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Astronomy: Easter Dates Determined by Astronomy

4-15-11 Egg Nebula
The Egg Nebula as seen by the Hubble Space Telescope. Credit: Hubble Heritage Team (STScI/AURA), W. Sparks (STScI) & R. Sahai (JPL), NASA

Easter falls very late this year, on Sunday, April 24. My own children's spring break is not until the last week of April, to coincide with the Easter holiday, which made this winter seem extra long. And we only have astronomy to blame.

The date of Easter is determined by the spring (vernal) equinox and the full moon. The spring equinox changes a bit depending on the year, and in 2011 the date of the spring equinox fell on March 20. Easter always falls on the Sunday after the first full moon that occurs on or after the spring equinox. Because a full moon occurred on March 19, hours before the equinox, we have to wait a month until the next full moon, which happens this weekend on April 17 at 10:44 p.m. EDT. Because this is a Sunday, it is the following Sunday that becomes the date for Easter, April 24. The very latest Easter can occur is on April 25, which will not happen until 2038.

The full moon this month is also called the Egg Moon. For those who don't celebrate Easter, the Egg Moon is also a perfect symbol of spring and rebirth after the long, cold winter. Even though there are many astronomical objects that are egg-shaped, such as asteroids and galaxies, there is only one celestial object I know of that was named for an egg. The Egg Nebula, shown above, is a star that is nearing the end of its life and casting off dust and gas to make a planetary nebula before the star eventually becomes a white dwarf. The Egg Nebula, also known as CRL 2688, is too dim for amateur observers to spot. The image here was taken by the Hubble Space Telescope, and the springlike pastel colors are a happy side effect of false-colorization to better show the dust shells.

-- Kelly Kizer Whitt loves clean, clear, and dark skies. Kelly studied English and Astronomy at the University of Wisconsin-Madison and worked for Astronomy magazine. She is currently the Feature Writer for Astronomy and Space at Suite101.com. You can follow her on Twitter at twitter.com/Astronomommy.

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