The Night Sky: June 2012
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The Night Sky in June 2012
By Harry J. Augensen
Professor of Physics & Astronomy, Widener University
Full "Strawberry" Moon on the 4th New Moon on the 19th
Stars and Constellations
As night falls in June, the stars of spring shine prominently, and the first of the spring stars to emerge from the evening twilight is Arcturus, the night sky's fourth brightest star, in the constellation of Boötes, the Herdsman. Arcturus has a distinct yellow-orange tinge, and by around 9 pm EDT lies high in the south. Also easily found is bluish-white Regulus in the constellation Leo, the Lion. Regulus stands high in the southwest in early evening, and sets around midnight. Spica, a star which possesses a similar color as Regulus but is somewhat brighter, lies in the zodiac constellation Virgo, and stands below Arcturus, about halfway up in the south shortly after nightfall. Regulus, Spica, and Arcturus form the Spring Triangle, which, though not as famous as its summer counterpart, is nonetheless a noteworthy celestial threesome.
Later in the evening, as the stars of spring sink toward the western horizon, the stars of summer begin to take center stage. One of the most famous of these is blue-white Vega, in the constellation Lyra, which can be seen rising in the northeast. Another is reddish Antares, in Scorpius, which is low in the southeast. In mid-June, Antares rises around 7:30 pm (before sunset) and crosses the south meridian just before midnight. After about 10 pm at midmonth, two other stars are high enough to make out easily: Altair, in Aquila, which is rising in the east, and Deneb, in Cygnus, which is rising in the northeast. Vega, Altair, and Deneb comprise the familiar Summer Triangle.
One summer constellation that seldom gets attention is Draco, the Dragon, which winds almost halfway around the north star, Polaris. Draco represents the dragon which Hercules killed as one of his 12 labors. Draco is the eighth largest constellation (out of 88), but it is comprised mainly of relatively faint stars. One of Draco’s apparently faint stars is Thuban, or Alpha Draconis, which can be located using the two inner pointers of the Big Dipper bowl, Phad and Megrez, or by following the arc of the handle of the Little Dipper. (Recall that the two outer pointers, Merak and Dubhe, point to Polaris.) In any case, Thuban will be hard to locate in the light polluted city or suburbs. Thuban’s apparent faintness is due to its substantial distance of 310 light years. It has a surface temperature similar to Vega or Sirius, but it is even brighter intrinsically, with a luminosity of over 300 times that of our Sun. If the Sun were placed at the distance of Thuban, it would be far too faint to see with the unaided eye.
Thuban is showcased in University of Illinois astronomer James Kaler's book, The Hundred Greatest Stars (New York: Copernicus Books, 2002). Thuban is famous historically, because it was once the polestar for the Northern Hemisphere, back in 2700 BC, at the time of the construction of the Great Pyramid of Cheops. However, due to the precession of the equinoxes, which is produced by Earth’s slow wobble over a period of 26,000 years, Thuban gradually, over the centuries, moved away from the pole. Today, Polaris marks the north celestial pole, but Thuban will eventually, in about 21,000 years, become the polestar again.
Naked-Eye Planets In the Evening and Morning Sky
Mercury starts off the month of June in the evening sky, just barely peeking above the western horizon at dusk. On the 1st, Mercury sets a mere half hour after sunset, and so would be inconspicuous were it not for the fact that it is only about half a full-moon's width from the much brighter Venus; the pair should be easy to pick out, especially in binoculars. As the month progresses, Mercury extends its angle from the Sun, and is in excellent position for evening viewing throughout the remainder of June and into early July. By midmonth, and for the duration of June, Mercury sets nearly an hour and a half after the Sun, and should be fairly easy to pick out against the evening twilight: look for a bright yellow star low above the northwestern horizon. If you have never seen elusive Mercury before, this is a perfect time to make your acquaintance with it.
After having put on a spectacular show in the evening sky for the first half of 2012, by the beginning of June Venus is poised to vanish from the evening sky. Before that happens, shortly after sunset on the 1st, Venus and Mercury stand close together (about half the width of the full moon) very low in the west. In the late afternoon of the 5th, Venus transits across the face of the Sun, an historic event that will not be repeated until 2117. Afterwards, Venus swings swiftly into the morning sky, joining Jupiter. Toward the end of June, Venus is rising nearly two hours ahead of the Sun and can easily be spotted low in the northeast before sunrise.
Mars is now nearly twice as far from Earth as it was at closest approach in early March. Hence, it appears much fainter than it was three months ago. As June opens, Mars glows like an orange star situated to the left of the true star Regulus; both objects share residence in the constellation Leo, which stands high in the southwest during the mid evening hours. Mars sets at just before 1 am at the start of June, and before 11:30 pm at month's end. Toward the end of June, Mars crosses the border from Leo into Virgo, joining Saturn in that constellation. The two planets are of similar brightness (somewhat brighter than Virgo's Spica), but possess different colors.
Saturn remains in fine position for viewing this month. Saturn resembles a bright cream colored star located above the fainter, whitish star Spica, Virgo's brightest star. At the start of June, Saturn is located moderately high up in the south when evening twilight ends, and sets by 3:30 am. By the 30th, Saturn is setting by 1:30 am.
Jupiter, having reached conjunction with the Sun last month, now resides in the morning sky. As June opens, Jupiter is rising only 45 minutes before the Sun, and will be hard to find against the background of morning twilight, unless you employ binoculars. By month's end, however, Jupiter is rising over two hours before sunrise, and should, along with morning newcomer Venus, be easily visible with the naked eye low in the northeast before the onset of dawn twilight.
The Sun reaches the June Solstice on June 20th at 7:07 pm, when the North Pole of Earth is tilted maximally toward the Sun. Summer begins in the Northern Hemisphere, and winter in the Southern Hemisphere. It crosses the constellation border from Taurus into Gemini on the 21st.
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/