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The Sky This Week, 2017 April 11 - 18

Computing Easter
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Jupiter, with moons Europa & Io, April 10, 03:34.5 UT
Imaged from Alexandria, VA with a 23.5-cm (9.25-inch) Schmidt-Cassegrain telescope

The Moon moves into the morning sky, waning from the Full phase to Last Quarter, which occurs on the 19th at 5:57 am Eastern Daylight Time.  Luna skirts the southern horizon as she passes through the stars of the summer constellations of Scorpius and Sagittarius.  She passes nine degrees north of the ruddy star Antares in the morning hours of the 15th.  She brackets the yellow-tinted planet Saturn before dawn on the mornings of the 16th and 17th.

Last week’s Full Moon was the first one to fall after the vernal equinox, which sets the date for the Christian feast of Easter.  The setting of this date is the most important function of the modern Gregorian calendar, as it fixes the dates of all of the “moveable feasts” celebrated by Christians.  The date has been fixed since the Council of Nicaea in the year 325 CE by a formula known as the “Computus”, a formula based on a fixed date for the equinox and the combination of several lunar cycles which allowed the dates to be calculated for many years into the future.  Prior to the Council, Easter was celebrated on the Sunday of the Jewish week of Passover, but irregularities in the Hebrew Calendar occasionally placed the observance of Passover ahead of the equinox.  The Council first fixed the date of the equinox as March 21st, then defined a set of cycles known as the “epact” and “golden number”.  These cycles closely approximated the cycle of lunar phases and set the ecclesiastical definition of the date of the Full Moon that followed the equinox.  Using these cycles the date of Easter could be accommodated in the strictly solar-based Julian and Gregorian Calendars without the complications introduced by the lunar-based Hebrew Calendar.  For the most part we can generally say the Easter falls on the Sunday after the first Full Moon following the vernal equinox, but there are occasional exceptions based on the dates of the Full Moon defined by the Computus.  Interestingly, Eastern Orthodox Christians still observe the older Julian Calendar, which now differs from the Gregorian Calendar by 14 days.  For these followers, Julian March 21 occurs on Gregorian April 4, so they celebrate Easter up to a month later!  

Mercury quickly falls from the evening sky, passing inferior conjunction with the Sun early in the morning of the 20th.  He has faded considerably from his peak brightness in late March and will be a challenging object to see in the western evening twilight sky.

Mars is still visible in the west as darkness settles in the evening.  The red planet is steadily moving eastward against the stars, and this week he crosses the boundary between the constellations of Aries and Taurus.  By the end of the week he’s within five degrees of the Pleiades star cluster as he drifts toward the bright star Aldebaran.

Jupiter is now visible at sunset in the east and crosses the meridian in the middle of the night.  He is the brightest object in the night sky right now, and offers a great target for the small telescope.  If you watch him over the course of a few hours you can see the changes in the positions of his four bright Galilean moons as the move in their orbits around the giant planet.  If you have a 4-inch or larger telescope you can see changes in his cloud belts over the course of a few minutes thanks to Jupiter’s 9 hour 50 minute rotation.  You’ll find the bright star Spica just over seven degrees southeast of the bright planet.

Saturn waits his turn to shine in the evening sky, so he’s still best seen due south as morning twilight begins to gather.  You’ll find him perched over the “spout” of the Teapot asterism formed by the brighter stars of the constellation Sagittarius.

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