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The ecliptic is a very special imaginary line in the sky, and it has been mentioned in previous videos, particularly as the location of eclipses. It’s where we see the planets and also the line the Sun lies on in the sky. The significance of this line was noticed even in ancient times, and as most of our constellations were created then, many of the most famous of them lie along this line as well. In this video, we take a closer look, first at the relation of the planets and their motion to the ecliptic, then the constellations that lie on it and how to find a few of them.

The motion of the Moon relative to the ecliptic is what gives it its name, ecliptic because it is where eclipses happen. The Moon, orbiting our equator, circles the Earth, and this circle intersects with the ecliptic circle twice, so twice every lunar orbit, the Moon will move across the ecliptic. When it does this in exactly the right place, we get an eclipse of some sort. Eclipses are an interesting topic, and they will get their own video soon.

The planets do follow the path of the ecliptic, much closer than the Moon does, as they orbit the Sun’s equator rather than our own. That is really what the ecliptic represents, essentially the Sun’s equator stretched out into space. The planets may deviate a little from this path (with dwarf planets like Pluto potentially deviating much more), but generally if you see multiple planets in the sky together they will lie on the same path, with the Moon being pretty close as well. Our axial tilt puts our equator a little off from lining up with the Sun’s, hence the different path of the Moon.

The planets don’t just sit on this line, as many stars do, they move along it. Of course, the planets appear to move across our sky as we move through the night, but this is due to the Earth’s rotation. The stars appear to move in the same way, but on a human timescale the stars can be considered to be fixed, they will be in the same places relative to each other night after night. Both planets and stars will appear at different times of the night through the year, but this is also due to our motion. At a given time of the night, each night objects will appear further and further west, due to the Earths orbit around the Sun. The other planets, however, are moving as well, orbiting the Sun just like the Earth. From night to night, this motion can be seen relative to the stars.

The combination of our moving position around the Sun and the planets themselves moving can make the apparent motion of the planets in the sky a little more complicated. Although planets always move around the Sun in the same direction, they can appear to move backwards. This normally happens with planets closer to the Sun, like Venus and Mercury, with Mercury in particular having a noticeable retrograde motion. Other planets, like Mars, can appear to make a loop in the sky as our orbit catches up with and then outpaces theirs. We’ll deal with these more complex motions, especially Mercury and Mars, in a different video. For now, we’ll look at the nightly motion of the planets.

We will start by looking at Venus. With Venus’s shorter year, thanks to it being closer to the Sun than we are, it’s motion is easier to see. Comparing the position of Venus to the stars beyond it, we can see it’s place shift night by night. This motion of Venus around the Sun is what takes it out of view at some times of the year, and causes it to swap from being visible in the evening, to being visible in the morning, and back again. Along with this pronounced variation, the shorter orbit of Venus means that amount it appears to move each night is greater than the more distant planets, allowing the motion to be noticed even over short timescales.

This motion of the planets is part of what allowed ancient people, like the Ancient Greeks and Romans, to notice that the planets were different to the stars. The stars remained relatively fixed night to night, and always returned to the same places at the same times each year, where as the planets would shift a little night by night and from year to year could vastly change positions. This difference is noticeable with no telescopes needed, allowing ancient people to recognize the planets as different, even though they didn’t necessarily know why.

Jupiter and Saturn are next up for a closer look, and although they are further from the Sun and orbit it much slower than the Earth or Venus, they still noticeably move independently from the stars. This is harder to notice on short timescales, but becomes very obvious from year to year, especially with the very bright Jupiter. Saturn may not be as bright, but it is still comfortably visible in most cities, unlike many of the stars in the constellations we’re going to look at next.

Some of the most famous constellations in the sky lie along the path of the ecliptic. With the Stellarium software I use in videos, we can see the lines or even images of the constellations in the sky, but in real life they can be difficult to spot, much more difficult than spotting the planets. Luckily, there is a group of constellations that sit along the ecliptic, originating from the same ancient peoples that noticed the planets motion along it. Given that the planets are always on the ecliptic, if you spot a planet, you must have also found the location of one of these constellations.

Saturn in Aquarius the Water-Bearer is a good example. As Saturn is a planet, it won’t stay in Aquarius forever, but it is there for the time this video is set. Aquarius is mostly quite faint stars, difficult to see from the city, and furthermore it is a difficult shape to piece together, the lines on their own don’t look much like a person carrying water. However, if Saturn is in Aquarius, then once you find Saturn you’ve found Aquarius, even if you can’t see the constellation in the sky.

