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[sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)
Section - C.05 Was 2000 a leap year?

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Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)
Previous Document: C.04 What's a Julian date? modified Julian date?
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Yes.

Oh, you wanted to know more?

The reason for leap days is that the year---the time it takes the
Earth to go round the Sun---is not an integral multiple of the
day---the time it takes the Earth to rotate once on its axis.  In this
case, the year of interest is the "tropical year," which controls the
seasons.  The tropical year is defined as the interval from one spring
equinox to the next: very close to 365.2422 days.

The Julian calendar, instituted by the Roman Emperor Julius Caesar
(who else? :), has a 365-day ordinary year with a 366-day leap year
every fourth year.  This gives a mean year length of 365.25 years, not
a very large error.  However, the error builds up, and by the
sixteenth century, reform was considered desirable.  A new calendar
was established in most Roman Catholic countries in 1582 under the
authority of Pope Gregory XIII; in that year, the date October 4 was
followed by October 15---a correction of 10 days.  Most non-Catholic
countries adopted this "Gregorian" calendar somewhat later (Great
Britain and the American colonies in 1752), and by then the difference
between Julian and Gregorian dates was even greater than 10 days.
(Russia didn't adopt the Gregorian calendar until after the "October
Revolution"---which took place in November under the new calendar!)
Many of the calendar changeovers elicited strong emotional reactions
from the populations involved; people objected to "losing ten (or
more) days of our lives."

The rule for leap years under the Gregorian calendar is that all years
divisible by four are leap years EXCEPT century years NOT divisible by
400.  Thus 1700, 1800, and 1900 were not leap years, while 2000 will be
one.  This rule gives 97 leap years in 400 years or a mean year length
of exactly 365.2425 days.

The error in the Gregorian calendar will build up to a full day in
roughly 3000 years, by which time another reform will be necessary.
Various schemes have been proposed, some taking account of the changing
lengths of the day and/or the tropical year, but none has been
internationally recognized.  Leaving a reform to our descendants seems
reasonable, since there is no obvious need to make a correction now.

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Top Document: [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)
Previous Document: C.04 What's a Julian date? modified Julian date?
Next Document: C.06 When will the new millennium start?

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Last Update March 27 2014 @ 02:11 PM