Same for the years. If a year actually is 365.24219 days (tropical year) and we’re adding 1 day every 4 years (.25 per year) there’s a difference of 0.00781 days or ~11 minutes per year. After a few years, aren’t we actually shifting hours? Is there a mechanism to adjust it?
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A sidereal day is 23 hours and 56 minutes. This is the time it take for the Earth to rotate 360°.
However, we don’t use sidereal days for our normal time keeping; we use solar day. A solar day is when the sun returns in the same place in the sky on the next day, which takes on average 24 hours. It takes a bit longer than a full rotation because of the Earth orbits around the sun as well. After the Earth rotates 360°, it is no longer is the same spot it its orbit. [It needs to rotate just a bit more in order to realign the sun](http://astronomy.swin.edu.au/cms/cpg15x/albums/userpics/solarday2.jpg).
So, we don’t lose those four minutes a day because don’t use sidereal days.
Leap years compensate for how long it takes to revolve around the sun. It has nothing to do with revolving on our own axis. For leap years, years divisible by 100 but not 400 are not leap years. For example, 2000 was a leap year because it is divisible by 100, and 400. 1900 was not leap year because it is not divisible by 400, neither will the year 2100 be. So, every 400 years, we skip 3 leap years.
You’re right that adding a leap day every four years adds ever-so-slightly too much time. This is accounted for by skipping the leap day in years that are multiples of 100. That’s a lot closer, but then we’re actually adding slightly too little time, so if the year is multiple of 400, then we do have a leap year. At that point, we’re close enough for it not to matter at all.
23 hours and 56 minutes is how long it takes the earth to revolve around itself relative to the stars. However, during that time earth also moved around the sun, approximately 1/365 of a revolution. Because of this, it takes the Earth approximately 4 more minutes to rotate to the same position relative to the sun.
Um. Yes…. this is what Leap Years are for. Every 4 years we add an extra day, February 29.
edit: Except there’s some junk about having to leave a few leap years every hundred out otherwise we’d end up out of synch that way too. So we skip like 3 every few hundred or something. There’s a formula for it.
> If a year actually is 365.24219 days (tropical year) and we’re adding 1 day every 4 years (.25 per year) there’s a difference of 0.00781 days or ~11 minutes per year
Yes, which is why every 100 years we don’t add a year on the leap year. So 1800 and 1900 weren’t leap years. But that means we aren’t adding enough days, so every 1000 years we don’t skip leap year, so 2000 was a leap year.
After that I think it is every 50,000 years you do need to skip the leap year on the 1,000. So the year 50,000 will not be a leap year, even it is a 4 year to be a leap year, and a 100 to be a skipped leap year, and a 1,000 to be an unobserved skip of a leap year. We will skip it.
Really what it comes down to is that the rotation of the Earth (length of a day) has nothing do to with the orbit of the Earth (length of a year). So we have weird clocks and calendars to try to compensate.
The day with reference to the sun is 24 hours. This used to be used in the exact definition of hours, minutes, and seconds, but since the Earth’s rotation is very gradually slowing over millions of years, more precise definitions have been created that rely on things like atomic vibtations. So it is no longer exactly 24 hours, just extremely close to it.
Since the earth revolves around the sun, the relative position of the sun in the sky changes by about 0.998° per day.
A *Sidereal Day* is 23 hours, 56 minutes, 4.0905 seconds. This is the amount of time it takes the earth to rotate exactly 360 degrees. This is the day with reference to the background of stars.
If you do the math and add this up, it works out to be 365.256 days, minus about 3 seconds. The former figure is accounted for by leap days.
The latter number of seconds requires an additional leap day every 26,000 years.
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