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
