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The Sky This Week, 2019 March 19 - 26

Equinox? Check. Full Moon? Check. Easter? Not so fast...it's complicated!
Orion setting, imaged from Mollusk, Virginia, 2019 March 16
Imaged with a Canon EOS Rebel SL2 DSLR and an Omegon MiniTrack LX2 sidereal tracking platform.

The Moon brightens the overnight hours this week, reaching her full phase on the 20th at 9:43 pm Eastern Daylight Time.  The Full Moon of March is popularly known as the Worm Moon, Crow Moon, Sap Moon, or Lenten Moon.  Luna begins the week in the eastern part of the constellation of Leo, the Lion.  She dives southward along the ecliptic, passing well north of the bright star Spica on the nights of the 21st and 22nd, and ends the week close to the planet Jupiter on the morning of the 27th.

The Vernal Equinox occurs on the 20th at 5:58 pm EDT.  This is the moment when the center of the Sun’s disc reaches an Ecliptic Longitude of zero degrees.  At this time Old Sol stands directly over Earth’s equator at a point in the middle of the Pacific Ocean.  From that time until the Autumnal Equinox Old Sol will be located in the northern hemisphere of Earth’s sky.  The Vernal Equinox is an important marker in many traditional calendars, and it is used to define the astronomical or “tropical” year.  Since it symbolizes the re-birth of plants and animals in the Northern Hemisphere, it is used in the computation of the most important feast day in the western Christian calendar.  The feast of Easter is fixed by an ancient formula known as the Computus, which has its origins in the 4th Century AD.  Chronological cycles used in the Computus may still be found in annual almanacs in the form of terms like the Golden Number, Epact, and Dominical Letter.  Ecclesiastical rules dictated that the feast occur on the Sunday following the first ecclesiastical Full Moon following the Vernal Equinox.  In AD 325 the Council of Nicaea fixed the equinox on March 21st and created tables based on the 19-year Metonic Cycle of the Moon’s phases to predict Full Moons.  At the time Christians were using the Roman Julian Calendar, which had “leap years” every four years.  The mean length of a Julian year was thus about 11 minutes longer than the tropical year, so after about 128 years the equinox regressed by one full day relative to the Sun’s position in the sky.  By the time that Pope Gregory XIII reformed the calendar to its current form, the calendar equinox differed from the real sky by 10 days.  Gregory’s reform shortened the year 1582 by 10 days, but it “re-synched” the equinox with the date of March 21, making the ecclesiastical equinox once again agree with the date fixed at Nicaea some 1200 years earlier.  For the computation of Easter the ecclesiastical equinox is still fixed at March 21, despite the actual time of its occurrence.  Complicating matters, the dates of the Full Moons in the Easter Computus are still based on tables constructed from the Metonic Cycle rather than the actual astronomical phase.  In 2019 the astronomical equinox occurs a few hours before the Full Moon, but the ecclesiastical equinox by definition has to fall on the 21st.  Using the Computus, the ecclesiastical Full Moon occurs before the ecclesiastical equinox, so Easter must wait until after the next Full Moon, which occurs on April 19th.  This places Easter on April 21st, one of the latest dates that it can occur.

The end of evening twilight finds the bright stars of winter heeling over to the western sky.  In their place are the more subdued constellations of spring, led by the distinctive constellation of Leo, the Lion.  Leo crosses the meridian at local midnight.  You’ll find its brightest star, Regulus, high in the south at this time.  Look past the zenith toward the north and you’ll have a great view of the “Big Dipper” asterism in the larger constellation of Ursa Major, the Great Bear.  

Mars moves eastward into the constellation of Taurus, the Bull this week, and by the week’s end he will be about five degrees southwest of the Pleiades star cluster.  His motion will carry him past the cluster next week. 

Jupiter lies just west of the meridian along the southernmost reaches of the ecliptic as morning twilight gathers.  The giant planet is slowly drifting eastward against a dense star cloud of the Milky Way, located between the constellations of Scorpius and Sagittarius.  He will spend the summer months in this general area before beginning a slow northward drift by the year’s end.

Saturn may be found in the gathering morning twilight to the east of the “Teapot” asterism in Sagittarius.  Like Jupiter, Saturn will spend the summer months languishing in this part of the sky.

Venus shines in morning twilight, greeting me from the southeastern sky despite the brightening conditions before sunrise.  She now rises after the beginning of twilight, but you should have no trouble spotting her if you have a clear view of the horizon.  

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