Caoimhín's Content

Today we are going to be looking at oppositions, specifically Saturn and Neptune coming to opposition, as they will both do so this month. I have taken a look at Neptune specifically quite recently and only a couple of months ago we took a close look at the planet Saturn as well, so although I will give a couple of close ups of each planet during this video I will be concentrating on their position and what these planets coming to opposition means.

Saturn will reach opposition much earlier than Neptune on just the 8th of September, although we won’t see it directly above the South from here in Ireland until a little after midnight, technically into the morning of the 9th. When Saturn is at opposition it is directly behind the Earth relative to the Sun, just like the Full Moon. This leads to a lot of similarities to the Full Moon, such as the fact that Saturn will be above the South at midnight, give or take a few minutes depending on your time zone and how close you are to the center of your time zone. Anything that is at opposition will be very much in a similar position to the Full Moon. When a planet is at opposition that means we are seeing it at it’s most illuminated, because planets face is facing us and the Sun. However, planets like Saturn and Neptune that are further from the Sun than the Earth, so we will always be looking at their illuminated face. The face of the planet facing us will always be the face of the planet facing the Sun because we are closer to the Sun. Luckily, thanks to the angles involved, it’s not exactly perfect. If we’re seeing Saturn when it’s not at opposition, if we’re seeing Saturn kind of out to the side of the Sun, then we may see a little bit more shadow we may see a shadow of Saturn on its rings. At opposition on the other hand, Saturn’s shadow is directly behind it on its ring and therefore we can’t see that shadow. How much shadow we see will vary over the course of the year and it’s one of the biggest differences, when it comes to Saturn, between opposition and not being at the opposition.

Planets are also closer to us at opposition, so if we take a look at the distance from Saturn to the Sun, it’s 9-and-a-bit Astronomical Units. The distance from the Earth to the Sun is 1 Astronomical Unit and because the Earth is between Saturn and the Sun here, we’re seeing Saturn at only 8-and-a-bit Astronomical Units away, one Astronomical Unit less than the distance between Saturn and the Sun. If we were to follow Saturn through it’s orbit around the Sun, we would see the distance slowly increase as we moved away from opposition. Without the ground or the atmosphere in the way, this is something we can simulate. Eventually, Saturn will end up on the fart side of the Sun, in a straight line with the Earth again. However, while we’re on the way, we will reach a point where Saturn, the Earth and the Sun will form a right angle triangle. With the Earth just 1 AU from the Sun, and Saturn about 9.5 AU, the hypotenuse of this triangle, the distance from the Earth to Saturn, is going to be pretty close to 9.5 AU as well. If the Earth was further from the Sun, the hypotenuse would be much longer. At this position, we get to see the shadow of Saturn on the ring there. Looking straight at Saturn, the Sun would be off to the side, shining on Saturn at an angle and casting Saturn’s shadow onto the ring in a way that we can see.

Continuing to move back through the year, Saturn will eventually reach the far side of the Sun. This is often called conjunction, and it is distinguished from planets closer to the Sun than we are lining up with the Sun. Mercury and Venus are said to have two conjunctions, one behind the Sun, the superior conjunction, and one in front of the Sun, the inferior conjunction. Conjunction is also the term used when two planets appear near each other in the sky, indeed anytime two planets appear to be in a line. To keep things clearer, I will call the conjunction of the outer planets with the far side of the Sun the major opposition and the conjunction on our side, what we would call opposition, the minor opposition, just like superior and inferior conjunction.

When Saturn is directly on the far side of the Sun compared to the Earth, of course we will no longer be able to see it, but Saturn’s shadow will be directly behind the planet again. We have to go through a lot of time here, a full six months, because although Saturn is itself orbiting we mostly see it change position due to our motion. It takes a long time for Saturn to orbit the Sun, 29 years for Saturn. Of course we’re moving as well, so we’ll see it come back to the same relative position in our sky at a slightly different time. Once Saturn is directly in line with the Sun, we won’t be seeing any shadow looking through a telescope. Pulling back to see the whole sky, without any atmosphere in the way, we can see that Saturn appears very close to the Sun in the sky. Saturn won’t appear to be directly behind the Sun, not exactly, but it’s definitely very close. Thanks to the tilt of the Earth and the inclination in Saturn’s orbit and the Earth’s orbit, the Sun doesn’t block out Saturn. I always say that the planets orbit the Sun along the plane of the ecliptic and generally speaking, they do. However, there is at least a slight inclination tp the orbits of the planets, they may go a little above or a little below the ecliptic over the course of their orbit.

Looking back to the last time Saturn was behind the Sun, if only we could see Saturn’s Rings when it was directly behind the Sun, it looks like they were at a very nice angle. Over the course of the year, the angle at which we see the rings does change thanks to the tilt of Saturn and the tilt of the Earth. The rings seem to have appeared their thinnest, at the steepest viewing angle, during June. I believe this was pretty close to the time when I made a video focusing on Saturn, and compared to the appearance of Saturn behind the Sun, the rings appear much thinner thanks to the change in the angle. Luckily we never see the rings exactly edge on, as they would be invisible. Saturn’s rings are so thin, even from the equator of the planet itself, those rings would be practically invisible. The changes in tilt let us see the rings at all and occasionally we see them at a good angle. Coming back up to the 8th, when Saturn is at opposition, the rings look pretty good, they’re certainly not as thin as they could be. When Saturn is at major opposition, on the far side of the Sun, it’s closer to 10-and-a-bit AU away from us, because it’s our distance to the Sun plus the distance to Saturn. Of course it doesn’t really matter, because we don’t get to see Saturn when it is at major opposition, forming a line with the Earth with the Sun in the middle, we only get to see Saturn outside of major opposition. The opposition we get to see is the minor opposition, also with Saturn, the Earth and the Sun a forming a straight line, but with the Earth in the middle instead.

