we add a day every 4 years, because a year is 365.25 days long.

now that`s fine, But it`s actualy 365.2564!

so how come we don`t add 3 days every 500 years?

or 1 day every 166 years etc...

we add a day every 4 years, because a year is 365.25 days long.

now that`s fine, But it`s actualy 365.2564!

so how come we don`t add 3 days every 500 years?

or 1 day every 166 years etc...

 

That's the sidereal year. The tropical year is ~365.2422 days (time between e.g. vernal equinoxes). They are different because of precession. You skip adding the leap day if the year is divisible by 100, EXCEPT if it's divisible by 400. Which is why 2000 was a leap year. 2100 will not be one.

 

"[T]he average Gregorian calendar year is 365.2425 days in length. This agrees to within a half a minute of the length of the tropical year. It will take about 3300 years before the Gregorian calendar is as much as one day out of step with the seasons." (from http://aa.usno.navy.mil/faq/docs/leap_years.html )

wow! fantastic answer, Thank You!

 

I gotta ask though, a Tropical year? what`s that?

The sidereal year measures how long it takes for the sun to return to the same apparent location against the "fixed" stars. We use a calendar to mark seasons. Summer would eventually occur in December if we used the sidereal year as the measure of a "year".

 

The driving factor that determines whether it is winter, spring, summer, or fall is the orientation of the Earth's rotational axis with respect to the vector from the Sun to the Earth. A tropical year marks how long it takes for the Sun to return to the same apparent orientation with respect to the Earth's rotational axis.

Now I`m with ya, Thanks

Also, much more precise corrections are made by scientists, financial institutions, and other orginizations that need to keep very precise time. In 2005 (and many times before that) a leap second was added to the clocks, in order to make sure everything agrees.

;318467'']Also, much more precise corrections are made by scientists, financial institutions, and other orginizations that need to keep very precise time. In 2005 (and many times before that) a leap second was added to the clocks, in order to make sure everything agrees.

 

Leap seconds are a rotation issue, while leap years are a revolution issue. They are a headache to many, because they are unpredictable, so you can't write software to automatically add them according to a schedule.