Caoimhín's Content

In this video, we take a look at the planets, particularly their motion through the sky. We’ve looked at the motion of various planets before, but mostly individually. Here, we’re looking at the planets all together, so we can compare their movements to each other and the background of the stars.

We’ll start by looking at the sky for this month, just the end of April. As the Sun sets, it’s clear that we can’t see Jupiter at all. If you have ideal conditions, you might still be able to catch Jupiter for another little while, but it is certainly leaving our evening sky. Pushing ahead to the morning, and we are looking at the sky from the city here, with all the light pollution that that entails, and there still isn’t any visible planets. At the moment, we are in a sort of gap without any planets to see here. This doesn’t always happen in a given year, or at the same time of year each year. It is also a little harder to see the planets from Ireland in general. In higher latitudes, in both the Northern and Southern Hemisphere, the angle of the ecliptic at sunrise and sunset can keep planets too low to the horizon to be seen, they get blocked out by the twilight glow of the Sun.

If we could remove the atmosphere, we would be able to see the planets even when they are right next to the Sun, even in the middle of the day. With no atmosphere to spread out the light of the Sun, even the stars would be visible. At the moment, this makes it very clear why no planets are visible at night, they are all close to the Sun. Jupiter, along with Uranus, is right next to the Sun on the Eastern side. Venus and Mercury are very close to the Sun as well to its West. Mars and Saturn and Saturn become visible, significantly further from the Sun than the other planets to the West. They aren’t as close to the Sun, but here in Ireland and at other higher latitudes they are still to difficult to see. By looking at the morning sky with no atmosphere, we can see that despite the fair amount of distance between the two planets and the Sun, they are very close to the horizon. This means they are only a little above the horizon when the glow of the Sun begins to creep into the sky. Without any light pollution, Mars and Saturn do come into view, however this may be down to the abilities of the software. In reality, the two planets would be very hard to spot, and using assistance such as a telescope or binoculars would be inadvisable. The glow of the Sun, even when it hasn’t fully risen, can be uncomfortable or even dangerous when focused by lenses or mirrors right onto your eye. At lower latitudes closer to the equator planets could be visible already, as they would be almost directly above the Sun, rather than lying at a low angle as we would see them here. Of course, as the planets orbit the Sun, they will appear to move away from the Sun, bringing them into view in the morning, even in light polluted conditions, in just a couple of months.

Coming back to our atmosphere free, midday view, we can see that the planets are just besides the Sun, they aren’t directly behind the Sun at this moment, and they don’t have to be in order to be obscured from our view. Moving forward through the year, we’re able to see that the planet Saturn appears to be moving from the Sun quickly enough, but the planet Mars isn’t moving as fast in the sky. This is especially compared to one another, we can see Saturn keeping pace with the stars as they move form East to West over the year, while Mars is lagging behind them. It is still moving westwards, away from the Sun, but much more slowly. Jupiter does end up behind the Sun after a while, as does Venus, one moving in from the East, the other from the West. Mercury also lines up with the Sun, as it moves from its westernmost elongation back to the East. All this time, Mars stays comparatively close to the Sun, while the planet Saturn has completely disappeared beyond the Western horizon. As we are looking at the day time sky, this really means that Saturn will be quite high in the morning sky. As continue moving forwards, Mercury appears on the eastern side of the Sun as it begins to be visible in the evening, Jupiter is now out on the western side of the Sun and becoming visible in the morning. By the time Venus starts coming out on the Eastern side of the Sun, Mercury is already turning back, moving retrograde back towards the Sun from its greatest Eastern elongation. Jupiter eventually passes Mars, following the background of stars while Mars, further from the Sun but not by a lot, continues to lag behind the background stars. As Jupiter moves West of Mars, Mercury starts coming back into the morning, while Venus remains completely to the East in the evening sky. it doesn’t take long for Mercury to begin turning back and moving in retrograde again, and now that we’ve moved through to the far end of the year, Mars starts to seem more significantly further from the Sun, having barely moved compared to the other planets.

Being able to see the Sun and some space either side of it, along with the planets, makes the retrograde motion of Venus and Mercury more easy to observe, especially when they appear to be moving opposite each other. Also, when looking at the whole sky like this, it’s easy to see the more normal motion of the other planets like Mars or Saturn and compare them to the two more inner planets as well. Despite the slower movement of Mars, it’s still visible that the planets further from us roughly travel from East to West in our sky. With Mercury and Venus, we can see them travelling one way, stopping, and beginning to move back the other way, over and over again. When the planets are moving East, against the background stars, they are moving in retrograde or backwards. This is only apparent motion, the planets are really orbiting the Sun just like us, but their orbit close to the Sun gives them the appearance of changing direction. This happens to both Venus and Mercury, but the short year of Mercury makes it a lot easier to spot and visible across shorter time periods. Venus has a longer year than Mercury and so it takes longer to see ti change direction. Both of them have much faster orbits than the Earth and other planets, so it is especially obvious that they are moving themselves, independently of the background stars.

The stars don’t move at all, and so their apparent motion in the sky is all down to the Earths orbit. Looking at a distant planet with a long year, like Saturn, it almost appears to follow this apparent motion of the stars, drifting East to West at about the same speed. If you were to look closely at Saturn compared to the background stars, you would see that Saturn is moving differently to them, but you would need to follow it through the sky for quite a while before a difference becomes really noticeable. Mars, on the other hand has a much shorter year. The quick motion of Mars around the Sun gives its motion a much more noticeable effect on its apparent motion in the sky. As Mars moves around the Sun, it noticeably moves against the background stars even over the course of just a few nights. Jupiter is technically between Saturn and Mars, but with a year several Earth years long it’s motion in the sky is much more similar to Saturn. This is clear when Jupiter appears to overtake Mars, moving past it following the stars as Mars lags behind.

The stars are always more fixed than the planets, they don’t speed up, they don’t slow down, they stay moving the same way at the same speed, all of the time, because the Earth is the only moving factor. With the planets, as both we the observers and they the targets are moving, they can appear to speed up and slow down, or even move backwards. Again, of course, all of the stranger parts are apparent motion, the planets all regularly orbit the Sun in fixed directions and pretty fixed speeds, but the multiple moving points complicate how things appear in the sky. Getting back to our real view of the sky, we may be missing planets at the moment, but inn just a few months we’ll have them visible in the sky again. If you want to catch the planets as they appear, you can subscribe to this website or my YouTube channel for updates, month by month, of what’s coming up in the sky. Whether you subscribe or not, I hope you’ll come back here to check out the next post.