When I was little, I had a friend who was born on February 29, the “lamp day” that we add this month every four years. I remember that we used to tease him saying that he was only three years old. I have lost contact with him over the years, but I suppose that now he has enough of the joke.
And here, we are again at the dawn of our quadrennial exercise in timing: the Jeap Day 2024 is almost on us. A handful of traditions have been associated with it; One estimated that it was the only acceptable day for – Gasp! – Women to offer marriage with men. Some people like to treat this day as a free day that gives them time to catch up with something that they have long pushed back.
I think it's a pretty good idea because, after all, catching up, that's what Jeap Day-astronomically, that is to say.
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There are two basic time units that we use that are based on astronomical events. One is the day, the duration of the earth to turn once on its axis. The other is the year, the time required for the earth to complete an orbit of the sun. Although it seems simple, these two units are actually quite complicated. For example, the earth turns once in relation to what? You need a reference framework compared to which measure this movement.
For our daily life, we use the sun. The time required in the sun to the south to go to bed, then get up and reach the southern meridian is once again a solar day, which we define as 24 hours, or 86,400 seconds. It is actually the mean Solar Day, which uses the center of the sun disc as a reference point and is on average every day of the year. The use of an average value is useful for the chronometer because the earth moves at different speeds at different times of its orbit, which changes the exact duration of a special day.
There are also several different ways to measure the duration of the year. Our current calendar uses the tropical year, the time of the vernal equinox with the Vernal equinox, to take into account subtle effects such as precession. Otherwise, the date of the equinox would change slowly over the years, and finally the December solstice would occur in July, which would be terribly confusing.
A tropical year is 365,2,422 solar days of length. Because the rotation of the earth and the orbital period are in no way linked, they are not divided uniformly. We end up with this rest of 0.2422, and this is the key to jumping days.
If we start to measure the day and the year at the same time, at the end of a year, the land will have turned 365 times, plus an additional 0.242 on the path at the start of the new year. After four years, this increases to 0.9688 days – very close to a full day. We built an additional day in the year!
This was known even of ancient peoples, and when Julius César decided to change the base of the Roman calendar of the use of the moon in the sun, he also decreed that each fourth year, an additional day would be added to keep everything in synchronization. Congratulations! Have a nice jump day! This is technically called the inter -lap day, which is added to the calendar to synchronize it.
Except that mathematics do not work quite. By adding a whole day every four years, we add too much: after four years, we only have 0.9688, not 1.0 days. This difference is 0.0312 days, or about 45 minutes. This means that every four years we still have about three quarters of an hour to take into account. Over time, it will accumulate and the calendar will be turned off again.
Enter Pope Gregory XIII, who reformed the calendar again in 1582. He declared that each 100th year (to make it simple, the years ending with 00) not Be a year of jump, so no jump day would be added. 25 days ago over a century, so this method DELETE 25 x 0.0312 = 0.78 days, and the calendar synchronizes a little better in the long term, but again, not exactly.
Using this algorithm, every 100 years, the calendar will operate from 1 to 0.78 = 0.22 days behind. It adds too! So, as part of its papal bull, Pope Grégoire XIII also said that each 400th year would acquire once again a day of jump. Until then, there are 4 x 0.22 = 0.88 days, so the addition of a day decently brings us closer to catch up with the annual annual irritating earth ratio.
This is the rule we use now. Each fourth year, which means that each year whose number is uniformly divisible by 4, is a year of jump and receives an additional day-that is to say, except every 100 years, when we do not know the jump day, except For every 400 years, when we reversed the rule and add a jump day again. The years 1700, 1800 and 1900 were therefore not years of jump. The year 2000 was because even if it is uniformly divisible by 100, it is also uniformly divisible by 400. The year 2100 will not be a year of jump, but the year 2400 will be, and so on.
It actually brings us closer to synchronize. I sometimes wondered, however, why Pope Grégoire XIII did not use the duration of every 500 years instead of 400. It would be better because the amount remaining after 100 years is closer to a fifth of a day. But here we are.
For this reason, however, our current rules always leave the calendar a little extinguished. We add a whole day every 400 years, but it is too much by 1 to 0.88 = 0.12 day. If we wanted, we could again modify the rule and say that every 3,200 years, we don't do it Make this year a year of jump this year. Why 3,200? Well, 8 x 0.12 = 0.96, so that we can jump a year of jumping at each eighth cycle of 400 years, which is every 3,200 years. This would mean that the year and the day would only be underway 0.04 days – just in less than an hour – all three millennia, which is quite dangerous.
As usual, when they treat astronomy and figures and calendar, things seem simple – until they are not.
So, anyway, Happy Leap Day, and if you have something you have been postponing for four years, it's now a moment as good as anything to get there. And to my old friend Ted, if you are there and you see this: happy birthday!
