The Night Sky: April 2009

Contents

The Night Sky in April 2009

By Harry J. Augensen
Professor of Physics & Astronomy, Widener University

Moon’s Phases in Month
  Full "Egg" Moon on the 9thNew Moon on the 24th

Stars and Constellations

As April arrives, the winter stars are gradually disappearing from view in the evening sky.  Now that the days have gotten longer, the bright stars and planets do not emerge from the evening twilight until around 8:30 pm. Aldebaran and the nearby Pleiades cluster in Taurus are now setting in the west, not to reappear until next autumn.  Betelgeuse and Rigel in Orion are in the southwestern sky, while the twin stars Pollux and Castor in the constellation Gemini are high in the south-southwest, to Orion’s upper left.  Blue-white Sirius in Canis Major and Procyon in Canis Minor also follow Orion.  The yellow star Capella is still quite noticeable, high in the northwest.

While the stars of winter are making their gradual departure, the stars of spring are moving into view.  Regulus in Leo is high in the southeast by 9 pm, and by 10 pm you will see yet another bright star, Spica, in the constellation Virgo, rising in the southeast. One overlooked winter-spring constellation is Cancer, the Crab, which lies between Gemini and Leo. Cancer is extremely faint, and yet it is significant because it, like Gemini and Leo, is a zodiac constellation, which means that the Sun moves through it during the year, specifically between July 21 and August 9.  Two thousand years ago, the Sun entered Cancer at the summer solstice, around June 21.  This explains the historical origin of the geographic term Tropic of Cancer to designate the latitude circle along which the Sun passes directly overhead on this date.  Because of precession of the Earth’s axis, the Sun is now in Gemini at this time.  Cancer also contains a famous cluster of stars known as Praesepe, or the Beehive, because the stars remind one of a swarm of bees around a hive.  A good pair of binoculars or small telescope will reveal this beautiful cluster.

 The largest of the 88 modern constellations, Hydra, the Water Snake, extends from the eastern border of Cancer to just below Spica.  In mythology, Hydra represented a nine-headed monster which Hercules battled as one of his twelve labors.  Cancer was a crab sent by Queen Hera to bite Hercules while he was battling Hydra, but Hercules merely stepped on the crab and crushed it. Hydra contains one fairly bright star, Alphard, which is now rising, a bit below Regulus.  Just above and to the left of Hydra are two small constellations, Crater (the Cup) and Corvus (the Crow).  The Big Dipper, which is part of the constellation Ursa Major, is now rising in the northeast, and its famous “pointer stars” point to Polaris, the North Star. The handle of the Dipper “arcs” to Arcturus, the bright yellow-orange star in the constellation Boötes (the Herdsman), which is rising in the east. The Dipper should be high enough now that you can spot Alcor, the faint companion to Mizar, the second star from the end of the Big Dipper’s handle. 

 Naked-Eye Planets In the Evening and Morning Sky

After having reached superior conjunction with the Sun on the last day of March, Mercury rises to prominence in the evening sky in April, making its finest appearance of the year.  Mercury is nearly invisible at the beginning of April, but by mid-month it is setting about 1½ hours after sunset, and two hours after it for most of the remainder of April.  Mercury resembles a yellow star hovering above the west-northwest horizon in the evening twilight.  Mercury reaches its greatest elongation with the Sun on the 26th.

 Saturn was in opposition with the Sun last month, and it continues to be in ideal position for viewing until early morning, reaching its maximum elevation above the southern horizon around 10:30 pm at midmonth.  Saturn continues to reside in the constellation Leo, and is easy to spot in the east-southeast., resembling a bright yellow star lying below Regulus. 

Jupiter continues to emerge slowly but steadily from the morning twilight.  For much of the continental US, it rises in the southeast about 2 hours before sunrise on the 1st and about 3 hours before the Sun on the 30th.   Jupiter is easy to spot, resembling a bright cream-colored star in the early morning sky.  Mars continues to keep a (literally) low profile in the morning sky, hovering inconspicuously above the eastern horizon just before sunrise.  Mars rises only an hour before the Sun during most of April.  It will take several more months for Mars to become even modestly attractive to the eye.

After having adorned the evening sky for several months and then reached inferior conjunction with the Sun at the end of March, Venus reappears in the morning sky during April.  Venus rises less than an hour before sunrise at the beginning of April, and about 1½ hours before the Sun at the end of the month.  Of greatest interest is that Venus will be occulted by the thin crescent Moon on the 22nd, but unfortunately this apparition will not be visible from the East Coast of North America. 

 For more information on astronomy and weather, visit the Widener University Public Viewing Website at http://www.widener.edu/stargazing/, then click on Web Links & Resources. A set of free sky maps can be obtained at http://www.skymaps.com/

Some content for this article has been obtained from US Naval Observatory Data Services