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The Sky This Week, 2019 April 16 - 23

The hopping dates of Easter
The Full Moon, imaged with a 102mm (4-inch) f/6.5 Explore Scientific AR102 refractor on 2017 December 4.
The Moon, imaged on 2017 December 4
with a 102mm (4-inch) f/6.5 Explore Scientific AR102 refractor
and a Canon EOS Rebel T2i DSLR.

The Moon keeps the night sky bright for most of the week, with the Full Moon occurring on the 19th at 7:12 am Eastern Daylight Time.  April’s Full Moon is popularly known as the Seed Moon, Grass Moon, Pink Moon, or Egg Moon.  This year it is also the Paschal Moon, since it is the Full Moon that defines the Christian feast of Easter.  On the evening of the 17th you’ll find the Moon just one degree north of the second-magnitude star Porrima in the constellation of Virgo.  Porrima is a beautiful but challenging double star for the small to modest telescope.  It has an orbital period of about 171 years, and it is one of the few double stars that shows perceptible changes in separation and position angle from year to year.  On the morning of the 22nd Luna will be seven degrees north of the bright reddish-hued star Antares, and the next morning she will be just west of bright Jupiter in the pre-dawn sky.  And this week marks the 47th anniversary of the flight of Apollo 16, the penultimate lunar mission.

Easter falls on the 21st this year, which many people think is very “late” in the season.  The date of this feast can occur as early as March 22nd or as late as April 25th.  The date is determined from astronomical phenomena, usually on the first Sunday following the first Full Moon falling after the vernal equinox.  Except when it isn’t.  Alert readers will note that this year the equinox occurred on March 20th with the first Full Moon following a day later.  By this reckoning Easter should have occurred on March 24th.  However, the formula for determining the date of Easter, known as the “Computus”, throws us a curve ball on occasion.  The Computus dates back to the Council of Nicaea in the year 325 CE, when specific rules were laid out to make the calculation of Easter “universal” in the Catholic world.  According to the rules of the Computus, the equinox always occurs on March 21st whether reflected by the stars or not.  The Full Moon was defined by tables of tabular dates based on the 19-year Metonic cycle of the Moon’s phases.  These tables were used since they were far easier to understand than the complex theory of the Moon’s apparent motion, once again aiding the computations of 4th Century church authorities.  If you look in an annual astronomical almanac you’ll find a section devoted to “chronological cycles”.  Among these are the “dominical letter”, “epact”, “golden number”, and “solar cycle”.  These numbers approximate the occurrence of the equinoxes and Full Moons over a period that lasts some 5,700,000 years, which is probably why they are still in use.  During the interval of the years 2000 through 2999 Easter will fall on April 21st 38 times.  The most frequent date for Easter during this interval is April 16th, when it will occur 43 times.  

With the Moon dominating the evening sky we’re not left with much to look at in the night sky.  The bright winter constellations are still visible in the early evening, but by 10:00 pm they are beginning to slip below the western horizon.  However, several of the bright winter stars hang tough.  By midnight Orion has disappeared, but you can still see an arc of bright stars in the west.  Capella hangs in the northwest, and as you sweep to the south you’ll pass Castor and Pollux in Gemini, and Procyon in Canis Minor.  One of my favorite sights at this time of year is the spectacle offered by the setting of Sirius, the brightest star in the sky.  Sirius disappears below the horizon shortly after 11:00 pm, and if you have a low, flat southwest horizon the star can put on a dazzling show.  Its light now must pass through the densest portions of our atmosphere, and the turbulence of the ever-shifting air causes the star to flash through all the colors of the rainbow.  The phenomenon is particularly noticeable if you have an ocean horizon, but the most spectacular views I’ve heard of come from Western Michigan, where Sirius sets over the cold waters of Lake Michigan.  The contrasting air masses over the lake not only distort the colors of Sirius, but also make it seem to shift places in rapid pulses of movement.

The evening’s only planet, Mars, continues to course his way through the stars of Taurus, the Bull.  By the end of the week he’s about halfway between the constellation’s brightest star Aldebaran and its second-brightest, Menkalinan.  

Jupiter now rises shortly after local midnight and crosses the meridian at the beginning of morning twilight.  The giant planet will be a tough target for northern hemisphere observers during this apparition.  He’s currently at his most southerly declination for the year, and he’ll never get more than 30 degrees above the southern horizon for most observers in temperate northern latitudes.

Saturn rises about two hours after Jupiter and may be spotted low in the southeast as the first rays of twilight appear in the east.  Like Jupiter, the ringed planet is at a southerly declination, so you’ll need very still air to enjoy a good view of his rings.

Venus continues to try to keep ahead of the rising Sun, rising just before 5:30 am.  Your best bet to find her is to look low in the southeast half an hour before sunrise.  Fortunately she should be fairly easy to spot if you have a cloudless sky.

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