are in a dark sky where they should otherwise be visible. The first thing I’m going to talk about is really only relevant in a dark sky, I don’t think this is something you’d really notice in the city. Without light pollution, there is a very big difference between a night with the Full Moon and a moonless night. It’s not something we normally notice in the city, but there’s a reason why it’s mentioned so much in older fiction.

You may have noticed if you’ve read a lot of older literature, I think in almost any language, people usually make a big deal if it is a moonless night. Either the author or narrator will say it, or characters will point it out, and this is because it matters. A moonless night is so much darker than a night with the Full Moon, if you don’t have any electrical lights to rely on, even if you’re relying on candles. A candle outside is nowhere near as bright as a modern LED flashlight some of which can illuminate a whole neighborhood. Even a smaler difference than the difference between a Full Moon and a New Moon can be significant. A slightly-past-full Moon just coming up from under the horizon makes a huge difference to the amount of darkness. In the attached video, I use a simulation to demonstrate this, but if you’re in the countryside, it is truly noticeable. On a night of an actual Full Moon the ground looks a lot brighter than it does when the Moon is out of the sky or even in a smaller, newer or older phase. If the Moon is very close to a narrow crescent, it’s not going to add as much light to the sky. Most importantly, on a night of the Full Moon, the Moon is bright enough to cast a shadow.

If you’ve ever been in the countryside on a night with the Full Moon, if you put your back to the Full Moon on a clear night with no clouds, in a dark enough area of the countryside, you will see your shadow. The Full Moon is bright enough to cast a shadow, you’re just not going to notice it in the city because of light. A lot of the sources of light that contribute to light pollution are pointing at the ground. Street lights sort of leaking into the sky is a big part of the problem, but most street lights are intended to point down, so of course they’re going to outshine the Moon. You’ll see your shadow based on the street lights, not on the Moon. However, if you are in a dark enough part of the countryside, the Moon is bright enough to cast a shadow all on its own.

In the simulation software Stellarium, the Moon and Venus are shown as a similar size. Stellarium uses the size of the circle in the sky kind of as a shorthand or indicator of how bright object is. Looking at the actual magnitude, the Moon is coming up at around negative 12. That’s a very low magnitude, which means it’s very, very bright. Venus is only negative four and a half, though of course Venus is not at peak brightness when the Moon is full this month, and it would be brighter if it was higher in the sky. Close to the horizon, Venus will be closer to just negative negative 4, due to the atmosphere. The Moon and Venus are clearly very different brightnesses, the Moon is significantly brighter than Venus. Venus at negative four and a half is much brighter than Jupiter at negative 2.6, down to negative 2.48 despite being quite high in the sky. This is not half as bright as I mentioned in the video, but about 5 times less due to magnitude being a log scale. For the magnitude of luminosity, a 1 is 100 times brighter than a six, making each magnitude about 2.5 times brighter than the last, or fainter depending on the direction. With Jupiter compared to Venus, Venus is significantly brighter, but it is even more significantly fainter than the Moon.

However, thanks to a comment on one of my recent videos, I have been made aware Venus can cast a shadow as well. This does make sense to me, Venus is so much brighter than the surrounding sky, especially when it’s far from Jupiter. On a night with no Moon, Venus being able to cast a shadow does make sense, because a night with all of the stars and the glow of the Milky Way, it is perceptibly brighter than a cloudy night. This is true if you measure the amount of light coming from the sky in a dark sky where there’s no light pollution. Your light meter or light detector will pick up more luxes, or lumens or whatever unit you’re using, if it’s on a clear night compared to a cloudy night. In a city, it’s the opposite, the more clouds you have in the sky, the more light pollution is going to be reflected back down at the ground, so the brighter the sky will appear, which does make things a little bit different. Regardless, if you’re in the countryside on a very clear night, it is much easier to see where you’re going, even on a moonless night, compared to a night with clouds in the sky. As such, I am willing to believe at least, that the light of Venus is significant enough to make a difference, enough difference to cast a shadow.

Another reason I’m bringing this up is partly because it’s so difficult to see Venus at its brightest. Figuring out when Venus is going to be at its brightest is a little trickier than other planets, mostly because it is a planet closer to the Sun than we are. As Venus moves around the Sun, at the moment coming out form behind the Sun, we can see its brightness continuing to climb as it reaches opposition. At a given time, this is complicated by getting reduced in magnitude by air magnitudes as it gets lower in the sky, and of course the glow of the sunset. However, even very much past the greatest elongation, we can see that Venus is climbing in brightness. With most objects, they’re going to look their brightest when they’re fully illuminated by the Sun, and for planets that are further from the Sun than we are, that’s easy. Planets further from the Earth appear full when they are at opposition, and they are going to be at their closest to us at the same time. For Venus and for Mercury, it’s the exact opposite. Venus is further from us when it’s fully illuminated by the Sun, it’s closer to us when it’s almost in front of the Sun. The difference that distance makes is significant, even though a much smaller portion of Venus is illuminated when it’s closer to us, we see it as brighter. As it moves away to the opposite side of the Sun, even though more of it is illuminated, it is significantly further from us. The orbits that these objects take around the Sun are very big. Going from one side of the Sun to the other side of the Sun, even for a much closer planet like Venus or Mercury, is a huge difference.

