Caoimhín's Content

Today, we are going to be taking a look at the galaxy, its equator, the ecliptic of our solar system and angles. As disparate as those topics sound, they do all flow together.

We are going to begin by taking a look at our galaxy, the Milky Way. It does require dark skies, but just after sunset in the middle of April, you might see the faint outer limb of the Milky Way over in the west. Of course the faint outer limb isn’t the most exciting part, and we don’t get to see it for particularly long. Given that the outer limb of the Milky Way is setting over in the west, by the time the western sky is getting dark, it’s already getting low. By the time the glow of sunlight has completely left, that outer arm of the Milky Way is arcing low above the horizon, which makes it more difficult to see. Following this arc brings us to Cassiopeia just over the north, and onto Vega and Deneb. As we push later, we are going to see more and more of the core of the Milky Way. The top of the summer triangle, Vega and Deneb, are already up when the Sun goes down, but we are waiting quite a while for Altair to come up, Pushing ahead, it is almost 2 o’clock in the morning by the time Altair is a reasonable height above the horizon. It will be another month or two before the summer triangle is with us all night long because we’re still not quite into the summer.

Still in the middle of April, just about 4 o’clock in the morning, we can see that the core of the Milky Way is clearly above the horizon. At this time the sky is still reasonably dark, but the Moon is quite close to the core of the Milky Way. We’ve now moved from the very middle of the month, the 15th into the 16th, as we move across midnight into morning time. If we were to pull back a couple of days from then, we would come to the Full Moon. The Full Moon this year is on the 13th of April, and it is more than bright enough to completely block out the glow of the Milky Way. Even without any light of the Sun creeping over the horizon, and at just about 4 o’clock there’s definitely no sunlight yet, but the light of the Moon is certainly enough to completely obscure the glow of the Milky Way. We can see that even when the Moon actually lines up with the galaxy, the effect isn’t even as strong. This is because it is later in April, and the Moon is just 77% full. It’s still over three-quarters full and we can see there’s still quite a lot of light coming from the Moon, but not as bright as a Full Moon. Of course, if we keep moving later into April, we can see that the Milky Way is clear of moonlight later in the month. It’s still completely above the horizon, and that gives us a period of time where the Milky Way is completely above the horizon before the glow of the Sun comes up in the east. We have a very definite, clear view of the Milky Way a little bit later in the month, early in the morning.

You may have noticed, as the Moon goes across the core of the Milky Way, that is almost the Moon going along the ecliptic. All of the planets pass along the ecliptic, and in Stellarium we can bring up the ecliptic line to compare with the planets and other features. The ecliptic is pretty much the Sun’s equator projected out into space. If we look towards the Sun with the atmosphere turned off, we see the planets on as well. Now that we’re later in April, we have a good number of planets either side of the Sun. On the east, or evening side there’s Jupiter, Uranus, and a little further east, Mars, all pretty close to being on the ecliptic. On the west, or morning, side we can see Mercury, Saturn, Venus, and the Moon, all reasonably close to being on the ecliptic as well. If we take a closer look we should have Neptune as well, even closer to being on the ecliptic. The ecliptic is roughly the Sun’s equator projected out from our perspective, so the Earth lines up perfectly with the ecliptic. We don’t line up perfectly with the Sun’s actual equator, the Earth’s orbit is a little inclined. The orbit of the Moon, of course, is a little different again, going around our equator rather than the Sun’s. By following the Moon, and the planets, we can see that the ecliptic line passes through the core of the galaxy.

Our galaxy, of course, is a collection of stars. If we turn first off, to the Andromeda galaxy, from here it looks a little bit fuzzy, it all kind of blends together here, but it is composed of trillions of stars all going around a common center. This is even easier to see with a grand design spiral galaxy like the Whirlpool Galaxy, which is way up in the direction of the Plough or the Big Dipper. There it can be seen even more clearly how there is swirling material going around a common center. These galaxies are of course other galaxies, outside of our own, but all of the stars that we can see in the sky, every single one of them is part of our galaxy. Being part of our galaxy means going around the black hole in the center, going around the core of the galaxy. That includes our Sun, our Sun orbits the black hole in the center of the galaxy, just like the planets orbit it. In the same way that we have the ecliptic line approximating the equator of the Sun, there is a galactic equator as well. Now, I’m not entirely sure if this is equivalent to the ecliptic, a projection from the Sun’s perspective, or if this is the actual galactic equator that the Sun’s orbit may be inclined with respect to. That is a only little difference, a very subtle difference, which doesn’t make any difference to what we’re talking about here. Essentially, the galactic equator plane is the line around which all of the stars we see in the galaxy orbit. Of course the same way that, Vesta for example, an asteroid in the asteroid belt, is orbiting around the Sun, but in a pretty inclined orbit with respect to the ecliptic, we can see there are plenty of stars who seem to be quite far from the ecliptic as well. They may be highly inclined, they’re orbiting around the same supermassive black hole, we’re all still orbiting roughly in the same direction.

