How could the year 2000 be a leap year when 1900 was not?

Question

How is that possible?

Answer 1

Actaully looking at several sources the third rule is every four hundered years.

If the year is divisible by 4 then it is a leap year
unless it is divisible by 100 then it is not,
but if it is divisible by 400 then it is again.

2000 was divisible by 4,100, and 400 so it was a leap year by the third rule
1900 was divisible by 4 and 100 but not 400 so it was not a leap year.

Most authoritative references are probably:
http://aa.usno.navy.mil/faq/docs/calendars.html
and
http://www.srrb.noaa.gov/highlights/sunrise/glossary.html#l

just google "leap year calculation" and either "navy" or "NIST".

Answer 2

This whole leap year thing is arbitrary based on calculations done for Pope Gregory long before I was born. The difference is that 1900 was the turn of a century but 2000 was also the turn of a millennium. Probably the extra day was needed to keep the calendar in balance.

Answer 3

Because the actual year is 365.25 something days. A leap year every 4 years just about keeps in in line. But because it is not exact, by not having a leap year every 100 years if the year is not divisible by 400 keeps it even closer.

Answer 4

Leap year is any year divisible by 4, but not years divisible by 100 but add back in years divisible by 1000. This makes the earths rotation match the calendar year.

Answer 5

hi

mean tropical year = 365.242 189 days

julian aprox = 365.2425 = 365 + 1/4 -1/100 + 1/400

each 4 years, yes
each 100 years no but ... each 400 years yes

bye