Caoimhín's Content

In this video we’re going to take a look at how the planets move across the sky. For most of the planets that orbit the Sun further out than the Earth, this is reasonably simple. However, for the two planets closer to the Sun than us, things certainly look a little more complicated. This video does mention certain specific times of year, but most of the points discussed are general, and happen every year, though often on different dates and in front of different stars in the sky.

To track to motion of the planets in the sky, it helps to see the paths that they are taking. Stellarium gives us the option of showing the orbital paths of the planets and the Moon in the sky. In fact, with this option in use, even the paths of the Moons around other planets become visible, though we won’t be looking at them here. The path of our Moon does show a bit more clearly how the Moons path crosses the ecliptic, which is conveniently approximated by the paths of the planets.

We do take a quick look at how the Moons path deviates from the ecliptic, showing how the path of the Moon appears to cross the paths of the planets. To follow the Moon’s path the whole way around, we need to get rid of the ground, otherwise it would get in the way. This will be useful for observing the planets closer to the Sun as well. Looking towards the Sun in this artificially improved view lets us see the difference, not just between the Moons path and the ecliptic, but the apparent paths of the planets relative to the ecliptic. As you may recall from a previous video (feel free to look back through the posts here if you haven’t see it), the ecliptic is roughly the Suns equator, and it is where the planets appear to move through the sky. It isn’t defined as being where we see the planets, as they don’t always line up perfectly.

To follow the paths of the planets, we’ll start with the further out ones, Jupiter and Saturn. Both are visible to the naked eye and both happen to be visible together at the time this video is set. We can see that the path of these planets takes them all the way around the Earth and the Sun, passing behind the Sun for part of their orbit. It is the planets closer to the Sun that we are more interested in, and even before taking a closer look it is clear that the orbits appear to be shaped very differently. The orbits are ellipses, almost but not quite circles, and this is true for the closer and further out planets. From our position on Earth they look a lot more elliptical, almost straight lines at some times. It is also visible that we can see the whole orbit, from behind the Sun to in front of it, without having to turn around. We don’t see to follow the paths of Venus or Mercury around the back of the Earth, because they orbit the Sun inside of our orbit, unlike the other planets.

We take a closer look at Mercury first. Mercury’s orbit is so close to the Sun that it doesn’t take long to complete, just 88 Earth days. Thanks to this, we can see its whole orbit, with its peculiarities, in a much shorter timeframe. As we follow Mercury here, we are hopping through weeks at a days at a time, so the Earth is moving through its orbit as well. This slightly alters our view, and so Mercury’s orbit appears to change from long narrow ellipse to practically a straight line. Again, Mercury’s orbit isn’t this shape or changing shape, it just appears to do so due to our movement, including changes due to our tilt. These videos are set to reflect the view from Ireland (bar some potential exceptions), so seasonal changes are a factor.

As we follow Mercury through its orbit, we can see it appear to move away from the Sun, and then turn around and and start returning towards the Sun. Of course, Mercury isn’t changing direction really, it is continuing on its orbit in the same direction the whole time, from the front to the back and around again. However, from our position on Earth, it appears to swap direction, both in the evening, first appear further from sunset, then rushing back in to appear next to it, and in the morning, first climbing out away from sunrise, and then falling back quickly into the glow of dawn. When the planet is moving contrary to how the planets usually move, this is called retrograde motion.

Importantly, what we seem to be seeing here isn’t really happening. The planets aren’t moving backwards. This is apparent motion, just like how the planets appear to move with the stars each night from East to West. Things can orbit the wrong way, some Moons do, and some planets around other stars appear to do so as well. We are seeing apparent retrograde motion, rather than an actual retrograde orbit. A normal orbit can be called prograde or direct, but usually aren’t distinguished. The rotation of a planet can also be retrograde or prograde, but we will see that in videos focusing an particular planets.

Venus also orbits the Sun closer than we do, just like Mercury, and we can see that its orbit is the same apparent shape, although it is much larger. The larger size of the orbit gives Venus a much longer year, closer to ours in length, at about 225 Earth days. This means its motion through the sky appears much slower, and it takes longer to change direction, and when it does it’s harder to notice. This may be one of the reasons that people speak about Mercury in Retrograde more often than Venus in Retrograde, although it does occur as well. As we hop through time here, we need to move much more quickly to notice the motion occurring, bringing us into the summer of the year after the winter we began. The later sunsets bring us into daytime as we watch Venus reverse its path in what would be the morning sky, coming closer to the Sun. This kind of confusing, impossible view is possible with the Earth rendered transparent in Stellarium. Removing the atmosphere is a handy feature of the software which stops it from being a problem.

Unfortunately, as we follow Venus around the Sun here, showing it move opposite to the expected direction of the planets, and then back again as it move through what would be the evening sky, it does get a little confusing whether Venus is in front of the Sun or behind it. Regardless of whether or not I can keep track, that is what’s really happening, just like Mercury, Venus is orbiting the Sun in almost a circle, going in the same direction all of the time. Thanks to our position of Earth, we get to see this apparent retrograde motion as the planets closer to the Sun go through their loops. Venus does take a lot longer and it is a lot harder to notice.

While going through the orbits of Venus and Mercury, you may notice times when the planets or their path appears to line up almost perfectly with the Sun. There are times when Venus or Mercury truly move in front of the Sun, known as transits. Both of the planets are too far away from us to cause an eclipse like the Moon, they never fully obscure the Sun. Instead, we get to see (through special viewing equipment, never look directly at the Sun) a small dark disc moving across the Sun’s face. A transit of Mercury happens pretty quickly, and the black dot is quite small, but certainly visible with low power magnification. A transit of Venus is slower, and the disk is a bit larger and easier to see, but they are also far more rare. We will probably take a proper look at transits at some point in the future.

For now, we’ll move onto Mars. Mars is outside of our orbit, but is also closer to us than the more distant planets. It is a bit further from us than Venus, but its year is only about twice as long as ours, instead of multiple years, so this phenomenon is a little easier to catch. We’ll still need to follow mars through a good portion of our year to see it. Mars needs to be at opposition, on the far side of the Earth relative to the Sun, for us to see it move backwards. Not quite as dramatically as the planets closer to the Sun, but as Mars gets close to being right behind the Earth, it appears to stop, move backwards, stop again, and begin moving forwards again in the direction we would expect. This little loop of retrograde motion is again apparent motion, it would be pretty strange for a planet to actually do a little loop like that. What is really happening is that the Earth is moving faster than Mars, so as we begin to draw level with it in the sky, we are really catching up causing it to seem to reverse as we draw level and move past it. Once we are a little bit past it appears to follow us in the direction we would expect. This little loop of retrograde motion happens bit by bit over the span of a couple of months, which may make it a little harder to notice, but it does occur when Mars is at its brightest.

This little loop of retrograde motion happens with all of the more distant planets, for longer periods for the more distant ones, but is does also get a little harder to observe without powerful telescopes. To notice apparent retrograde motion, you need to compare the planet to the background of the stars over successive nights, at a time when they are easiest to see close to the middle of the night. Mars is one of the easier ones to observe as it happens a little more quickly, and its red color helps it to stand out from the stars around it.

In the video, I go on to show when these instances of retrograde motion will next occur, but these will change from year to year and a few are quite near to the time of this being posted. If you got here quickly, the details are at the end of the video. There’s a good chance that I’ll come back to retrograde motion when various planets are at opposition, and transits as well as eclipses are going to get dedicated videos in the future as well. Hopefully I’ll see you here when I post about them.