Jupiter is here in the constellation Aries the Ram. Although the stars in Aries may be visible. Turning a couple of stars in a line into a ram with horns is pretty tricky. As long as Jupiter is in Aries, you don’t have to worry about recognizing the shape, all you need to do is find the second brightest planet, which in this case is the brightest object in the sky besides the Moon. Of course, Jupiter too will eventually drift across the sky, but with five planets visible to the naked eye there will usually be at least one visible at some stage during the night, and it will have to be in some constellation.

For those of you already familiar with the group of stars lying on the ecliptic, the group that includes Aries and Aquarius, you may know that there is another constellation between them in the circle. The lines are visible in the video, and this constellation is Pisces the Fish. The constellations don’t drift across the sky or swap positions, so whenever you find Aries and Aquarius, Pisces must be in between them.

The Sun of course also lies on this line. As we orbit the Suns equator, we will always see it on the ecliptic, but it will appear to move along the ecliptic as we orbit it through the year. Of course, you should never look directly at the Sun, and even if you did, you wouldn’t see constellation that it sits in. In real life, you can figure out what constellation the Sun is in by looking at what else is visible. The Sun will always appear to return to the same constellation at the same time of the year, so you can also learn what positions are associated with what times, but we won’t deal with that much here.

For the time of this video, late October, the Sun is just at the end of the constellation Virgo the Maiden, and will appear to move into Libra the Scales as we move into November. Mercury can be seen moving further out from the Sun into Scorpius the Scorpion, while Mars continues behind the Sun in its orbit, all still staying close to the ecliptic line. If the Sun is in Virgo, then starting next to Sun at sunset is Libra, followed by Scorpius, though some people say Scorpio. The next famous constellation on the line is just over the Milky Way’s nucleus, Sagittarius the Archer. We discussed the shape and name of Sagittarius in a previous video dedicated to constellations more generally. You can check out one of the previous posts here to find it.

After Sagittarius we have Capicornus, or Capricorn, the Hippocamp. A hippocamp is a mythological creature, often half-horse half-fish, but here half-goat half-fish, similar to a mermaid. This brings us back around to Aquarius with Saturn, followed by Pisces and then Aries with Jupiter. The next constellation, Taurus the Bull, contains a famous and recognizable group of stars, the Pleiades or the Seven Sisters. These stars have also featured in a previous video about deep sky objects. They are a young open cluster of stars and are quite prominent when they are in the sky. This means that even without a planet to help, you can find the constellation of Taurus by finding the Pleiades. Similarly the core of the Milky Way always has Scorpius on the west and Sagittarius on the east, so if you have dark enough sky to see the galactic core it can help. You can learn more in another previous post on this site.

Moving on from Taurus, we have a constellation where the lines actually resemble the picture it is meant to be, Gemini the Twins. The lines do look like two stick figures holding hands, if a little wonky, and this is meant to be two people together. They are usually called the Twins, though in some stories they are just brothers, and in one story their names are Pollux and Castor, which is where we get the names for the two brightest stars, each the head of their respective twin. After that we have Cancer the Crab, another faint and hard to recognize constellation, followed by Leo the Lion which has much brighter stars and almost looks like the side view of a lion lying down. Venus is just at the end of Leo in this video, which brings us back around to Virgo and the Sun.

Those are twelve of the most famous constellations in the sky, the twelve constellations on the ecliptic, sometimes called the Zodiacal Constellations. They are where we see the Sun, the Moon, the planets, they’re where we see eclipses and even the core of our galaxy. However, there is one other constellation, just barely on this line. The one of the lines of this constellation can be seen crossing the ecliptic, right between Scorpius and Sagittarius. This is the famous thirteenth Zodiacal constellation, the other constellation that the Sun moves through, even though it only takes a couple of days in December to do so.

This is Ophiuchus the Snake-Bearer. Really we see three constellations in one for the image of Ophiuchus. The human figure and the portion of snake on or around them is the Snake-Bearer. The little bit of tail sticking out to the side is Serpens Cauda, the Snake’s Tail, and the front of the snake on the other side is Serpens Caput, the Snake’s Head. It is only Ophiuchus proper, and then only the end of a leg, that actually crosses the ecliptic, so it might be understandable that it is often left out of the famous group.

In the video we go through the Zodiacal Constellations again, but with the images being shown rather than just the lines, to give a better idea of what they are supposed to look like. I also mention again what planets are visible in what constellations, but do be warned that this can change over time.

That’s all for this video, but now you should have a better chance of finding these famous constellations in the sky and a deeper appreciation for why this region of the sky is so interesting. Hopefully, I’ll see you again on the next post.