Caoimhín's Content

The analemma has been the subject of the past two Irish language videos so if you’ve been reading along here, then you already know that it is a figure eight shape traced out by the Sun over the course of the year. We’re going to take a further look at this motion, from Ireland and the equator here. I will of course, quickly describe what will be visible in the sky tonight as well. As the Sun is sets on the equinox here on the 20th of March, the Moon will be just a few days from its fullest, with the bright planet Jupiter high to the West above the sunset. Much lower to the West is Mercury, a very tricky planet to spot. It is quite bright, though not as bright as Jupiter and the brightest stars, but it is very low in the sky, so a clear view of the western horizon is more or less essential to be in with a chance of seeing the smallest planet. Jupiter is still more or less at the end of Taurus, a bit West and below the Pleiades, while the Moon will be moving into the start of Leo. This leaves us with Gemini high to the South, and Orion a little to the West of South and a little lower in the sky. All of this will be visible nice and early, with the sky being mostly dark by 8:30.

Of course the analemma is the motion of the Sun during the day, so we will be focusing on daytime for this show. I will be breaking up the sky based on the azimuthal grid. This grid uses the point right above your head, the zenith, as the middle of the sky. As you change location, as we will be doing, the zenith changes as well. This will help keep our view consistent when we move from Ireland to the equator. The main alternative is the equatorial grid, which uses the poles as the centers. This means the center of the grid will vary up and down as we change latitude, and it can look a little odd at the equator itself. With the azimuthal grid projected on the sky, we can see that the Sun is lining up with the middle of the sky at about 12:40. I’ve explained in a previous video that we are close to the edge of our time zone, so the Sun reaches it’s highest a little later here than it would in Greenwich in London, the middle of our time zone. This gives our time zone, the one all others are measured from, the name Greenwich Mean Time, but this time is also called UTC or Universal Coordinated Time, as it is used all over the world, especially in computing. The International Astronomical Union decided to set the acronym at UTC, otherwise English speakers might say UCT while French speakers might say TUC for “temps universel coordonné”, which could get confusing.

The equinox is a handy place to start for looking at the analemma, as the Sun is right in the middle of it’s journey up and down between it’s highest and it’s lowest. In Ireland, that doesn’t look particularly dramatic. As we move forward in time towards summer, the Sun rises up into the sky, but also noticeably drifts to the West of the South. As it continues to rise it begins to turn back towards the central line, and it is close to it on the Summer Solstice. The Sun will briefly move to the East as it begins to drift back down, but will quickly curve back, cross over it’s own path, end end up noticeably towards the West for the September or Autumnal equinox. It will continue dropping down, reaching its lowest for the winter solstice, still a little West of the center, before continuing to cross over and begin rising back up towards the March or Vernal Equinox. I go through this path first in a normal view of the sky, and then with a very artificial view of the sky. With no atmosphere, we can see the Sun, stars and planets during the day, which fills our sky with objects. However, Stellarium also lets you remove the stars, letting us concentrate on the Sun. When the Sun reaches it’s lowest here in Ireland, it has to shine through a lot of atmosphere, just like any object close to the horizon, and this can make it hard to follow. With no atmosphere it gets significantly easier to track the Sun as it bottoms out.

I’m starting with the Spring Equinox’s midday Sun at the center point of the sky, but this happens earlier than the Autumn Equinox, and a little earlier than the solstices as well. If we started with the other equinox centered, we’d see the Sun peaking, or troughing, a little earlier on the solstices and significantly earlier on the Spring Equinox. If we used one of the solstices to center the Sun, then both equinoxes would have the Sun either side of center, lagging a little behind or reaching a little ahead of midday on the solstices. This slightly lopsided figure eight shape means that sundials run a little fast or a little slow at different times of the year. Some sundials have a little graph attached to them, known as the equation of time, which will tell you when to add or deduct time and how much to get a more accurate reading. The speed of our rotation doesn’t change over the year, but the speed of our orbit does, and that effects the speed at which the Sun moves in our sky. When we are closest to the Sun in January, Northern Winter, the Earth orbits a little quicker. When we are far away at the opposite end in July, we orbit a little slower. This variation is causes the bulk of the analemma’s side to side motion, while the up and down portion is of course due to our tilt, the same thing that causes the seasons. Looking at the sky with no atmosphere as we do in the video, lets us see a comet! the comet 12P/Pons-Brooks has been visible through telescopes for a while, but will be between Jupiter and the Sun in the sky during the eclipse on April 8th, visible from North America.

