The Night Sky: July 2012

June 29, 2012, 6:11 pm
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The Night Sky in July 2012

By Harry J. Augensen

Professor of Physics & Astronomy, Widener University

 

Moon’s Phases
            Full "Thunder" Moon on the 3rd                          New Moon on the 19th

Stars and Constellations

As July opens, most of the spring stars are disappearing into the twilight, not to return to the evening sky until early in 2013.  Bluish-white Regulus, in the constellation Leo, is moving toward the western horizon in early evening and sets around 10 pm in mid-July.  Of similar color but slightly brighter is Spica, in Virgo, which sets in the southwest around midnight.  Yellow-orange Arcturus is usually the first star to be spotted as twilight fades into night.  Arcturus, in the constellation Boötes, is located high in the south in early evening, and remains visible until it sets around 3 am at midmonth.  Although it is considered a spring star, Arcturus will remain visible in the early evening through October.

 Regulus, Spica, and Arcturus comprise the Spring Triangle, which this year is accented by two bright interlopers: the planets Mars and Saturn.   As this bright trio of spring stars slowly descends toward the western horizon, a trio of summer stars is rising above the eastern horizon.  This is the famous Summer Triangle, comprised of Vega in the constellation Lyra, Altair in Aquila, and Deneb in Cygnus, in descending order of stellar brightness.  All three stars are whitish or bluish-white in color.

Ascending in the south-southeast in early evening is Antares, the brightest star in the magnificent summer constellation Scorpius. The name Antares translates to mean "rival of Mars," because of the similarity of its reddish color to that of the Red Planet.  Antares is featured in University of Illinois astronomer James Kaler's book, The Hundred Greatest Stars (New York: Copernicus Books, 2002).  Antares is classed as a red supergiant, with a diameter of over 800 times that of the Sun, a visual luminosity of 12,000 suns, and lies at a distance of over 600 light years from the solar system. A moderately large telescope reveals that Antares possesses a faint companion star, Antares B, of blue (some say green) color, but which is hard to see since it lies in the glare of the much brighter Antares A.   In reality, this comparatively dim star has the luminosity of over two hundred Suns, but is so far away that its apparent brightness is considerably diminished.  Astronomers have determined that Antares A and B are locked in orbit about a common center of gravity in a period of 878 years.

Antares lies within a few degrees of the eclipic, the apparent path that the Sun traces out on the celestial map during the year.  On around December 1st of each year, the Sun passes just north of  Antares.  This means that Antares is opposite to the Sun, and hence crosses the meridian at midnight (standard time) six months earlier or later, around June 1st.  Three other first-magnitude stars – Aldebaran (in Taurus), Regulus, and Spica – are similarly positioned along the ecliptic at various points, with the Sun passing near to them on or about May 30th, August 22nd, and October 17th, respectively.

Scorpius is one of the oldest constellations, dating back to several centuries BC. According to one myth, Orion the Hunter boasted that he could hunt down any creature on Earth.  Angered by this remark, Artemis, the goddess of the hunt, sent the Scorpion to kill Orion.  The Scorpion caught up with Orion, and mortally stung him in the foot.  Afterwards, the gods placed both Orion and the Scorpion in the sky, but on opposite sides to keep them apart, so that when one of the two is rising, the other is setting.  As it happens, Orion contains the only other first magnitude red supergiant star in the sky – Betelgeuse.  Antares and Betelegeuse are very similar in their properties.  Both are massive, enormous stars, reddish in color and variable in light output.  Both were once hot, luminous blue stars that bloated up into cooler red supergiants as their nuclear fuel ran out and are now in an advanced stage of evolution.   Both will most likely explode as supernovae within the next million or so years. 

 

  Naked-Eye Planets In the Evening and Morning Sky

 After a very favorable appearance throughout much of June, Mercury remains in relatively good position for viewing in the evening sky as July opens, hovering low above the western horizon at dusk.  On the 1st, Mercury sets about an hour and a half after the Sun, and should be fairly easy to pick out against the evening twilight:  it looks like a bright yellow star low above the northwestern horizon.  The viewing circumstances decline rapidly thereafter, as Mercury sinks toward the horizon and simultaneously fades in brightness.   After about a week, Mercury will be setting only an hour after the Sun, and will be difficult to locate without binoculars.  By midmonth Mercury will be setting only 30 minutes after the Sun, and with much reduced brightness, it will have essentially vanished from the evening sky.  Inferior conjunction with the Sun awaits Mercury on the 28th.

 Saturn resembles a bright cream colored star located above the fainter, bluish-white true star Spica.  At the start of July, Saturn is moderately high up in the south when evening twilight ends an hour or so after sunset.  Farther to the right, Mars glows like an orange star low in the southwest.  Having just crossed over the constellation border from Leo in late June, Mars resides at the far western end of Virgo. As July opens, Mars is positioned between the planet-star pair of Saturn and Spica in eastern Virgo and the star Regulus in Leo.  Mars and Saturn remain up until just before 12:30 pm and 1:30 pm, respectively, at the beginning of July.  By the end of the month, Mars will have drifted much closer to the Saturn-Spica pair, forming a neat triangle of orange Mars, yellow-white Saturn, and bluish-white Spica.  Mars and Saturn both set before midnight, at around 11 pm and 11:30 pm, respectively, on the 31st.

 After its historical transit across the face of the Sun in early June, Venus has taken up residence in the morning sky, joining Jupiter.  Jupiter resembles a very bright golden star located above the brilliant Venus. The Jupiter-Venus pair should be easily visible with the naked eye low above  the northeastern horizon before the onset of dawn twilight. As July opens, both Venus and Jupiter are rising roughly two hours ahead of sunrise.  As the month progresses, the separation angle between Venus and Jupiter increases steadily, and at the same time both planets extend their distance from the Sun.  By the end of the month, Venus and Jupiter are rising, respectively, over three and four hours ahead of the Sun, so Jupiter has ascended well above Venus.

Earth reaches aphelion, or farthest distance in its elliptical orbit from the Sun, on July 4th.  Earth was closest to the Sun, or at perihelion, back in January. 

 

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/

Glossary

Citation

Augensen, H. (2012). The Night Sky: July 2012. Retrieved from http://www.cosmosportal.org/view/blog/177241