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The Sky This Week, 2011 March 15 - 22

Passing planets, a Super-Moon, and the equinox(es)
 Tycho-Clavius region of the Moon
Imaged 2011 FEB 13

The Moon brightens the nighttime sky this week, with Full Moon occurring on the 19th at 2:10 pm Eastern Daylight Time.  The March Full Moon is variously known as the Worm Moon, Sap Moon, or Crow Moon in various skylore traditions, and often it is used to fix the day of Easter.  However, this year the Full Moon occurs just before the vernal equinox, so the Paschal Moon won’t occur for another month.  Easter will therefore be almost as late as it can theoretically be this year.  What is particularly interesting about this year’s Full Moon is that it occurs within an hour of lunar perigee, and this perigee is the closest one for the entire year.  This so-called “super Moon” last occurred on March 8, 1993, when the difference between times of perigee and Full Moon were just over an hour.  Luna’s apparent disc varies in size by about 15% between extreme apogees and perigees, so this Full Moon will appear about as large as it possibly can as it rises on the evening of the 19th.  If you watch Luna rise that night against a distant horizon dotted with trees and houses you may have a particularly good experience of the “Moon illusion", where Luna’s disc looks enormous when close to the horizon.

The vernal equinox occurs on March 20th at 7:21 pm EDT.  This is the moment when the center of the Sun’s disc reaches an ecliptic longitude of zero degrees.  Astronomically this defines the beginning of spring, but this definition doesn’t apply when figuring the date of Easter.  Thanks to rules first laid out in the Third Century the date of Easter is determined by a formula known as the Computus.  Over the centuries the Computus has been modified, most recently in 1583 by Pope Gregory XIII.  Under the Gregorian Calendar Computus the date of the equinox is fixed at March 21st, no matter what the actual sky is doing.  The Paschal Moon is not fixed astronomically either.  According to the Computus it always occurs on the 14th tabular day of the lunation that begins after the equinox.  This year that Full Moon occurs on April 17th, a Sunday, so Easter falls on the following Sunday.  This will be the latest Easter since 1943!

This week will be the best time to look for the most difficult planet to find with the naked eye.  Fleet Mercury passes by Jupiter over the course of several evenings as the week opens.  The pair are closest on the 15th, just two degrees apart, but even by the week’s end they are still only about 7 degrees from each other.  Look for bright Jupiter in the twilight glow over the western horizon about half an hour after sunset.  Mercury will appear to the right of Old Jove and gradually pull above the giant planet as the week passes by.  On the 17th Mercury will receive a gift from us Earthlings in the form of its first artificial satellite.  If all goes well, the MESSENGER space probe, which has been en-route to the solar system’s innermost planet for some seven years, will slip into a looping orbit around Mercury to conduct the first global mapping mission for this sun-scorched world.  MESSENGER’s cameras should be able to map the entire planet’s surface to a resolution of a few dozen meters and probe its enigmatic magnetic field for several years once it arrives. 

Thanks to the switch to daylight Time the rising of the ringed planet Saturn has been pushed back by an hour, but this is just a temporary setback.  By the end of the week Saturn rises at around 8:30 pm, around the same time that Jupiter sets.  Late-night skywatchers can catch a good glimpse of Saturn by midnight through a small to modest telescope.  The sight of this world never ceases to amaze me whenever I get a chance to look at it.  The rings, made up of billions of chunks of water ice and interplanetary dust, have a diameter that would go from the surface of the Earth to about 2/3rds the distance to our Moon, yet their thickness is measured in handfuls of meters.  They form the flattest and thinnest structure we have yet observed in the universe!

Now that sunrise is later again, look for Venus low in the southeast before dawn.  What she lacks in altitude is more than made up for in dazzle as she cheerily gleams in the gathering morning twilight.