Easter is a Time Traveler, Too!

Do you know when Easter is?  It’s the first Sunday after the first full moon after the spring equinox.  That can be a complex calculation to make.  In 2011, the full moon in March was the 19th.  About 36 hours before the spring equinox.  29.5 days later will be the next full moon – April 18th.  It happens to be a Monday, which means that Easter is six days later on April 24th, 2011.  I wonder what the latest possible Easter Sunday is?

Astropotamuses deal in astronomy.  Easter, while perhaps based in astronomical ideas, came to us from the Christian Church.  So it bends the rules a bit with what’s the equinox and what’s the lunar schedule.  Astronomically speaking, the equinox can be on March 19, 20, or 21.  And the full moon is between 29 and 30 days from one to the next.  So, astronomically speaking, the full moon could be on the 20th, the equinox on the 21st, the next full moon on April 18th, and then the Sunday following would be Easter Sunday.  If the lunar calendar lines up with the solar calendar correctly, that could be as far into April as the 25th.  The earliest date could be just the opposite:  The equinox on the 19th of March, the full moon on the 20th, and Easter Sunday would be the next day, the 2st.

Remember that the church messes with things a bit?  One way is that they define the equinox to be March 21st.  So that means astronomy and clergy don’t always agree on when the first day of spring is (i.e., the first day of the equinox).  Given this fuzzy math, Easter can be March 22 through April 25.  It’s doubtful that you remember the last time it was on the 22nd (it was in 1818), and it’s doubtful you’ll be here for the next March 22nd (it will be in 2285).  As for the April 25th date, the Easter Bunny last hopped on that date in 1943 (maybe you were here for that one), and it will next do so in 2038 (Astropotamus hopes to be here for that one).

For some reason, the most common date is April 19th, even though it’s not in the middle of the range of possible dates (being only six days away from the latest possible date).  That just goes to show you how important our moon has been to people throughout the ages.  We not only plan our calendar around it (the months are based on lunar cycles, not solar ones), but also our holidays involving chocolate (the most important ones!).  So Easter Sunday is a time traveler just like the Astropotamus – it moves its date every year!

So it’s not surprising, then, that at this time of the year, Astropotamus turns his Time Machine towards the moon as much as possible.  It’s getting warmer out, so it’s more fun to be outside at midnight, the moon is getting bigger every night, and it’s also more overhead, which means it’s easier to see since the light doesn’t have to travel through as much atmosphere first.  More on why that’s bad in a future posting.

In the meantime, I hope you’ll enjoy some of the more striking images of our moon as it nears being half full.  The terminator, or the line between light and dark, is always where you can see the most detail because of the shadows.  See if you can pick out the ones where the crater walls are lit but their floors are still dark.

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