Moving later into the month will bring us up to Neptune’s opposition. Certainly with the atmosphere removed, and a little bit zoomed, you might just about see the faint little dot of Neptune quite near to Saturn. Neptune is an incredibly distant planet. Moving from Saturn to Neptune, we’re going from 9-and-a-bit AU to almost 30 AU, 29-and-a-bit AU. As we try to get Neptune up to opposition, which will occur around the 20th of September, we may not see the absolute maximum of the opposition. As with eclipses, as with absolute fullest point of a Full Moon, we may not get to see Neptune, or indeed Saturn, at the exact moment of peak opposition from here in Ireland. Just like the maximum of the Full Moon, these things happen quickly enough and transiently enough that we may actually miss it here in Ireland. Taking a look at the distances, we are almost exactly 1AU closer to Neptune than it is to the Sun, almost. Unfortunately, as we move through the days, we get to see Neptune at 28.896 AU and 28.898 AU, but we don’t get to see exactly 28.897, at least not form Ireland at around midnight.

Neptune should be directly behind the South when it’s at opposition, and it does reach the South at midnight, adjusted for our position here in Ireland, without being at the exact moment of opposition. We’re going to go through the same process, we’re going to zoom in on Neptune and follow it through its orbit. Neptune has a very thin ring there which is incredibly difficult to see, but opposition is the best time to try to see it. You may be able to tell that Neptune’s ring appears to be tilted in a different way to Saturn’s. When we’re are looking at Saturn, we’re seeing the leading edge of its ring lower than its equator, but when we’re looking at Neptune here we’re seeing the leading edge of its ring, the side of its ring that’s pointed towards the Earth at least, higher than its equator. Neptune’s ring seems to come out from behind planet, then up and around in front of the planet, whereas Saturn’s ring seems to come out from behind the planet and then down in front of the planet. That’s just a difference in the relative tilts between these planets and Earth.

Moving back through the year, there is a moment where the thinner outer ring of Neptune appears to be visible, just barely. Any of Neptune’s rings are only barely visible, how much we get to see it depends on how it’s reflecting light back to the Earth so the angle can matter. Changes in the angle, just like the changes with Saturn’s ring, can effect the visibility of the ring around Neptune. Neptune will also form a right angle with the Earth and the Sun, but with two sides of with a length of almost 30 AU and a base of just 1AU, it is a triangle very close to a straight line. We’re not going to see much shadow, at least not easily, but with a massive telescope you might see a little bit of Neptune’s shadow falling on its ring. Neptune’s so far away and its ring is so thin that it’s going to be really hard to pick out the shadow. Continuing to move through Neptune’s orbit, we can see the distance form the Earth climb up past 30 AU, up to the distance from Neptune to the Sun plus the added AU to get all the way back to the Earth.

Neptune is 29.9 AU from the Sun during the opposition that we’re looking at, though it averages around 30.03 AU thanks to the elliptical shape of its orbit. From Ireland, we’re seeing it at 30.897 AU, at most, with the distance decreasing the day either side. This is an indication that Neptune will reach it’s major opposition, on the far side of the Sun, while the Sun is up for somewhere other than Ireland. Even working with a tenths, hundredths, even thousandths of an AU, it’s hard to get it things to match up perfectly. One thousandth of an AU is still almost 150,000 kilometers. From here in Ireland we may just miss the exact point of the major opposition between Neptune, the Earth and the Sun. Then again we’re not going to see it at the major opposition anyway, only the minor opposition.

There will always be almost exactly six months between Neptune at its furthest from us, on the opposite side of the Sun, and the opposition that we care about, the one that we get to see when Neptune’s on the same side as the Earth. This is because it’s half a year, half of our orbit. Even more so with Neptune compared to Saturn, the apparent change in position is thanks to our movement around the Sun. Neptune’s movement around the Sun takes so long that it ends up being pretty subtle, it takes years for us to notice that Neptune is in front of different stars at opposition, or any given date and time. The year on Neptune is closer to 165 years here on Earth, so it takes a long time for Neptune to get around the Sun, of course our motion around the Sun will bring it back to the same relative position a little sooner than that, just like it did with Saturn.

Those are the up coming oppositions of Neptune and Saturn, and every planet further from the Sun than us has a time where it is at opposition. The most important thing about the opposition is that it’s when the planet is at its closest to us so it appears its largest in the sky. That’s especially useful for brighter planets like Jupiter they’ll be that little bit more bright and prominent. They’re also with us for the entire night just like the Full Moon, so when these planets are at their closest to us, they’re also in the sky for the longest possible amount of time, the entire night. This gives us plenty of time to observe them and we’re observing them at their closest so they are at their easiest to observe when we have the longest to observe them. the opposition causes all those things to align nicely. Saturn of course will be visible to the naked eye, you’ll need a telescope or binoculars to take a closer look at Neptune.

I hope you enjoyed this quick piece looking at the oppositions of those two planets, and the different types of opposition or conjunction. If you did enjoy this piece then you can like it, make sure to like it to let me know, you can also subscribe to this website or to my YouTube channel, or both, to encourage the production of more videos. Hopefully, I’ll see you back here next time.