Far out cosmology: why leap days (Introduction)

by David Turell , Saturday, March 02, 2024, 17:50 (264 days ago) @ David Turell

To always keep our seasons on schedule with sun's position:

https://bigthink.com/starts-with-a-bang/most-bizarre-facts-leap-day/?utm_campaign=swab&...

"without a leap day, the physics of planet Earth would quickly cause the seasons to move out of phase with our annual calendar, and the equinoxes and solstices would drift around the days, months and seasons. In fact, if we simply added a leap day to our calendars every four years without fail, things wouldn’t line up very well, either, which is precisely what happened for the centuries under which humanity followed the Julian calendar. Only if we properly account for our planet’s axial rotation and revolution around the Sun can we keep our calendar correct, and that’s what leap day is all about. Here are seven of the most bizarre, but still true, scientific facts that everyone should know.

"1.) Our calendar requires them, in part, because a “day” isn’t 24 hours long. Completing one 360° rotation is not the same as one day, as the Earth’s motion through space relative to the Sun means that rotating once on its axis actually leaves you a little bit “behind” where you needed to be.

"2.) A year is not defined by the Earth completing a revolution around the Sun. This means that by the time one sidereal year (a complete 360° revolution of Earth around the Sun) has passed, the Earth is now oriented slightly differently with respect to the Sun versus how its axis was pointed the sidereal year prior. Instead of a sidereal year, our calendar needs to be based on the tropical year, which differs from a sidereal year by about 20 minutes, with the tropical year being slightly shorter than a sidereal year. In other words, the day and the year both are not defined simply by a constant celestial motion, but also by changes, as the Earth-Sun distance and the relative Earth-Sun orientation are inconstant.

"3.) There aren’t an even number of days in the year, no matter how you measure it. You’ll find that there are 365.242188931 days in a true calendar year, to the best currently-known precision.

"4.) Our current, Gregorian calendar, complete with its prescription for leap days, isn’t quite perfect either. Under the Gregorian calendar, if your year ends in the numbers “00,” as in you’re experiencing the turn of the century, it’s only a leap year if that number is also divisible by 400. In other words, 2000 was a leap year, as was 1600 and as 2400 will be, but 1700, 1800, and 1900 weren’t, and 2100, 2200, and 2300 won’t be, either...Instead of 365.25 days in a year, which the Julian calendar gave us on average, the Gregorian calendar then gives us 365.2425 days in a year, which is much closer to the actual number of days we experience in a year. In fact, whereas the Julian calendar drifted by about a day with each 150 years that went by, the Gregorian calendar won’t drift by a full day until around 3200 years pass.

"5.) Earth’s orbit actually is changing, which means our current system of leap days will need to be revised down the road. Our moon doesn’t just exert a gravitational pull on the Earth, it pulls ever so slightly more on the part of Earth that’s closer to it than the part that’s in the center, and pulls even less on the part that’s farther away.

This creates a tidal bulge on the Earth, and as the Earth spins and the moon orbits around the Earth, those tidal forces from the moon cause two very small changes that occur together.

Earth’s rotation rate, ever so slightly, slows down. it turns out that this effect — known as tidal braking — causes Earth’s rotation rate to slow down by about 14 microseconds per year. This means that, even now, Earth’s Day is getting slightly longer. As time marches onward, we’ll need to add fewer and fewer leap days to keep our calendar in sync.

"6.) In about 4 million years, leap days will be unnecessary, and the calendar will contain exactly 365 days for a time. Tidal braking might be almost imperceptible on human timescales, but its effects — very importantly — are cumulative, meaning that they add up over time.

"7.) The long-term changes in leap days coincide with the loss of total solar eclipses. It’s difficult to believe, but the same tidal braking from our moon that causes Earth’s rate of rotation to gradually slow causes the moon to spiral away from the Earth: it’s a consequence of the law of conservation of angular momentum. Both “spinning” and “orbiting” are types of angular momentum, and as the Earth’s spin slows down owing to the moon’s tidal forces, it gets transferred in a fashion that increases the Earth-moon distance.

***

"For now, however, this leap year means we simply get an extra day for 2024, and that when 2096 rolls around, we’ll have enjoyed a long streak of every 4th year being a leap year since 1904. However, 1900 wasn’t a leap year and 2100 won’t be either, so to those of you with February 29th birthdays — an estimated 5.5 million of you — enjoy celebrating your special day once every four years until then. After that, you’ll have to wait for eight of them to go by between 2096 and 2104!"

Comment: quite a complicated mess, but not to worry. Each year buy a new calendar and you will be OK. Just remember Homo sapiens evolved on time, wherever it was on the measure of days of the year.