The Earth fully rotates in 23 hours and 56 minutes. Where do those extra four minutes go??
I know the answer is supposedly leap day, but I still don’t understand it from a daily time perspective.
I have to be up early for my job, which right now sucks because it’s dark out that early. So every day I’ve been checking my weather app to see when the sun is going to rise, and every day its a minute or two earlier because we’re coming out of winter. But how the heck does that work if there’s a missing four minutes every night?? Shouldn’t the sun be rising even earlier, or later? And how does it not add up to the point where noon is nighttime??
It hurts my head so much please help me understand.
In: Earth Science
Look up 1752, I think the year was? There’s a September where 11 whole days just *poof* vanished. VSauce did a video on this very subject.
It’s not leap day.
If the earth weren’t spinning at all, the sun would still move across the sky because the earth is going around the sun — a day would be the same length as a year. The sun would move about 1 degree East per day, so it’d be going backwards.
We spin the same direction we orbit the sun (counter clockwise if you’re looking down from the north pole), so in the time we’ve made a full rotation, we’ve also moved about 1/365^th of our orbit around the sun. That means the sun is about a degree “back” from where we’d expect it to be. So we need to rotate an extra degree, which takes about 4 minutes.
Or another way to look at it… In a year, we spin ~366.25 times, not 365.25. But moving around the sun “unwinds” one of those spins. That unwound day is spread across the days of the year. 1440 minutes in a day, split among 365 days, is about 4 minutes.
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There are plenty of other great replies in here, but I thought I’d add a gif that shows what others have already described really well:
https://imgflip.com/gif/3o07r3
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A sidereal year is 365.256363004 days. A tropical year is 365.256363004 days. A sidereal year is the time it takes for the Earth to return to the same position in relation to a certain star (the Point of Aries). A tropical year is 365.24219 days. A tropical year is the time it takes the mean position of sun to advance 360 degrees. This is what the calendar is based on.
Leap years are added to correct for the difference of the odd length of the year compared to an integer number of days. We add an extra day to the year every 4 years, unless the year number is evenly divisible by 100, then an extra day is not added, unless the year number is evenly divisible by 400, then an extra day is added after all.
Thus:
1996 was a leap year, it follows rule 1
1900 was not a leap year, it follows rule 2
2000 was a leap year, it follows rule 3
This system has been working for the last about 500 years. There is a small error but it will not amount to a full day until about 3800 AD.
Ok putting together some of these responses, I think the main idea is that we don’t define 24 hours as the time it takes for the earth to spin exactly 360 degrees, but instead as the time it takes for the sun to re-appear in the closest position in the sky from one day to the next. Therefore the 4 minute time difference is irrelevant to daily timekeeping with respect to the sun. (?)
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