Travelling to the equator to get a slightly different view, we are going to go exactly to the equator and prime meridian, just like in the last Irish language video. One degree of latitude or longitude is about 111km long at the equator, which is a reasonably big area. To be more precise, every degree of a circle or arc can be broken into 60 minutes of arc or arcminutes, and those can be broken into 60 seconds of arc or arcseconds. An arcsecond is only about 30m here on Earth, so that lets us be much more precise. We’re going to exactly the equator, or 0 degrees North, and exactly the prime meridian at 0 degrees East. These are equally 0 degrees South and 0 degrees West, but we don’t usually refer to it that way.

Exactly at the equator, the Sun reaches the zenith on the equinoxes. At the right time, the Sun will be almost exactly overhead. I say almost exactly because it is very similar to a conjunction, with the Sun smoothly changing position over time, making the exact moment where the Sun is exactly at the zenith occur only for a certain point on the equator, with every other point being just very very close, certainly close enough. We’ll also see that on the Spring equinox, the Sun reaches this peak at about 12:07, and we are exactly in line with Greenwich here, the physical midday is late. When we push through to the Autumn equinox, the Sun reaches the zenith earlier, close to 11:50, the physical midday is early. This is a consequence of the analemma, the Sun will only be exactly on the North-South axis at 12 when the analemma crosses it, once near the winter solstice, once near the Summer solstice, and twice more between the summer solstice and the equinoxes. Like any figure eight, the analemma has a waist or point where it crosses itself, and this point occurs while the Sun moves northward between the March Equinox and June Solstice, and again as the Sun sinks from the solstice towards the September equinox. This does mean that the figure eight remains a little lopsided, one loop, the top or northern summer loop, remains a little smaller than the bottom, or northern winter, loop.

As we move from solstice to solstice at the equator, you should see that the Sun isn’t at it’s highest for either of them. The Sun will be lower than the zenith for any date that is not the equinoxes at the equator, so the Sun is at it’s lowest for both solstices in the year. This low is of course very high, higher than the midsummer Sun gets for many higher latitudes. This also limits the variation in temperature throughout the year compared to other locations on Earth. Our temperature change is almost exclusively down to the tilt in our axis, hence northern winter occurring when we are at our closest to the Sun. With the Sun having such a minimal impact, many areas near the equator are described as seasonless, though some still have a wet season versus a dry season. In high latitudes, the seasons can be very important, so knowing exactly where we are around the Sun is important. This is one of the reasons our calendar is based on the Sun, one year is the same as one orbit, it is a solar calendar. IN parts of the world where this is less important, it is practical to split up the year based on the Moon instead. We get 12 lunar months about every 355 days, and if you use that to break up you year, then the Full Moon will always occur on the same dates every year. You can easily tell what part of the month you are in by looking a the phases of the Moon, and by counting Full Moons you can track how many months have passed. That kind of calendar is a lunar calendar, and it is still used in some cultures today, although the solar calendar is often used as well. By mixing the two, you get a lunisolar calendar. Lunar calendars fall out of sync with the Sun, the same way a solar calendar will fall out of sync with the Moon. In a solar calendar, the winter solstice is always around the 21st of December, but that says nothing about the phase of the Moon, it will vary based on solar year. In the same way, in a lunar calendar, you might always know the date of the last Full Moon of the year, but whether that happens in summer or winter will vary on a long cycle.

Many famous calendars, such as the Islamic lunar calendar and the Hebrew lunisolar calendar, mark holidays whose dates vary based on the solar calendar we use, but of course they are regular if you measure by the Moon instead. They may end up being the topic of a future video all on their own. With all that, I believe I have the what and why of the analemma covered, and from two “where”s to boot! There may yet be another Irish language video this month, even though Seachtaine na Gaeilge is drawing to an end. If you enjoyed this little article then I encourage you to subscribe to the website and my YouTube channel, I have something new twice a week, every Thursday and Tuesday. You can also catch special notices on my Instagram, under Caoimhín’s Content, as I am every where. Hopefully, I’ll see you back here soon.