Caoimhín's Content

Welcome to Seachtain na Gaeilge! Seachtain na Gaeilge, Irish Week or the Week of Irish, is a celebration of the Irish language in Ireland, and for a while now it has lasted a fortnight, from the 1st of March till the 14th. This gives me time to put up an few extra Irish videos, at least one each week. Given that Seachtain na Gaeilge has already grown in length, hopefully it will be a whole month eventually, and who knows, maybe one day it will last all year long. I’m certainly going to continue posting Irish videos all year, but they will most likely return to a monthly schedule after this month. For some of us, Irish is not just for March or Saint Patrick’s day.

March is also the month of the equinox, and so we will be looking at the Sun. I didn’t mention this in the video this time, but of course, never look directly at the Sun! In real life, you won’t have the handy grid that Stellarium provides to divide up the sky, so unless you have a very good sundial, figuring out what time the Sun is exactly in the middle of it’s path across the sky can be tricky. With the sky divided into degrees, we can see where the center should be, 0 degrees or -180 degrees. There are 360 degrees in a circle, so once we go 180 degrees past 180, we’re back at zero. Positioning the Sun directly on that line from here in Ireland, we might expect it to be midday. The Sun is at it’s highest in the South, halfway between rising in the East and setting in the West. However, looking at the time on the clock shows us that we’re closer to 12:45. Even if we push ahead to the equinox on the 20th of March, the Sun is still a little late getting to that center point, about 12:40.

The Sun is certainly higher on the equinox than it was on the winter solstice, and if we keep pushing ahead, the midday Sun will continue rising, until it reaches it’s highest point at midsummer. It is in the middle of the sky, measuring from East to West, but of course it isn’t quite at the zenith, it’s highest point still won’t be directly over our heads here in Ireland. As the midday Sun rises and falls across the year, if we keep our local time the same, it also shifts a little bit from side to side. If the Sun is on the center line for 12:40 in March, it will shift a little ahead of the line as we move towards the Summer Solstice. As we begin reach it, the Sun will shift back, lining up with the South at the time we expect. It will then continue to shift back as it sinks down towards the autumnal equinox, eventually crossing the line. It will then shift ahead of the line for a while, before shifting back to the position we’d expect for the Winter solstice. I’ll get back to this yearly shift further on, but the Sun still doesn’t line up with due South at exactly 12 for us here in Ireland, despite the shifting of it’s apparent position.

This is without making any major alterations to the time, or at least to the clocks. Some countries around the world practice Daylight Savings Time, and Ireland is, as of yet, still one of those countries. As we move from Winter into spring, the clocks are changed by an hour. This means that midsummer brings our Sun to the middle of the sky when our clocks say 1:40, a big difference from the 12 o’clock you might expect. As we move into autumn time and winter, the clocks are moved back by an hour, so that our midday Sun on the winter solstice is at the slightly less strange time of 12:40. This is not an actual physical change in time, it is purely a human thing. Us deciding to skip an hour makes no difference to the position of the Sun, and that’s something that can be easily seen in Stellarium. With the Sun at it’s highest point on midsummer’s day, with daylight savings in effect it’s about 1:40 according to the clock, with daylight savings turned off, the clock say’s 12:40, and the Sun stays in the exact same position, because the passage of time itself hasn’t changed at all.

