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Credit & Copyright: Classical Numismatic Group, Inc.,
Wikimedia
Explanation:
In 46 BC Julius Caesar reformed the
calendar system.
Based on advice by astronomer
Sosigenes of Alexandria,
the Julian calendar included one
leap day
every four years to account for the fact that an Earth
year is slightly more than 365 days long.
In modern terms, the time it takes for the planet to orbit the Sun
once is 365.24219 mean solar days.
So if calendar years contained exactly 365 days they would drift
from the Earth's year by about 1 day every 4 years and
eventually July (named for
Julius
Caesar
himself) would occur during the northern hemisphere winter.
By adopting a leap year with an extra day every four years,
the Julian calendar year would drift much less.
In 1582 Pope Gregory XIII provided the further fine-tuning that leap days should
not occur in years ending in 00, unless divisible by 400.
This Gregorian
Calendar system is the one in wide use today.
Of course, tidal friction
in the Earth-Moon system slows Earth's rotation and gradually
lengthens the day by about 1.4 milliseconds per century.
That means that
leap
days like today will not be necessary, about 4 million years from now.
This Roman silver coin, a
denarius,
depicts Julius Caesar (left) and Venus, Roman goddess of love.
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NASA Web Site Statements, Warnings, and Disclaimers
NASA Official: Jay Norris. Specific rights apply.
A service of: LHEA at NASA / GSFC
& Michigan Tech. U.
Based on Astronomy Picture
Of the Day
Publications with keywords: Leap Days
Publications with words: Leap Days
See also: