Caoimhín's Content

For today’s piece, we are going to look ahead a couple of days to the equinox. The day of the equinox will be next week on the 20th of March, and on the date of the equinox, the night and the day are of equal length. That is the meaning of equinox, from the Latin *aequinoctium*, the night or “nox” is equal or “aequam” to the length of the day.

Looking towards the east on the morning of the equinox, the Sun should come up exactly in the east. However, to the naked eye it looks like the Sun rises a little to the side of east. It takes a little extra time for the sun to rise from behind whatever obstructions are on the horizon. If we remove everything from that line so that we can see the ground itself, the Sun is still coming up a little to the north of east. This is because the atmosphere is bending the sunlight and due to that we’re able to see the Sun before the it is really above the ground. With the atmosphere removed, the Sun is almost cut in half by the ground, and if we put the atmosphere back the Sun jumps up above the ground again. Due to that, the Sun appears as though it is not coming straight up in the the east. It appears that the Sun is coming up a little to the north, but when we remove the atmosphere, it is really still below the ground. We have to wait a little bit longer for the Sun to come up directly in the east.

The Sun also goes down directly in the west for sunset, they are equal. If we go across to the other side of the sky, as the Sun rises up from the east it eventually reaches its midpoint. This gives us the Sun during the day, halfway up the sky at midday when the Sun is directly in the South. It will be at the point halfway between its lowest point at midday and its highest point at midday. These are the points that we get on the solstice, the winter solstice and the summer solstice respectively. Continuing through the day, the Sun begins to go down over in the west. If we wait until the Sun is right on the line, just about to set, then it will be directly in the west, as long as we’re looking without any atmosphere. When we turn the atmosphere on again, the Sun jumps up and we have to wait longer for the Sun to go down. That’s one of the key features of the equinox, the Sun reaching its highest directly in the middle of each extreme, and going from the east to the west right on a line, right to the correct points. This is related to the Sun shining on the equator. Going to the equator, I have to be a bit more accurate then usual. Usually, if we are going to the equator, a couple of minutes or seconds of arc to the side is fine. However to look at the equinox, and the Sun going up and down in the right places, we have to be exactly in the right place.

From right at the equator, firstly looking at the Sun setting, and even with the atmosphere it is exactly in the right place. Even with the small difference that the atmosphere makes, still the Sun is going down due west. If we come back to morning, the Sun will pass very high in the sky, as it does when you are at the equator. Looking back in the direction of morning, the Sun will go straight down exactly in the east. Again, the Sun is below ground a little bit before it appears to be, so if we remove the atmosphere, it’s almost completely below ground when it appears to be just setting. If we line up the Sun, with the atmosphere, so that the bottom of the Sun is just arriving at the ground there, then if we remove the atmosphere, it will be fully under the ground.

If we widen our field of view and look right up into the middle of the sky, we’re looking at the whole sky as if it was a flat circle. The Sun is going to come up from the east, go straight through the zenith and straight down in the west. The Sun going straight through the middle of the sky from the equator on the equinox, that’s truly the definition of the equinox. Bringing up a polar coordinate or equatorial grid on the sky, this creates a straight line going from east to west, the celestial equator. If we focus on the Sun through the day, it follows that line without any deviation, without moving from that line in the middle of the sky. The way in which the sky is cut up by the equatorial grid, we’re cutting up the sky from north to south, based on each pole. If we change the type of grid that we have on the sky, to a horizontal or alt-az (altitude-azimuth) grid, then the centre will be the zenith above us, rather than the poles. Using this, we can tell that the Sun is really exactly in the center, exactly at midday. We’re a couple of minutes to the side of noon, so the Sun isn’t exactly in the center, but it’s very hard to be very accurate with stuff like this. Regardless, that’s the Sun right in the middle of the sky atr midday as long as you are at the equator on the equinox.

Also, when we are going from northern winter to summer, the Sun passes from south of the zenith at midday at the equator, to north of it. When we are talking about this equinox, the top or the north side of the world, is going into summer, and the other side of the world, the south, is going into winter. We’re at least going in the direction of winter and summer, really spring and autumn still. Due to the atmosphere, on this day and a few days either side, the Sun is up for both poles, North and South. Taking a look at the equinox sky from the north pole, the Sun is above the horizon, with the atmosphere. If we remove the atmosphere, the Sun is only coming up, still partially below the ground. If we go straight down to the other side of the planet, straight down to the South pole, the Sun will appear to be in the same position, still up.

With Stellarium, clicking on the actual North or South pole is more or less impossible. As such, we need to type in 90 degrees and change everything else to zero’s. We have to be at 90 degrees 0 minutes or seconds, or really 90° 0′ 0.00” if we are very accurate about it. For South, this brings us straight down to the other side of the world and the Sun is still up. It’s practically touching the horizon, right on the level, but still up. This is due to the atmosphere, if we remove the atmosphere the Sun is already partially down. This is the Sun setting for the south pole, because the south side of the planet is going into winter. From exactly at the pole, the Sun is setting around the 21st/22nd and it won’t rise again for months. On the other side of the world, up on our side, at the north pole, the Sun is only coming up, the Sun is rising at this time. With the atmosphere turned on, from the North pole, the 19th is when the Sun appears to rise for the first time in months. The Sun is up for both sides of the planet at this time of the year, on the equinox, due to the atmosphere bending that light. If we push ahead on the north side of the planet, the Sun is only going to rise more and more until we hit the summer solstice. Then the Sun is going to start going down, but the Sun isn’t going to set again until we reach the next equinox, the autumn or September equinox.

That is the equinox, from Ireland, the equator and both poles, the four most important places on Earth I’m sure. I assume you will be able to see the equinox from your location. Whatever location you are in on the planet, the equinox will be on the 20th of March. All the same, now you know how the equinox will look from a few other places as well and I hope you enjoyed it. If you did enjoy it, make sure you like this piece and if you are interested in astronomy then make sure to subscribe to this website and my YouTube channel as well. Thanks a million for reading this piece, and hopefully I’ll see you back here next time.