The Night Sky: April 2010

Contents

The Night Sky in April 2010

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

Moon’s Phases
New Moon on the 14thFull "Pink Moon" on the 28th                            

Stars and Constellations

The month of April bids farewell to many of  the winter stars as they gradually disappear from view in the evening sky.  Aldebaran in Taurus and the nearby Pleiades cluster are now setting in the west, and will not reappear until next autumn.  Betelgeuse and Rigel in Orion are in the southwestern sky, while the twin stars Pollux and Castor in 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, in Auriga, is now high in the northwest.

While the stars of winter make their leisurely exit, the stars of spring are taking center stage.  Regulus in Leo (the Lion) is high in the southeast, and another moderately bright star, Alphard, in the constellation Hydra (the Water Snake), is now rising, a bit below Regulus.  Hydra is the largest of the 88 modern constellations, and represents a nine-headed monster which Hercules battled as one of his labors.  The name Alphard means the “solitary one,” which makes sense because Alphard is situated in a region of the sky with almost no other nearby stars of comparable brightness.  Alphard is an orange giant star, similar to Arcturus, but about five times further from our solar system, and so it appears noticeably fainter, comparable in apparent brightness to Polaris.

If you wait until after around 10 pm, you will see yet another bright star, Spica, in the constellation Virgo, rising in the southeast following the planet Saturn.  The constellation Virgo is especially noteworthy telescopically because it contains over 2000 galaxies, including the famous Virgo Cluster of Galaxies, which is about 55 million light years away.  Our own Milky Way and most of the galaxies in its vicinity are gravitationally bound to this cluster.  
 

The Big Dipper, a part of the constellation Ursa Major, is now rising in the northeast, and its handle “arcs” to Arcturus, the bright yellow-orange star in the constellation Boötes (the Herdsman), which is rising in the east.  Arcturus lies about 37 light years from our solar system, and has a diameter over 30 times larger than that of our Sun.  Arcturus, Spica, and Regulus form a “Spring Triangle” which is larger though not as famous as its summer counterpart.

Naked-Eye Planets In the Evening and Morning Sky 

Mercury is at its best in early April this year, reaching greatest evening elongation on the 8th.  Look for Mercury around this date, shining like a yellow star in the west after sunset.  On this date, Mercury sets about one and a half hours after sunset.  Toward the end of the month, however, Mercury sinks rapidly into the western sky, getting closer to the Sun.  On the  28th it reaches inferior conjunction with the Sun., and is therefore not observable.

Venus looks like a glittering yellow diamond floating above the western horizon about a half-hour after sunset, and getting higher with each passing night.  As April opens, Venus sets around 9 pm EDT, or an hour and a half after sunset.  By the 31st, Venus has extended its duration after sunset to just under two and a half hours, setting a little after 10 pm.   During the first week of April, brilliant Venus and much fainter Mercury appear close together low in the sky at dusk.

Mars resembles a bright, copper-colored star just to the east of Pollux and Castor in Gemini.  Mars stands nearly overhead during the early evening hours of April, remaining in view until well after midnight.  Mars sets around 3 pm at midmonth.  Mars remains quite bright during April, though it slowly fades all month.  Mars has about the same brightness as the nearby star Capella on the 1st, and about the same as Betelgeuse on the 30th.

Saturn, continuing its residence in Virgo, is situated roughly between the true stars Regulus  and Spica. Saturn resembles a bright yellow star as it rises above the eastern horizon in early evening.  Saturn was in opposition with the Sun last month, and so it remains in fine position for viewing for the entire month of April, setting around 5 am at midmonth.

Jupiter, which was in conjunction with the Sun back in late February, is too close to the Sun to be easily seen until late April, when it sets about one and a half hours before sunrise.   Look for what appears to be a bright star hovering above the eastern horizon about 45 minutes before sunrise.

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

Times given apply for observers near to the latitude and longitude of Philadelphia, USA: 40 degrees North latitude, 75 degrees West longitude.