Caoimhín's Content

Happy April Fool’s Day everyone, don’t worry I won’t be pulling any pranks. Today we are going to take a look at Mercury, Saturn and Venus in the morning. In a recent piece I mentioned that Mercury is coming to its greatest elongation and how that wouldn’t really be visible for us here in Ireland. We are mostly going to look at why and look at where Mercury can be seen from.

Taking a look at the Sun at midday, with no atmosphere to block out the stars and planets, almost all of the planets would be visible in early April. By moving through time day by day, we would see Mercury moving further away from the Sun, for a few days. Then, as it draws close to Saturn, it seems to stop moving away from the Sun and start coming back in towards it instead. Just as Mercury stops to turn around, that’s Mercury’s greatest elongation, that’s when it’s reaching its farthest from the Sun. In this case, it’s stretching to the west, so it is the greatest western elongation. Then it starts to turn around and come back in towards the Sun, Mercury’s retrograde motion. Mercury is reaching this point on the 22nd of April, though in the video I do say that it looks like it’s getting to the greatest elongation at around the 20th. Around the elongation, Mercury appears to stay about the same distance from the Sun for a couple of days. This is all a lot easier to see with no atmosphere, or ground, in the way.

During this time, Venus is nice and bright, a little bit further from the Sun than Mercury. If we come back to a few days before the greatest elongation, Venus is about as far from the Sun as Mercury will get at it’s greatest elongation. However, Venus is still a little bit higher in the sky. This is easier to see with the ecliptic draw across the sky. The ecliptic is almost the Sun’s equator projected out into space. It’s definitely the plane around the Sun where everything is seen from our perspective. With the ecliptic up, it looks like Jupiter is dead on the ecliptic and Mercury is pretty much dead on the ecliptic as well, just around the second week of April. Venus is quite a bit above the ecliptic and that is important for visibility, particularly for those of us in the Northern Hemisphere. In the morning, while the Sun is still below the horizon, Mercury and Saturn are much lower in the sky than Venus, even at comparable distances from the Sun. Looking at this from slightly earlier before sunrise, it’s clear that Venus is still above the horizon while the Sun is under the horizon. However, by the time Mercury and Saturn are up, there’s nearly a limb of the Sun already coming above horizon at that point. If we move forward to the greatest elongation of Mercury, Mercury ends up very much under the ecliptic. This keeps it pretty much on the horizon as the Sun is coming up. It may be as far from the Sun as Venus was, but it’s much lower in the sky than was Venus.

A lot of that is all only visible with the atmosphere turned off. We will move to a view with an atmosphere, but we’ll keep the ecliptic drawn on the sky because it is a good guide. With the atmosphere back on, Mercury and Saturn disappear, even though they are above the horizon. This is mostly due to the atmosphere scattering out the sunlight. We can’t really see them at all. This is from here in Ireland and with the light pollution that a city may have. As usual we’re assuming the view from a city like Cork, so a medium amount of light pollution. If we had an incredibly dark sky with no light pollution at all, it looks like Saturn and Mercury are still practically invisible. Perhaps a little later in the month they may be faintly visible, but is as visible as they get. Certainly in comparison to Venus they are already faint, Mercury’s not quite as bright as Venus, neither is Saturn. They are also lower in the sky, and the sky is a little bit brighter down closer to the horizon, thanks to the way the Sun’s light is getting scattered through the atmosphere as it rises.

This all adds up to make Mercury pretty impossible to view for us here in Ireland. Thankfully we shouldn’t have to move too far south to get it visible. We’ll start by moving just down to about 45 degrees north. This is pretty much level with France, the very top of the USA and the southernmost parts of Canada. Immediately, there’s Mercury and there’s Saturn, from the countryside at least just that little bit of a jump to the South. One of the reasons why Mercury and Saturn here are so much easier to see is because they are higher from the horizon. This means they are higher above that thicker portion of the atmosphere. The atmosphere is kind of the same thickness all the way around, but when light is shining through the atmosphere at a very oblique angle, rather than straight down, that light is passing through more atmosphere due to the angle. The two planets seem just about visible from this latitude, but I have an odd feeling that if we were to come back into a city, even a smaller city like Cork, then they might disappear. Indeed, with the same level of light pollution, they are much easier to see with binoculars. Saturn and Mercury are definitely harder to see from the city, but of course that is pretty obvious. Of course they are harder to see from the city, everything is.