With the ecliptic, based on our Sun, cutting through the core of the galaxy, you can tell that it doesn’t line up with the equator of the galaxy. This brings me to a tool that Stellarium offers called the angle measurement tool. I’ve been told that the angle at which the Milky Way and the ecliptic cross is about 60 degrees, and the narrower angle, the more acute angle of the intersection, does look like it could be around 60 degrees. It at least looks a little bit more than 45 degrees to me, but still much less than 90. The ecliptic and the galactic equator are perpendicular, but not at a perfect right angle. The opposite obtuse angle then filling out the other half of the circle, would be 120. There are two 60 degrees for 120, and that leaves behind 240 from the total circle of 360, so that would mean 120 degrees on either side. However, these are not the angles that the angle measuring tool measures.

When we measure the angles of objects in the sky, we’re measuring degrees of arc, distance across the sky. The angle measurement isn’t a default component of Stellarium, so it needs to be turned on. If you are using Stellarium, go into the configuration window, which is on the left hand panel with the wrench. Clicking the wrench brings up a menu, one tab of which is plugins. In the plugins tab, you’ll see the angle measure in the list, and it needs to be loaded at startup. Click the checkbox and then restart Stellarium, with Stellarium restarted, down in the bottom tray is the angle measure symbol where it wasn’t before, just next to the exoplanet button. What this tool does is it measures the distance between two points in the sky in degrees of arc. This gives a measurement like this, 9° 9′ 40.13”, which is 9 degrees, 9 minutes and 40.13 seconds of arc. That’s the important thing, that these are an arc measurement. I usually say minutes of arc and seconds of arc, especially with a number, but you can say arcminutes and arcseconds as well.

Just to give an example, measuring from the top of the Plough to the North Star, they are about 28 degrees apart in the sky. With a line drawn in the sky joining them, no matter what way we turn around, it doesn’t curve much. Maybe it gets a little bit curved, but that line isn’t going to get too curved regardless of how we turn around, because of the way it lines up with our coordinate system. Bringing up a grid to break up the sky, for example the altazimuth grid, the lines radiate from the North Star. Those lines that point to the north star, at least close to the North Star when we’re looking it, they are going to look pretty straight. A grid on the sky will show curved lines in some places, for example looking south there are some curved lines. Measuring from the heart of the scorpion, Antares, over to the bottom of Corvus, you can really see the curvature of the line. That’s because we are sort of looking at the inside of a curved surface. Now, of course, space is an infinite void, with great depth. However, one of the best ways to visualize the sky is as a curved dome over our heads. It doesn’t matter too much to us if Deneb is way farther away from us than Vega, we just want to know how far apart they are in the sky. About 23° 52′ 25.63″ or 23 degrees, 52 minutes, and 25.63 seconds seems to be how far apart they are. This doesn’t tell us that they’re a certain number of meters apart. As we zoom in, the distance between them seems to get bigger, and more distant stars will appear closer together in the sky. As we move around, the line between two stars may also appear curved, which would change the distance in a flat plane, but not on the curve. Either way, the angle measurement that we use in Stellarium isn’t going to help us measure the angle at which our ecliptic and the equator of the galaxy line up.

If we had the markings of the ecliptic and the galactic equator in the sky, the angles between them would of course be visible. It’s easier to visualize with the galaxy, so we’ll need to move out into the countryside. Of course, we won’t be able to see any part of the galaxy otherwise. In mid April of course the Moon is going to be in the way but that’s not too bad as it indicates where the ecliptic passes. Depending on where we measure, we will get different angular distances between the lines, even though they preserve the same angle. The angle measurement tool could measure 2 degrees, but that’s how far apart the lines are in the sky, with reference to the 360 degree sphere in which we live. For the degrees of arc of a circle, the intersection is definitely closer to 60 degrees than 2 degrees. Of course, this angle measure won’t tell us the exact answer, so someone else might have to tell me instead.

Hopefully you enjoyed this piece investigating those aspects of Stellarium and our sky. If you did enjoy it, please do like it, you can also subscribe to this website and my YouTube channel to see more. Thank you very much for watching and hopefully I’ll see you back here next time.