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The earth orbits the Sun in 365.2564 days.  This is the sidereal year.

However, the procession of the Equinoxes moves the effective position of the vernal equinox every year, so the difference in times between successive springs is 365.2422 years.  This is defined as the tropical year (analogy between sidereal and solar days)

Since it is important to know when spring is to plant crops, and it is easier to count days than to build Stonehenges everywhere, the Roman Emperor Julius Caesar established leap years.

Every 4 years, (years divisible by 4--1992, 1994, etc.) we add one day to the year (e.g. Feb 29).

In 1582 Pope Gregory XIII further reformed the calendar to get even closer to the tropical year.  Under the Gregorian system, only centuries divisible by 400 are leap years (e.g. 2000 is a leap year but 1900 wasn't).

What is the length of the Gregorian year?  How many years until it is 1 day off?

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