Contents

The Night Sky in May 2011

By Harry J. Augensen

Professor of Physics & Astronomy, Widener University

Moon’s Phases
New Moon on the 3rdFull "Flower Moon" on the 17th 

Stars and Constellations

Clear May nights are among the most appealing for skywatching.  Not only are the nighttime temperatures pleasantly cool, but also the air is perfumed with the scent of newly sprung blossoms of lilac and viburnum.  On the other hand, in May you need to wait until around 8:30 or 9 pm to see the first stars emerge from the evening twilight. After it becomes dark, try to catch a last glimpse of Aldebaran, Rigel, Betelgeuse, Pollux, Castor, Sirius, and Procyon.  These bright stars of winter are all fading into the evening twilight, not to reappear in the night sky until next autumn.  Yellow Capella is setting in the northwest, but because of its far northern declination you will still be able to see it through June.

The evening sky is now firmly dominated by the stars of spring. The brightest of these is Arcturus, the yellow-orange star in the constellation Boötes, the Herdsman, which is high in the east-southeast. Arcturus is easily found by following the arc of the Big Dipper’s handle. The Big Dipper, which is part of the constellation Ursa Major (Big Bear), is now rising in the northeast, and its pointer stars Merak and Dubhe, point to Polaris, the North Star.  Regulus, the bright blue-white star in Leo, lies high in the south-southeast.

Of the 88 constellations which populate the celestial sphere, the three largest (i.e., that cover the largest area in square degrees) can be found in the spring skies:  Hydra, Virgo, and Ursa Major.  Hydra has the distinction of being the largest of all the constellations, although most of its stars are pretty faint.. According to British astronomer Chris Kitchin, Hydra dates back to at least the time of Ptolemy's Almagest in 145 AD, but may go back even further to the ancient Babylonians (2000 BC). In Greek mythology, Hydra was a multi-headed serpent which haunted the river Amymone, and was eventually slain by Hercules as one of his twelve labors.   Hydra winds across the southern sky on spring nights, and its brightest star, orange Alphard, lies a bit below Regulus.

By around 10 pm, you will see yet another bright star, Spica, rising in the southeast.  Spica, or alpha Virginis, is featured in University of Illinois astronomer James Kaler’s book, The Hundred Greatest Stars (New York: Copernicus Books, 2002).  Spica, which is the 16th brightest star in the sky, actually consists of four stars each more luminous than our Sun.  The two most massive stars form the core of the system, orbiting each other in a period of just over four days at a distance of just 0.12 AU, where one astronomical unit (AU) is the average Earth-Sun distance.  The more massive of the pair has a luminosity and radius of, respectively, 13,000 and 8 times the solar values. The entire Spica system lies at a distance of over 262 light years from our solar system.

Note that Regulus, Spica, and Arcturus form a "Spring Triangle," with the right angle at Spica.  Though not nearly as famous as the soon-to-rise Summer Triangle (consisting of Vega, Altair and Deneb), the  Spring Triangle is larger and is fairly easy to discern during the evening hours of May.  This year, the triangle contains within it an interloper, lying just above Spica – the planet Saturn (see below).  Also noteworthy is the fact that during the months of spring, the three brightest stars in the northern skies, each representing a different season – yellow-white Capella (winter), orange-yellow Arcturus (spring), and blue-white Vega (summer) – can all be seen on the same night at the proper time of night.  During the middle of May, that time is around 10 pm, when Capella is setting, low in the northwest, while Vega is in the opposite part of the sky, rising in the northeast.  Arcturus is ideally situated nearly overhead.

Naked-Eye Planets In the Evening and Morning Sky

Saturn  was closest to Earth and in opposition with the Sun last month, and it continues to be in excellent position for viewing this May.  Saturn is already well up in the east by the time darkness falls.   Saturn resembles a cream colored star situated above the fainter white star Spica and to the right of the yellow-orange star Arcturus, fourth brightest star in the night sky. 

Mercury and Venus pair up low in the east just before sunrise in early May. Venus is by far the brighter of the two, and can be picked out of the dawn sky without any optical aid, so long as buildings or trees do not obstruct the view.  It rises about an hour before the Sun all month.   Mercury lies just below Venus, but is probably too faint to spot against the bright morning sky; binoculars will help to pick it out of the dawn twilight.  Mercury reaches its greatest elongation with the Sun on the 7th, but thereafter begins to sink toward the Sun.  Venus remains at relatively the same elevation above the horizon, though its position does migrate slowly northward. 

Mars rises only about a half-hour before the Sun as May opens, and is even fainter than Mercury; binoculars or a small telescope will  help to pick both planets out of the dawn twilight.  Toward the end of the month, Venus passes just below Mars, and will be a fine sight in binoculars.

Jupiter begins to emerge from its conjunction with the Sun last month, and, as May opens, it lies only a full-moon's width from Mars in the dawn sky.  Jupiter, though much brighter than Mars, will, like both Mercury and Mars, probably need optical aid to be picked out from the bright sky background.  Jupiter rises less than an hour before the Sun on the 1st; by the end of May, Jupiter has drifted further to the west of the Sun, and rises nearly two hours before sunrise. 

Information on lunar phases and rise/set times of Sun and planets is obtained from the US Naval Observatory Data Services at http://www.usno.navy.mil/astronomy/. Times given apply for observers near to the latitude and longitude of Philadelphia, USA: 40 degrees North latitude, 75 degrees West longitude.

For more information on astronomy and weather, visit 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/