Caoimhín's Content

Today we are taking a look forward to the 23rd of September, the Equinox. The equinox is the date in the year where we transition from wintertime to summertime. Instead of the Sun shining more on the north or more on the south, the Sun is shining directly on the equator. The the term equinox comes from equal night, an equal length of night and day. However, if we take a look at our length of daytime for this date, it’s 12 hours and 9 minutes here in Ireland. 12 hours and 9 minutes is bigger than half of 24 hours, it’s bigger than 12 hours, so something is a little bit wrong. The equinox is the day where we transition from summer to winter, but if we move forward, just a couple of days to the 25th, then the day is only 12 hours and one minute, much more close to equal with the night.

If we move back, of course, we’ll get much higher numbers. We get up to about 16 and 3/4 hours in the day once we’re at the summer solstice and if we moved in the opposite direction, we’d come down to just about 7 and 3/4 hours in the day by the time we’re through to the winter solstice. We’re in the middle in terms of the date, but 12 hours and 9 minutes is not exactly 12 hours. The reason is the atmosphere. There’s two equinoxes a year and they both result in the day length being approximately, but a little over, 12 hours. We’re going to use a perfectly flat horizon so we can get a closer look at the Sun just as it is rising, without worrying about the trees and the buildings getting in the way. A zero horizon, perfectly flat, is unlike the view you’d get from almost anywhere on Earth. If we take a closer look at the sunrise, we should see the Sun rising pretty much due east, and it is. The Sun pretty much rising due east. It’s a little bit off east, but of course we’re not seeing the exact equinox here in Ireland, it’s very similar to the exact Full Moon. Often we only see a 99% Full Moon, as the Moon is only 100% for a short period of time and as such is only visible from some locations.

The sunrise is a little bit off the east on the 23rd. In fact, we are a little bit closer to due east here on the 24th. The Sun comes up from due east on the 24th, and it doesn’t look like it’s exactly due east on the 23rd. What we’re going to do is we’ll get the Sun directly above the east on the 23rd and we will remove the atmosphere. This causes the Sun’s apparent position to drop, leaving it rising directly in the east, exactly in the east. With the atmosphere on, the Sun rises, or appears to rise, before it actually has. With no atmosphere and the Sun just below the horizon, only a little bit of the Sun’s corona is visible. The Sun is definitely not above the horizon. However, if we bring back the atmosphere, it is half risen at least. That difference, that extra bit of light that the atmosphere refracts, makes the Sun appear to be higher in the sky than it actually is, that’s enough to give us a couple of minutes of extra light. That brings or equinox up by 9 minutes, as the same effect occurs at sunset, with the Sun appearing to be higher than it is and therefore up for longer.

We will very quickly jump down to the equator so that I can show that this is the date where the Sun is crossing the equator. Of course, I don’t need to change my longitude to get to the equator, I only have to change my latitude. I will change both, to bring the viewpoint to the Null Island, which is just off the coast of Africa and doesn’t exist. The Null Island isn’t actually a landmass, but it is marked by a buoy that is just off the coast of Africa. The exact zero point, 0 degrees in latitude and longitude, is just off West Africa in the Gulf of Guinea. Using the equatorial grid in Stellarium brings up a grid on the sky centered at the zenith. The Sun is right in the middle, almost exactly right in the middle, at midday on this date. If we move back in this case, we can see that the Sun moves more to the north. If we move forward, the Sun drops more to the south.

At midday on the equator, the Sun is only directly overhead on the equinox and the reason we’re not seeing the Sun exactly directly overhead here is because the equinox, just like the eclipse, just like the Full Moon, it does happen over a specific location. By changing our location a little, we can get a little bit closer to the Sun being exactly in the middle. That does put us a little bit off the equator, just a tiny bit, to get the Sun directly above us on this date at this time. Again, if you were in the exact right location, the Sun would be directly above your head on the equinox, but because we’re not in the exact right location, we have to be a little bit off the equator to get the Sun directly above our heads. Most importantly, even though the Sun is a little off the exact zenith, it is further off a day before or after, showing that the Sun has crossed the zenith at the equator, somewhere, on this date.

If we zoom out to see more of the sky, the Sun makes a sort of figure 8 shape as we move from solstice to solstice through the equinoxes. Moving forward to southern summer, the Sun drops down from the zenith at midday, but it also moves to one side, further west. At its lowest, it drifts back east, coming around and then back up. The Sun doesn’t directly move up and down in our sky, because we are also moving closer to and further from the Sun over the course of the year. The Sun’s path at over successive local noons describes a loop known as the analemma. That does complicate things a little bit. As we move through years, the Sun loops a little bit to the north and then back down to the south. It’s not going straight up and down. There is a curve to the apparent motion of the Sun, and again, this is apparent motion. The Sun isn’t really moving in the sky. We are both turning and orbiting around it.

I mentioned in a recent piece that the terminator of the Earth is a little bit tilted when it is not the equinox. If we’re closer to the winter or summer solstice, then the terminator won’t be straight up and down on the Earth. During the equinox, it should be. I wanted to check the view of the terminator from the Moon, but for some reason Stellarium had an issue with that. Luckily, the view from the Sun will do. Looking at the Earth from the Sun on the Equinox, you can to tell that we are looking pretty much straight down on the equator. If we move through time a little bit the Earth will rotate, bringing the Null Island into position directly under the Sun. Again, this is just off the coast of Africa. If we move through the year, of course, we’ll see the tilt changing. As we come back towards midsummer we’re seeing a lot more of the North Pole. The center of our view is very much North Africa, so that is where the Sun would be overhead. If we move forward, we’ll come back to our current equinox or the equinox that is coming up very soon. From that pretty much equal view we’ll keep moving forward, until Antarctica is coming into view. As we come up to our winter solstice we’re seeing a lot more of Antarctica and the South Pole. What looks to be directly underneath us is Southern Africa. I won’t say exactly South Africa, but Southern Africa, around the latitude of Madagascar.

That is the Earth’s apparent tilt and we can see how that would affect our view of the terminator. So coming back to our equinox, the line that divides daytime from nighttime on the Earth here is pretty much going straight through the North Pole and straight through the South Pole. It is following the line of longitude between London and that zero point off the coast of Africa. However, when we are at the winter solstice or the summer solstice, the Earth is tilted, so the terminator doesn’t follow that line. It instead crosses the Earth at much more of an angle. Coming back to our most recent solstice, if we get things lined up just right, the line of shadow will still pass through London. However, by the time the line of shadow is actually passing through London, the terminator is so tilted that it’s almost hitting the bottom of South America, with africa mostly already in darkness. That is because of the tilt of the Earth.

I hope you enjoyed this description of the Earth tilting. Of course, this is something we can really only see in simulation, unless certain satellites point in particular directions. I hope you enjoyed this piece overall, and if you did then please do like it. If you enjoy this kind of content, then please subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.