One calendar common year has 365 days:

1 common year = 365 days

One calendar leap year has 366 days:

1 leap year = 366 days

Leap year occurs every 4 years, except for years that are divisible by 100 and not divisible by 400.

So the mean length of the Gregorian calendar year is:

1 mean year = (365+1/4-1/100+1/400) days = 365.2425 days

Julian year is used for astronomical calculations (light year definition).

One Julian year has 365.25 days:

1 year = 365.25 days

One sidereal year is the time it takes the Earth to do a single rotation around the sun.

One sidereal year has 365.25636 days:

1 year = 365.25636 days

One tropical year is the time it takes the Earth to complete a single cycle of the 4 seasons.

One tropical year has 365.242189 days:

1 year = 365.242189 days

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