Caoimhín's Content

In this video, we will finish dealing with the strange motion of the Sun in our sky by travelling to the equator and observing the analemma, from (almost) the sub-solar point. Of course, we will also be heading forward in time to the equinox, so while that happens at the beginning of the video we do take a quick look at the evening sky in March. The Moon and Jupiter have a reasonably close conjunction in the next few days, followed by the Moon moving quite close to the Pleiades. As you’ll see next week, it is technically possible to catch Mercury as the Sun sets, but once the sky is dark we are left with some familiar constellations. Orion is reasonable high and clear, but certainly past due South once darkness falls. Leo is still a little bit East of due South, with the Sickle of Leo coming close to the center of the sky as it gets later.

I don’t want spend too long before getting to the meat of the video, which is the behaviour of the analemma from the equator. We already looked at the analemma from Ireland in the previous Irish language video, but the shape looks even more distorted from high latitudes than it really is. In the video, I do take the time to get to exactly 0 degrees North and 0 degrees East, which is what they are called, despite exactly zero degrees being right in between North and South, and East and West. It is a little like the number 0 in most cases. It isn’t really a positive or negative number, but it does often start the number line. Even if we moved one second South of zero, we would be in the southern hemisphere, but 0 degrees latitude is usually called 0 degrees north, and similarly for longitude and 0 degrees East. Degrees of arc aren’t the most precise measurement, and I deal will deal with this further in the next video. Essentially, if you divide up the world in 64,800 sections, based on 360 degrees of longitude and 180 degrees of latitude, each of those sections will be roughly squares 111km long on each side at the equator. The shape and size varies as circles of longitude shrink as you approach the poles, but those sections are big no matter what. To be more precise, degrees are broken down into minutes and seconds, there are 60 minutes in a second and 60 seconds in a minute, just like with time. These measurements are often called arcminutes or arcseconds and they help to provide more precise information on location. I do make sure the view point here is exactly 0 degrees, 0 minutes and 0.00 seconds North and East, but in reality a few seconds or fractions of a second off isn’t going to make that much of a difference.

At this position on 0° 0′ 0.00” to use the proper notation, on the equinox, at almost exactly 12 o’clock, the Sun will be almost exactly at the zenith of the sky, directly above your head. This is the middle of the sky in the azimuthal grid used in the video, which is what we need for our purposes here, where as the equatorial grid uses the poles as the center.

Thanks to a little visual glitch in Stellarium, there is a tiny flaw in the exact middle of the zenith, which is useful here to make it very clear that the center of the Sun doesn’t quite line up with it. Even though we are at the exact center of the coordinate system, the Earth does have a slightly wobbling axial tilt, among other complicating factors, but luckily getting the Sun to the exact center isn’t the focus of this video. We’re here to see the Sun trace a figure eight shape across the sky. Looking at it sideways it could of course look like an infinity symbol, and it is a little longer and thinner than most 8’s. That shape or symbol is a leminscate, which describes the shape even when it doesn’t mean infinity.

Moving through the year, we can see the Sun moving up and down across this central point, moving from it’s furthest North in the sky to it’s furthest South in the sky. With a close enough look, you can see that the Sun’s motion isn’t straight, but curves from side to side as it drifts to and fro. It still doesn’t trace a perfect symmetrical figure eight in the sky, it’s a little tilted and a little bigger on one side, but that is as close to it’s true shape as we can get, without any distortion coming from our latitude. The lopsidedness is partly to do with the shape of the Earth’s orbit. As the Earth’s orbit is elliptical, the Earth spends some time closer to the Sun and time further away. This influences the speed of the Earth, as the orbit is quicker closer to the Sun, at perihelion in January. During the period of time when the Earth is further away, towards aphelion in July, the Earth is moving slower. It is a tricky phenomenon to capture, especially on a screen significantly smaller than the sky, so in the video I go through a couple of years, both with the normal blue sky and an atmosphere and starless view, showing only the Sun, Moon and planets. The slightly lopsided nature is the tricky part to see, the fact that the Sun is not simply moving up and down is usually pretty clear, and of course even that has much lesser of an effect than the simple changes in height. Going through the years, you’ll see that the Sun doesn’t reach the same place in the sky during the autumnal equinox as it does during the vernal equinox, or at least not at the the same time. This means the crossing point or waist of the figure eight shape is a little “higher” than the center of the sky. I say “higher”, but really at the equator, it’s just off center.

At the equator, it is the equinoxes when the Sun reaches it’s highest in the sky, practically the exact center. This means that the Sun can’t be at it’s highest during the summer, northern or southern, because the Sun moves away from the zenith at midday towards those dates. For the equator, the winter and summer solstices bring the Sun it’s furthest from the center, you could call either point the lowest, and it is of course still very high compared to Ireland, and indeed many parts of the world. Thanks to the variation, the areas close to the equator can experience the Sun directly overhead as well, away from the equinoxes, which is one of the reasons that delving into that topic will need to wait for another video.

Given that the equator experiences much less seasonal temperature variation, generally hot pretty constantly throughout the year, dividing the year into summer and winter often doesn’t make as much sense. Many areas have a rainy season, but in some areas, such as the drier areas near the equator, the yearly variation in the weather can be pretty minimal. This may be one of the reasons that lunar calendars are used or were used in many parts of the world, particularly in cases when they are purely based on the Moon. There are several calendars that are mixed, lunisolar calendars, and just using the Sun to measure the year as we commonly do is using a solar calendar. The lunar cycle of 29.5 days doesn’t line up with the solar year, leading to multiple Full Moons in one calendar month, or four in one season (a Blue Moon). In the same way, midsummer and midwinter would drift relative to a calendar based purely on the Moon. This wouldn’t make much difference in location where seasonal variation isn’t too significant. This is why many religious festivals, such as those from the lunar Islamic calendar and lunisolar Hebrew calendar, change date from year to year. To be accurate, they change date based on the Gregorian calendar every year, and are a lot more regular in their original system.

As with almost all my videos, I do on a few tangents, and in this case it is thanks to a comet. As we move backwards, after looking at a few years worth of analemmas in both directions, a comet appears quite close to the Sun in April. This comet, 12P/Pons-Brooks, is visible with the right equipment right now, but I will discuss in in a later video. Also in April is the upcoming eclipse on the 8th of that month, visible from the Pacific and North America, along with some of South America. I’ll take a closer look at that little closer to April as well, but they are a couple of interesting topics coming up.

This is the second Irish language video for Seachtain na Gaeilge, which is really a fortnight, so one a week. there may be another Irish show this month anyway, given that we’re so close to Saint Patrick’s Day, I don’t need that much of an excuse to speak Irish. However, the next video will be in English, and it will talk about some of the things covered in the past two Irish videos, but as always things will be a little different. It you liked this description of viewing the analemma, you can subscribe to this website, my YouTube channel and/or Caoimhín’s Content on Instagram, whatever you would prefer. Hopefully, I’ll see you back here next time.