Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                              2020 February 29

                         Julius Caesar and Leap Days
     Image Credit & License: 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 14 milliseconds per century. That means that leap days
   like today will not be necessary ... about 4 million years from now.

                     Tomorrow's picture: a hole in Mars
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                             & Michigan Tech. U.

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