# Actual year/day vs earth's movement

1. Jul 15, 2010

### Good4you

1) So i found on Wikipedia that the earth takes 365.256363004 days to orbit the sun. If you include leap years then the average calendar year is 365.25 days. So the calendar year is off in accuracy by the difference. Calculating that out would mean that in 157 years the calendar would be off by one day. So i guess my question is: is there a means to correct for this, or will over the centuries the seasons slowly change when they are supposed to occur?

2) My guess is that over time as technology develops we incrementally refine measurements of things like the time it takes for the earth to rotate. So this would lead me to believe that when the first clocks were invented we did not know to the same precision the amount of time it takes for the earth to rotate.
a) So is time defined in a way that there is exactly 24hours in a day? ...and would't this mean that a second measured by an old clock would not be equivalent to a second on a new clock?
b) or if the definition of time is not based on there being exactly 24hours in a day; wouldn't this imply that as we refine our measurement of the earth's rotation, there is more or less than 24hours in a day; therefore over the centuries, wouldn't there be some "creep", and 12:00 would no longer correspond to noon, perhaps it would eventually correspond with dusk? or is there some method to correct for this "creep"?

2. Jul 15, 2010

### Staff: Mentor

You're not using the full rule for leap years.

If the year is divisible by 4, it's a leap year,

UNLESS

it's also divisible by 100, in which case it's not a leap year,

UNLESS

it's also divisible by 400, in which case it's a leap year after all.

So 1700, 1800, 1900 and 2100 are not leap years (despite being divisible by 4), but 1600 and 2000 are leap years.