The Night Sky in April 2012
By Harry J. Augensen
Professor of Physics & Astronomy, Widener University
Full "Egg" Moon on the 6th New Moon on the 21st
Stars and Constellations
As the month of April begins, the beautiful stars of winter are gradually disappearing from view in the evening sky. The days have gotten longer, and so the bright stars and planets do not emerge from the evening twilight until well after 8:30 pm. Sirius, the brightest star of all in the night sky, is now setting in the southwest, not to reappear until next autumn. Aldebaran and the Pleiades cluster in Taurus are setting in the west, as are Betelgeuse and Rigel in Orion. The twin stars Pollux and Castor in Gemini are high in the south-southwest, to Orion's upper left. Procyon in Canis Minor also follows Orion. And yellow Capella is still quite prominent, high in the northwest.
As the stars of winter vanish into the evening twilight, the stars of spring are emerging from below the eastern horizon. By 9 pm, the constellation Leo is high in the southeast, and its brightest star Regulus, or Alpha Leonis, shines with a slightly bluish white tinge, compared with the distinctly orange-red color of nearby planet Mars. Regulus, which lies 77 light years from our solar system, actually consists of four stars, even though it looks like a single object to the naked eye. The main component of the Regulus system possesses a diameter of about 4 times that of the Sun, and is intrinsically nearly 300 times more luminous than the Sun. Regulus lies close to the ecliptic, the same path that the Sun follows throughout the year. The Moon also follows the ecliptic fairly closely, and occasionally it occults (eclipses) Regulus.
In addition to Regulus, Leo contains a prominent feature known as the "sickle," which consists of several stars, the brightest of which is Algieba, or Gamma Leonis. Algieba is, as a telescope reveals, a beautiful double star. The two stars are both giants, the brighter being orange and the companion yellow. Their diameters have been deduced to be, respectively, 23 and 10 times larger than our Sun. The two orbit each other in a period of approximately 500 years, and the entire system lies 126 light years away from our Sun.
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. When the Dipper is high enough you can spot Alcor, the faint companion to Mizar, the second star from the end of the Big Dipper's handle.
After around 10 pm, another bright star, bluish-white Spica, in the constellation Virgo, rises in the southeast followed closely by the planet Saturn. 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 April. This year, the triangle contains a very noticeable interloper, lying just to the left of Regulus – the planet Mars.
In 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 month of April, that time is around 12 midnight, 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
The giant planet Jupiter, which had been such a glorious fixture of the evening sky since last autumn, can still be seen, resembling a bright star, low in the west. As April begins, Jupiter glows brightly well below the much brighter Venus, during the evening hours. On the 1st, Jupiter sets in the west around 10 pm EDT, or about two and a half hours after sunset. However, Jupiter is sinking closer toward the western horizon with each passing night, and by month’s end, it is setting a little after 8:30 pm EDT, or less than an hour after the Sun. In May, Jupiter will reach conjunction with the Sun, and then move into the morning sky, so that it can delight early risers throughout the summer and fall months of 2012.
This April, magnificent Venus radiates like a yellow beacon high over the western horizon shortly after sunset, and continues to dominate the evening sky until it sets at around 11:30 pm EDT. Venus reached its greatest evening elongation with the Sun in late March, and it continues to put on a celestial show that is nothing short of spectacular. Venus sets its maximum four hours after the Sun as April begins, and about three and a half hours after sunset at month's end. As a bonus, Venus passes within a full-moon’s width of the Pleiades star cluster in early April. Binoculars will show both Venus and the Pleiades in the same field of view. On the 30th, Venus reaches its maximum brilliance. However, Venus’s days as an evening showpiece are numbered (at least for 2012); during May, Venus will plunge toward the horizon, so that by the beginning of June it will have all but disappeared from the evening sky. Venus (much like Jupiter) will then swing into the morning sky where it will remain through the end of the year.
Venus is so bright that it can be seen in the daytime. Try spotting it high in the south, not quite overhead, preferably around 4 or 5 pm, when Venus is near the meridian (due south) and the Sun is well past it. Choose a location shaded from the Sun by a tree or building to the west, but with a clear view of at least the upper half of the sky above the southern horizon. Binoculars will help greatly.
Mars glows like an orange-red star high in the south in the constellation Leo during the early evening hours of April. After reaching opposition with the Sun early in March, Mars is now being left behind as the Earth pulls away in its orbit. Consequently, Mars's brightness diminishes with each passing week., so that by the 30th it will be only about half as bright as it was on the 1st. At midmonth, Mars will be roughly the same brightness as the star Arcturus, and much brighter than nearby Regulus to its right.
Saturn reaches opposition with the Sun on the night of the 15th, at which time it will rise as the Sun sets, and remain visible all night long. Saturn resembles a bright cream colored star located to the left of the fainter, whitish star Spica, Virgo's brightest star. A small telescope or even powerful binoculars will reveal Saturn's fine ring system.
Mercury is up in the morning sky roughly 45 minutes before sunrise all month, but this month's apparition is a very poor one. On the 18th, Mercury reaches greatest morning elongation with the Sun, but even then it will be difficult to spot low above the eastern horizon unless you use binoculars.
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/