Contents

The Night Sky in June 2011

By Harry J. Augensen

Professor of Physics & Astronomy, Widener University

Moon’s Phases
New Moon on the 1stFull "Strawberry Moon" on the 15th 

Stars and Constellations

During June, twilight lasts until after 9:30 pm (EDT), although a few of the brighter stars (and planets) can be seen before that. Although summer officially begins this month, several stars of spring are still viewable during the early evening hours. One of these is bluish-white Regulus in the constellation Leo, which stands high in the southwest in early evening and sets around midnight. A star with similar color and brightness is Spica, which stands about halfway up in the south shortly after nightfall. Spica is the brightest star in the constellation Virgo, the second largest constellation in the sky (neighboring Hydra is the largest). Virgo, like Leo, is a zodiac constellation, which means the Sun passes within its boundaries at some point during the year (specifically between September 16 and October 30). Virgo was known to the ancient Greeks as Demeter, the goddess of agriculture and fertility. To modern astronomers, the constellation Virgo is of great importance because it encompasses several thousand distant galaxies known collectively as the Virgo cluster; some of these galaxies are over 60 million light years from our own galaxy, the Milky Way.

The first true star to become visible in evening twilight is Arcturus, the fourth brightest star in the entire sky, in the constellation of Boötes, the Herdsman. Arcturus has a distinct yellow-orange color, and at around 9 pm EDT lies high above Spica in the south. To find Arcturus and Spica, first locate the Big Dipper, and follow the arc of the Dipper’s handle outward until you encounter Arcturus, then continue on to meet Spica. Regulus, Spica, and Arcturus form the "Spring Triangle," which is larger than, though not as famous as, its summer counterpart.

To the east of Virgo is the next zodiac constellation Libra (the Balances). Its two brightest stars, Alpha and Beta Librae, have proper names from the Arabic: Zubenelgenubi and Zubeneschamali. These mean “southern claw” and “northern claw,” respectively, and refer to the fact that they were once part of neighboring Scorpius. Zubenelgenubi, or Alpha Librae, is featured in University of Illinois astronomer James Kaler’s book, The Hundred Greatest Stars (New York: Copernicus Books, 2002). It is a famous double star, lying at a distance of 77 light years. Zubeneschamali’s claim to fame is that through a telescope it appears to have a greenish color, and also that it may have faded significantly since ancient Greek times.

If the sky is especially dark where you live, try to locate the semicircle of stars representing the constellation Corona Borealis (Northern Crown) just a bit above and to the left (east) of Arcturus. In mythology, Corona Borealis represents the crown of Princess Ariadne, daughter of King Minos of Crete. The crown’s brightest star is blue-white Alphekka, also known as Gemma. To the left of Corona Borealis is Hercules, the fifth largest constellation in the sky. The brightest star in Hercules is alpha, also known by its proper name Rasalgethi. It is both a variable star and a double star.

Astronomical summer begins on June 21st at 1:16 pm, at which time the Northern Hemisphere of Earth is tilted at its greatest angle toward the Sun, thereby resulting in a higher concentration of solar radiation than at any other time of the year at Philadelphia’s latitude of 40 degrees. This, coupled with the fact that the length of daylight is also at its maximum, approximately 15 hours, is the underlying cause for seasonal changes.

Naked-Eye Planets In the Evening and Morning Sky

Saturn, the only naked-eye planet in the evening sky this June, continues to be in fine position for viewing as summer begins. As darkness falls in late evening, Saturn is already well up in the southeast, resembling a cream colored star well above Virgo’s brightest star, Spica. Saturn remains above the horizon until it sets at around 3 am in early June, and by 1 am on the 30th. As an added attraction this month, Saturn lies within a full-moon’s width from the star Gamma Virginis, also known as Porrima. Porrima, when seen through a telescope, reveals itself to be a beautiful double star with yellow-white twin components.

As June opens, Mercury lies in the early morning sky, rising less than one hour before the Sun and therefore is difficult to locate unless one has an unobstructed eastern horizon. Mercury sinks ever closer toward the eastern horizon during early June, and reaches superior conjunction with the Sun on the 12th; thereafter it shifts into the evening sky and begins to appear above the western horizon at dusk during the second half of the month. By the end of June, Mercury is setting over an hour after the Sun and can be spotted low in the northwest about 30 minutes after sunset resembling a bright star against the evening twilight, to the left of the true stars Pollux and Castor in Gemini.

Back in April, Jupiter was in conjunction with the Sun, and subsequently began to appear in the early morning sky. As June begins, Jupiter’s separation from the Sun has increased measurably, and hence so has its visibility. Look for Jupiter in the east-northeast before dawn begins; it resembles a very bright cream-colored star. Jupiter currently lies in the opposite part of the sky from Saturn, so that when one is rising, the other is setting. On the 1st, Jupiter rises nearly two hours before sunrise, or at about 3:30 am, well before the start of dawn twilight; by month’s end, Jupiter is up just after 2 am.

Mars, like Jupiter, is beginning to improve its visibility in the early morning sky. Mars rises in the northeast only about an hour before the Sun as June opens, but the optimal time to view it will be about 30 minutes later after it is well above the horizon. Mars is much fainter than any of the other morning planets – Mercury, Venus, and Jupiter, and will be difficult to pick out of the dawn twilight. By the 30th, Mars rises at about two hours before sunrise, and should be more easily identified against a darker sky.

 Venus’s low elevation above the northeastern horizon at dawn has not changed much since last month. To see Venus, find a location where buildings or trees do not obstruct the view, and look about 30 minutes before sunrise. Venus rises roughly one hour before the Sun all month, but this interval is slowly decreasing; by August, Venus will vanish into the morning twilight.

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/