The Night Sky in May 2012
By Harry J. Augensen
Professor of Physics & Astronomy, Widener University
Full "Flower" Moon on the 5th New Moon on the 20th
Stars and Constellations
The month of May offers starlit nights which are often pleasantly cool, with just a light jacket required for outdoor viewing. But with the Sun setting around 8 pm or later during the month, you will need to wait until nearly 9 pm for the sky to get dark enough to make out the constellations. Take one last look at Aldebaran and the nearby Pleiades star cluster, along with Rigel and Betelgeuse, Pollux and Castor, and Sirius and Procyon. Bright yellow Capella is getting lower in the northwest, but will still be visible through June. These bright stars of winter are fading into the evening twilight, not to reappear in the night sky until autumn.
The celestial stage now belongs to 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, is now high in the northeast, and its pointer stars Merak and Dubhe, point to the North Star, Polaris. Mizar, the second star from the end of the Big Dipper's handle, actually possesses a fainter companion star, Alcor. Spotting Alcor was used as a test of vision in ancient times.
Regulus, the bright blue-white star in Leo, lies high in the south-southeast, with the planet Mars to its left. Another fairly bright star, Alphard, in the constellation Hydra, the Water Snake, is now rising, a bit below Regulus. After about 10 pm, yet another bright star, Spica, in the constellation Virgo, can be seen in the southeast, with Saturn to its upper left. Regulus, Spica, and Arcturus form a "Spring Triangle," which, though not nearly as famous as the soon-to-rise Summer Triangle (consisting of Vega, Altair and Deneb), is larger and fairly easy to discern during the evening hours of May. This year, the Spring Triangle is contaminated by two planets lying on or near its perimeter: Mars and Saturn (see below).
One of the largest and most spectacular constellations in the night sky at this time of year is Centaurus (Chiron in Greek mythology) but unfortunately only the uppermost portions of this group can be glimpsed from latitudes north of the Gulf Coast states. Centaurus lies just below the tail end of Hydra, and skims the southern horizon around midnight in May. Like Orion, Centaurus boasts two first-magnitude stars: alpha and beta Centauri. Alpha Centauri, also known as Rigel Kentaurus, is a triple-star system, and has the distinction of being our Sun's nearest stellar neighbor, at a distance of 4.3 light years. Even more interesting is that the brightest of the three components, alpha Centauri A, is nearly identical in chemical composition and intrinsic brightness to the Sun. Beta Centauri, by contrast, is a blue-white giant star many times larger than the Sun and it lies much farther away than alpha, around 200 light years. If your travels take you to Mexico, Hawaii, or, even better, South America, southern Africa, Australia, or New Zealand, watch for brilliant Centaurus in the night sky in May.
Naked-Eye Planets In the Evening and Morning Sky
Jupiter's residence in the evening sky comes to a close this May, when it vanishes into the evening twilight. As May opens, Jupiter is setting only a little over half an hour after sunset, and is located well below the much brighter Venus. Jupiter reaches conjunction with the Sun on the 13th, and then shifts into the morning sky. By month’s end, Jupiter rises a little more than a half hour before sunrise, but it won't really become noticeable to the unaided eye until the end of June.
Venus continues to dazzle the eye during the first half of May, shining like a brilliant yellow star high over the western horizon shortly after sunset. At the beginning of May, Venus sets about three and a half hours after sunset, which is around 11:30 pm EDT. As the month progresses, however, the angle between Venus and the Sun shrinks rapidly, so that by month's end Venus is setting less than an hour after the Sun. A small telescope or even strong binoculars will reveal that Venus has a distinct crescent phase, similar to the Moon. By early June, Venus will have all but vanished from the evening sky, although it can still be spotted through binoculars or a telescope. Note that in the late afternoon of the 5th, there will be a rare transit of Venus across the face of the Sun. Toward the end of June, Venus swings into the morning sky, joining Jupiter.
Mars remains in good position for evening viewing during May, although the Red Planet continues to fade after reaching its maximum brightness and closest approach to Earth back in early March. Mars glows like an orange-red star situated to the left of the fainter blue-white star Regulus; both objects are located in the constellation Leo, which rides high in the south during the early evening hours of May. A moderate-size telescope will reveal distinct surface markings on the Martian surface. Mars sets at around 3:30 am at the start of May, and before 1 am at month’s end.
Saturn reached opposition with the Sun last month, but it remains in fine position for viewing this month. Saturn resembles a bright cream colored star located to the upper left of the fainter, whitish star Spica, Virgo's brightest star. At midmonth, Saturn is located due south around 11 pm, and sets by 4:30 am.
Mercury rises in the morning sky roughly 45 minutes before sunrise at the beginning of May, but its appearance is fleeting, and by midmonth it vanishes into the dawn twilight. At the very end of May, Mercury pops up low in the western sky at dusk, and will subsequently be in excellent position for evening viewing throughout June.
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/. Additional information comes from Astronomical Calendar 2012 by Guy Ottewell (Raynham, MA: Universal Workshop, 2012)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/