We never get to see Venus at opposition, it’s never directly behind us and when it’s fully illuminated side is facing us, it’s on the far side of the Sun. We don’t get to see Venus at would theoretically be the brightest perspective. Probably the best candidate for the brightest perspective of Venus would be if we were on Mercury looking back at Venus. From the right location on Mercury, we would be able to, every now and again see Venus as fully illuminated in the sky. There are a couple of tricky points to this. Luckily, Stellarium provides a vantage point known as the solar system observer which can help. The solar system observer allows us to look down on the planets from above, and that will let us see when Mercury and Venus are aligned with each other in such a way that from Mercury’s perspective, Venus would be full or at opposition. With the Sun in the center, Mercury becomes invisible if we zoom out enough to see Venus, with Mercury in the center it is possible to see all three and make sure that they appear in a straight line. They may not actually be in line with each other, due to differences in inclination, but as long as they appear to be in a straight line from above it will be close enough. Even if they were in a perfect line, we wouldn’t get an eclipse or anything on Venus, Mercury’s shadow is not going to be noticeably cast on Venus. Unfortunately, Mercury is just too small and so much closer to the Sun, you would only get a transit, similar to how we see Venus or Mercury transiting in front of the Sun from here on Earth. If you don’t have the right glasses, if you don’t have the right equipment, you’re just not going to see it, it’s not visible to the naked eye. What we need to do is go to Mercury so that we can stand on Mercury and look back at Venus.

Unlike the Earth, there is a problem for Mercury and for Venus, the issue of seeing an object at its moment of opposition. It’s even more difficult to do for a random location on Mercury than it would be for a random location on the Earth, because Mercury turns around so slowly. I just need to make sure that we’re not on the side facing the Sun, but on the exact opposite side of Mercury. We’ve checked form the Solar System observer, so we know Venus will be there, opposite the Sun. However, because Mercury rotates so slowly, because its day is so long, by the time the part of Mercury which is looking at the Sun while Venus is at opposition turns around to be facing Venus, there’s a good chance that it won’t be at opposition anymore. This is both because these two planets are, A, moving around the Sun so quickly, and B, have such incredibly long days compared to all of the other planets in the solar system.

The gas giants have very, very short days, compared to ours, while the Earth and Mars’s days are pretty similar. Venus and Mercury are the weird ones, their days are a lot more complicated. I have spoken about the day length on Venus and on Mercury in the past, in pieces dedicated specifically to those planets. They are back on post history of the website somewhere. I will go over all of the planets again in the near future, once I’ve looked at a few more trans-Neptunian and other objects. Mercury at night time looks incredibly dark, with a very clear view of the stars. Of course, there’s no atmosphere on Mercury, and we can clearly see Venus. It is just past its opposition b ut still very close, at 99.5% illuminated, and we’re seeing it at magnitude negative 7.

This leads me to believe, I think I can say with some amount of confidence, that the planet Venus at opposition as viewed from Mercury is not as bright as the Full Moon when viewed from the Earth. This something that I have now learned, in the process of creating the video attached to this piece. I didn’t know this beforehand, but it does make sense. Venus is so much further from Mercury than the Moon is from us, even though it is so large and so close to the Sun. Of course, the calculations provided by Stellarium could also be wrong, but they sound right to me. Anyone who wants to go into the maths of calculating the brightness can, with Venus’s albedo and the amount of insolation it’s receiving, it’s interesting maths. It’s fun maths to play around with, but I’m not going to be doing it myself. I would accept if an astronaut or a robot was to ever go to Mercury and perceive its own shadow due to the illumination of Venus at opposition, at magnitude minus 7, I would believe that. Of course, believing something isn’t the same as double checking yourself, but I have no intention of ever going to Mercury daytime or nighttime to check for myself.

I hope you enjoyed coming along on this little investigative journey with me. This whole thing was inspired by a comment on the previous video, so thank you to the commenter in question. If you did enjoy this piece, please do like it and make sure to subscribe to this website and my YouTube channel if you’d like to see more from me. Thank you for reading and hopefully I’ll see you back here next time.