Caoimhín's Content

Today we are going to be taking a quick look at the equinox. Of course the equinox is the date where the length of the day and the length of the night are equal and the equinox is occurring on the 20th of March. The equinox occurs on the 20th of March pretty much every year, despite the fact that leap years can change when equinoxes happen, there isn’t really much variation in the date of the March equinox. In the northern hemisphere the March equinox is the vernal equinox or the spring equinox, but for the southern hemisphere, it is the autumnal equinox, the autumn or fall equivalent.

Firstly, we will look at the sunset on the night of the equinox. The sky will be starting to get dark at 7 o’clock, with a couple of bright objects already visible, Venus and Jupiter. These are the two brightest planets in the sky and the two brightest objects besides the Moon and the Sun. The Moon will be up just next to Venus, although it will be just waxing, it is going to be a pretty narrow waxing crescent. Taking a look at the percentage of illumination, it’s at just 3.8%, so the Moon is barely illuminated. Moving further into darkness, Sirius will be the next object to appear. As the brightest star, it will be the second type of object to come out, after the planets Jupiter and Venus. Soon after Orion will be coming into view, already over towards the southwest as it emerges. There is still plenty of orange-yellow glow in the sky as we come up to 8 o’clock. We’ll be a little past 8 o’clock, about 8:15, before the sky truly gets dark. By then Gemini and Procyon are due south, with the twins of Gemini above Procyon. Leo will be completely above the horizon as well, making it nice and easy to see in the south-east. Further east and higher in the sky, we can already follow the handle of the Plough, the arc of the Plough, to Arcturus, but we will need to wait until a little bit later before Virgo rises into the sky. Most of Virgo, most importantly Spica, wll be up by a bit before 11.

By then, we’re coming close to the end of the equinox, or coming close to the 24 hour date that we’re calling the equinox. The equinox is similar to an eclipse or an occultation in that the exact moment it occurs may not be visible from everywhere. Of course, the exact time when it occurs is going to be a time for everywhere on Earth, but the Sun may not be up. The exact moment that the equinox occurs is a specific moment, it is a specific time. We’ll get to it in just a second, but before we do, we’ll look at sunrise on the equinox.

By just 5:30 we’re already getting a little bit of an orange glow in the sky from sunrise. We’ll come back a little bit earlier, a little closer to 5 o’clock to keep the sky dark. We’ve got the summer triangle completely above the horizon, nice and easy to see and we’ve got Antares in Scorpius down towards the south just as the Sun is rising. If we head out to the countryside, some of you will already know where this is going, but the glow of the Milky Way should be particularly visible. It stretches from the south down with Sagittarius and Scorpius, up across the sky through the summer triangle, then past Cassiopeia and down the other side. The Milky Way is visible very early in the morning, around 5 o’clock, if you are in a perfectly dark sky in the countryside. For example, if you’re down in the dark sky park on the Iveragh Peninsula, which is having its dark sky festival this weekend, beginning on the equinox. If any of you are in Ireland, that is something you can look forward to. We’ll continue to move forward and allow the Sun to rise for the equinox. As I’ve mentioned in the piece posted last week, the equinox is the date when the Sun rises directly in the east and sets directly in the west, but it doesn’t look like it, it looks to be a little bit off. That optical illusion is explained in the previous piece and the attached Irish Language video.

We’re going to move forward to 2:45, because this is the moment of the equinox, this is the moment where the Sun shines directly on the Earth’s equator this year. The actual moment of the equinox is really when the Sun shines directly on the Earth’s equator. This is when the Sun’s position in the sky at midday crosses from the southern celestial hemisphere to the northern celestial hemisphere as viewed from the equator. That’s really the moment of the equinox, even though the equinox is thought of as a whole night and day equal in length to each other, there are various definitions for the equinox and the more exact scientific definition is the crossing of the Sun from one celestial hemisphere to the other. It doesn’t happen at midday for Ireland, that means that, because the equinox is really happening almost two and a half hours after our local midday, that means that the 24 hour span of the equinox is really lasting until a little after midnight on the 20th and it starts a little bit after, a couple of hours after, midnight on the 19th. We have to wait until we’re a few hours into the 20th before what we define as the equinox truly begins.

If look at some of the details regarding the Sun, we will get our rising and setting times. The Sun will be rising at 6 hours and 37 minutes, setting at 18 hours and 47 minutes. This gives us 12 hours and 10 minutes, not exactly the 12 hours we would expect. If we move forward to the 21st, the times deviate further, the sunrise is earlier and the sunset is later. If we move back, we can see the opposite effect. The timing of sunset and sunrise is definitely closest to 12 hours on the equinox, but we’re not in the exact right location to catch it. Not at the peak of the equinox, certainly not catching the peak of the equinox at midday. We need to be in a time zone that is a couple of hours behind where we are now, so that the Sun doesn’t reach its midday (12) until it’s already 2:45 here in Ireland. That means we need to be ahead on the planet further forward in its rotation. Hopping over to Greenland, that jump is a tiny bit too big, giving a 3 hour and 10 minute difference in local time. Coming back a little bit towards Iceland gives us a difference of 2 hours 31. It looks like only a very small patch of land close to the equator will be using that time, the very eastern edge of Brazil. If we move things around a little bit in that location, we can get to a local time of UTC -2:46, just about a minute off.

This is a tiny bit off exactly where we want to be, and we are a little bit off the equator as well, but we will correct that momentarily. This is giving us a day starting at 6 hours 4 minutes, ending at 18 hours 11 minutes. So that’s giving us a little bit closer to exactly 12, but it’s still a little bit off. We will move to the equator to begin just to make sure we’re in the exact right place. At the equator, we’re still getting the same length of day, 12 hours and 7 minutes in total for the Sun to cross over the sky from one horizon to the other. That’s really about as close to the 12 hours night and 12 hours day that we’re going to get because the Sun appears to rise before it actually has risen. This is down to atmospheric distortion and I go into more detail about it in the previous piece. This is almost exactly the right place at almost exactly the right time to see the Sun as the equinox is occurring. Coming up to midday, the Sun at its midday height is exactly where it should be, at the zenith from the equator. We can test that by bringing up a grid in Stellarium to check. We are a tiny bit off exact midday, we are just at 11:58 rather than exactly at 12, and because we are at UTC -2:46 instead of -2:45, that may put us a couple of seconds off as well. Either way, just as we come up to 12:07, the Sun reaches the exact middle of the sky, exactly at the zenith. That’s the real moment of the equinox, perfectly visible from roughly the Southern Caribbean.

So that is a little bit about the equinox, a little bit about what you can see at the equinox at sunset and sunrise, but also a little bit about the actual moment of the equinox. For us here in Ireland that’s at 2:45, but if you are in the right time zone, it’ll be at your midday. I hope that you enjoyed this piece, I hope that you found it interesting. If you’d like to learn more about the equinox, you can check out the previous post and the Irish language video on the Caoimhín’s Content as Gaeilge YouTube channel. It is subtitled in English so you can still enjoy it if you are not an Irish speaker. If you did enjoy this piece, 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.