This variation over the year helps to show the difference between what’s on our clocks and the actual (apparent) motion of the Sun across our sky. It doesn’t explain the 40 or so minute difference between our physical midday and the time on our clocks, but that too is down to a human decision. Due to the difference in the local time of midday, the Earth is divided into time zones. After all, midday in London is close to midnight in the Pacific, so using the same time would lead to some difficulties. However, any amount of distance that you move will cause a difference in local midday. A difference of a few hours could lead to problems, but a difference of just a few minutes isn’t as bad. Time zones could be a different size, but they are usually about an hour or so wide. For us here in Ireland, we are a bit further West than the center of our time zone, so our local midday is a little later than the time on the clock. In the very center of the time zone, everything should work as expected, and for us that means going to London. I gloss over this issue in the video, but the London Metropolitan area, the built up region around the River Thames, isn’t technically called the City of London. The City of London is the old city center, a collection of historical buildings and skyscrapers now surround by the urban sprawl of Greater London. The City of London puts us about 0.09 degrees West of the center of the time zone, and the center of all time zones, the Prime Meridian. This (imaginary) line splits the Earth from pole to pole, and runs through the Royal Observatory in Greenwich, pretty close to the O2 venue. At this location, the Sun would be directly above the South at midday, if it weren’t for that shifting it does over the course of the year. This is location that we get Greenwich Mean Time from, which is now usually called UTC or Coordinated Universal Time. You can blame the IAU for the acronym not matching the English, the same group that decided Pluto was no longer a planet.

The Prime Meridian is set in Greenwich for purely historical reasons, the center of international time could have been anywhere, all that would change is what we call the time zone. If the Prime Meridian ran through Cork, then it would be London’s physical midday that would be off by 40 minutes, just 40 minutes ahead rather than behind. In the modern world, connected by the internet, making sure that people are calling the same moment 12 o’clock in Cork and in London is more important than people doing things when the Sun is exactly at it’s highest point. The time that we use on our clock’s is almost always different from what the physical position of the Sun would tell us, and how much can vary a lot from place to place.

In fairness, it isn’t entirely our fault. Time zones can cause a big difference between the local midday and the official time of noon, but this does vary over the year, As we followed the Sun up and down the sky, from it’s lowest winter solstice point to it’s highest summer solstice point, I mentioned that the Sun drifts a little from side to side if we keep the time the same. The Sun rises and falls due to the tilt of the Earth, but the earth doesn’t orbit the Sun in a circle. Our elliptical orbit means that we’re moving a little faster around the Sun at certain times of the year. This causes the highest time of physical noun to vary a little from side to side, falling behind the clocks and then outpacing the clocks as we come closer and move further away in our orbit. this causes the Sun to trace a slightly wonky figure 8 shape in the sky. This phenomenon is known as analemma. In other videos I will look at why it seems so wonky, it is a very bottom heavy figure 8 as we see it here in Ireland.

Quickly going through the process here again, starting with the winter solstice, the Sun will start to lag behind where we expect as we move into January and approach our closest to the Sun. In late February this difference reaches it’s maximum, before the difference starts to decrease moving towards the equinox and becoming 0 some time in late April. This is no difference from where the Sun should be for local midday, in Ireland that would still be about 40 minutes late based on the time on the clock. The Sun then starts move ahead of where we expect, up until mid May. The difference will then get smaller as we approach the Summer solstice, with the Sun starting to lag behind of where we would expect again as we approach our farthest from the Sun in early July. As we come into august the difference will peak, before it starts to decrease again, with the Sun passing it’s expected midpoint on the way to the autumnal equinox, with the Sun being the Furthest ahead of where we would expect again around the first of November. The difference will gradually lessen as we come back to the winter solstice, and then it starts all over again. The shape of the analemma can very quite a lot depending on where you are, so this description of the shape will only work for mid to high northern latitudes. Luckily, I will cover more in a future video.

The word analemma, as I mention in the video, comes form Ancient Greek, apparently from word meaning “support”. As with many technical and scientific terms, especially in astronomy, the English language has borrowed this word from the Ancient Greeks, and Irish has done pretty much the same, using the word “anailéim”. It sounds a lot like the word in English, but really both are borrowings from Ancient Greek. Granted, Irish almost certainly got the word via English, as is true for many technical terms. We will continue to look at the motion of the Sun in the next few videos, including observing the analemma from other locations, such as the equator. If you enjoyed this little description about the Sun’s motion then I invite you to support me by subscribing to the website, the YouTube channel or Caoimhín’s Content on Instagram, but most importantly, by coming back next time to read more. Hopefully I’ll see you then.