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The Sky This Week, 2020 April 7 - 14

Easter and Yuri's Night
Venus and the Pleiades, imaged 2020 April 5 from Alexandria, Virginia.
Venus and the Pleiades, imaged 2020 April 5, 01:50 UT from Alexandria, Virginia
with an Antares Sentinel 80mm (3-inch) f/6 refractor and a Canon EOS Rebel SL2 DSLR

The Moon brightens the evening and overnight hours this week, waning from a bright perigee full phase to Last Quarter, which falls on the 14th at 6:56 pm Eastern Daylight Time.  Luna dives down into the rising summer stars as she dips low along the ecliptic.  You will find her several degrees north of the bright star Spica in the wee hours of the morning of the 8th.  On the 11th she passes north of the ruddy star Antares in the pre-dawn sky.  By the week’s end she will be moving to join Jupiter, Saturn, and Mars as morning twilight gathers.

Sunday, April 12th is the first Sunday to occur following the first Full Moon after the vernal equinox, which means that it is the time for the feast of Easter for most of the world’s Christians.  The formula for determining Easter has ancient roots, with its primary rules defined at the Council of Nicaea in the year 325 CE.  At this time it was decreed that Easter follow the first Full Moon to fall after March 21st, which was defined to be the traditional date of the equinox.  At the time clerics were using the Julian Calendar, which added a leap day every four years.  This approximation of the true length of the astronomical year caused an error of one day every 128 years, so the actual equinox occurred several days before the tabular one.  This accumulating error prompted the Gregorian Calendar reform of 1582, when ten days in October were summarily deleted from the Julian Calendar to bring the equinox “back in line” with the sky.  Today the equinox can fall on either March 20th, 21st, or 22nd, so the ecclesiastical definition of March 21st still figures into the determination of Easter.  Even today the ancient “Computus” formula is still employed in the reckoning of Easter.  If you look in an astronomical almanac you will find a number of seemingly arcane terms like the Golden Number, Epact, and Dominical Letter.  These terms were used to define different solar and lunar cycles that would enable local priests and other church officials to determine Easter for themselves if they were far from their central administrative centers.

It also happens that April 12th is the anniversary of the first flight of a human in space and the first flight of the American space shuttle.  Named Yuri’s Night in honor of the cosmonaut Yuri Gagarin, the observance has become very popular over the past decade as a celebration of our exploration of space.  This year’s events have been limited by the spread of the coronavirus, but you can still participate either virtually on your computer or with your own telescope or binoculars from your home.  The wonderful thing about the night sky is that it’s always there for you to explore by whatever means you have; all you need is imagination and a little sense of wonder.

The early evening sky offers Venus, blazing away in the west after sunset.  As the sky darkens you should be able to watch her move toward the setting stars of the Great Winter Circle.  This week she heads of the gap between the bright stars Aldebaran in Taurus and Capella in Auriga.  When she sets at around 11:30 pm the winter stars seem to follow her as the springtime constellations take over the sky.

Early risers have a trio of planets to greet them before sunrise.  Jupiter, Saturn, and Mars are arrayed in a line in the southeastern sky, just east of the “Teapot” asterism of Sagittarius.  During the course of the week Old Jove and the ringed planet maintain nearly equal spacing between themselves, but ruddy Mars continues to charge off to the east.  This week Mars crosses through the middle of the constellation of Capricornus, one of the faint constellations that populate the autumn sky.  Mars is gradually brightening, and by the end of the week he should be comparable to the glimmer of Saturn.

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