We will move a little bit closer to the equator again, down to about 39 degrees North. This is down by Spain, a lot of the Mediterranean, the northern part of the United States. From here, Mercury and Saturn are visible even from a city sky. I didn’t put the light pollution back down while jumping further south, but they are definitely visible. This is still not very far south, just 39 degrees north. 39 degrees north is very much in the northern hemisphere, it’s not even in the tropics. We’re not even that close to the equator, just a little bit closer than we are here in Ireland. However, over a range of days, Saturn and Mercury they stay visible along with Venus in the morning. Even Mercury gets quite visible, or at least quite high above the horizon. However, each day forward is getting closer and closer summer, meaning an earlier and earlier sunrise. Pushing ahead until Mercury is if as far out as it will get, and then coming back earlier in the morning, from this latitude Mercury and Saturn become much easier to spot. Even though they are as low in the sky as they would be from here in Ireland, the sky is darker and so they remain visible.

We will keep pushing down, almost to the tropics around 29 degrees north. This brings us all the way down to the north of Africa, around the middle of the USA. The planets end up nice and high above the horizon. Coming back earlier in the month, from here you can even see Saturn and Mercury getting pretty close together. Saturn and Mercury have a pretty close pass just before Mercury’s greatest elongation. Just as Mercury starts to sort of turn around and Saturn continues on its path away from the Sun, Mercury and Saturn get quite close. This is around the 14th-16th of April, and it’s something we’re not going to be able to see from here in Ireland. However, from this position significantly closer to the equator, it’s absolutely visible.

We’ll continue southward and jump right down into the tropics. We won’t go all the way to the equator just yet, we’ll take one more jump in between to 13 degrees north. Here Saturn and Mercury are nice and close together, and we can see that they do get quite high above the horizon in that formation there as well, creating a nice kind of grouping in the early morning. Just before sunrise, this is super clear from closer to the equator, right in the middle of the more northern tropic, The tropic of Cancer. Even from quite early in April, there’s the three planet’s, Venus, Mercury and Saturn all together. There is a chance in April that, if you are close enough to the equator or on the equator, you will get to see more planets in the morning than you do in the evening. It was really the evening sky that was dominated by planets for all of the beginning of the year, with the parade of planets going on, but we can see here that, from close enough to the equator, over the course of April the morning sky is filling up. We’ve got three planets in the morning sky, and only got two in the evening sky. We’ve only got Jupiter and Mars in the evening sky, although we do technically have Uranus in the evening sky as well, it’s just incredibly difficult to see. If we are going with technically, then we do have Neptune in the morning sky. It is technically out there, of course Neptune is going to be very difficult to see without a telescope.

If we move into the Southern Hemisphere, things will change. However, again because of the angle of the ecliptic, we’re going to have to jump pretty far into the Southern Hemisphere to see a major change. Even at 20 degrees south, the ecliptic almost looks like it’s going straight up into the sky. We are here after moving into southern autumn, a different season from the northern spring we’re in. The different season is also going to effect the angle at which we see the ecliptic, and therefore how we see the planets in the sky. We’ll keep moving, as we’ll need to go significantly further South to see the ecliptic arcing back towards the north. Finally, by around 43 degrees south, we can see that Saturn and Mercury appear a little bit higher in the sky than Venus. The angle isn’t quite extreme enough yet to block out Venus, so it’s not as extreme as the comparable angle from here in Ireland. Of course we’re heading towards summer time here, so we’re going to see the ecliptic at quite a shallow angle at night in comparison. Even going bit further, all the way to 60 degrees south, Venus doesn’t disappear. This is even farther South than we are North here in Ireland, but Venus, Mercury and Saturn are all still visible. It looks like the angle doesn’t get extreme enough, more than likely thanks to the season, to completely block out Venus, as it did when we were seeing the double Venus.

Last month, at the end of March, we were able to see Venus in the evening and in the morning because Venus was pretty much above the Sun. Just before the beginning of April, in the far south Mercury and Saturn are up in the morning, but Venus isn’t visible yet, it’s still under the horizon. This is the case because of the angle. If we move back into the end of March Venus is gone, but it looks like by the time we’re into April, Venus is coming down towards the ecliptic again and that keeps it visible. This all leads to a really nice view of those three planets in the morning, if you are far enough into the south. Those are the planets that we’re going to get to see in the morning, or at least the planets that some people are going to get to see in the morning over the course of April. April is significantly different from March in terms of what planets we can see and where we can see them, or where we can see them, from. We’re not going to get all the planets together in the sky at the same time, but as long as you are in the right location, you might get a good few. For here in Ireland, being in the countryside will really help with the visibility of Mercury and Saturn, but as long as you’re in the right place, we’re going to have a fair few planets becoming visible over the course of the April.

Moving through to the end of April, for us here in Ireland, Saturn will climb up into visibility. Once Saturn has sort of overtaken Venus, we are getting to see it in the sky, but even though Mercury is quite far from the Sun at its greatest elongation, we’re definitely not catching it from within the city. Hopefully you’ll get to catch it if you’re in a different location.

I hope that you enjoyed this piece, if you did please do like it. You can also subscribe to this website and my YouTube channel if you’d like to read more of this content. Thank you very much for reading and hopefully I’ll see you back here next time.