The Night Sky: July 2009

Contents

The Night Sky in July 2009

By Harry J. Augensen
Professor of Physics & Astronomy, Widener University

Moon’s Phases in Month
  Full "Thunder" Moon on the 7thNew Moon on the 21st

Stars and Constellations

July evenings are not usually considered ideal for stargazing, especially in urban and suburban regions.  One has to wait until well after 9 pm for the sky to become really dark, and, moreover, the atmosphere is often hazy due to the abundance of water vapor and pollutants.  Nevertheless, the stars of summer are well worth the effort.  Usually the first star to be spotted as twilight fades into night is orange Arcturus, located high in the south.  Arcturus remains visible until it sets around 3 am at midmonth.  Although usually regarded as a springtime star, Arcturus remains in good position for evening viewing until October.  However, two other bright stars of spring are disappearing into the evening twilight.  Blue-white Regulus in the constellation Leo is heading toward the western horizon in early evening and sets around 10 pm in mid-July.  Further to the east lies Spica in Virgo. Spica has about the same color and brightness as Regulus, and it sets around midnight in the southwest.

While the spring stars are setting, the Summer Triangle, comprised of Vega in the constellation Lyra, Deneb in Cygnus, and Altair in Aquila, is rising in the east. Ascending in the south-southeast in early evening is Antares, the brightest star in the 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 listed 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 apparently dim star has the luminosity of over two hundred Suns, but is so far away that its 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 meridan 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 30, August 22, and October 17, 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.  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 in supernova within the next million or so years.

 Naked-Eye Planets In the Evening and Morning Sky

Saturn’s reign as an evening planet is nearly over for 2009.  Saturn sets around midnight at the start of July and at 10 pm, or less than an hour after twilight ends, by the end of July.  Look for Saturn low in the southwest shortly after sunset during July.   A telescope reveals that Saturn’s rings are nearly edge-on.  Jupiter takes over the mantle of evening prominence from Saturn this July.  At midmonth, Jupiter rises around 10 pm, which is just about an hour before Saturn sets.  Even novice sky watchers will have no trouble spotting Jupiter once it rises in the southeast, looking like a brilliant cream-colored star.

In the morning sky, Mars begins the month in fairly close proximity to Venus, which lies to its lower right.  Mars is far fainter than Venus, and will over the next several weeks and months be steadily pulling away from Venus, eventually making its way into the evening sky.  Mars rises in the northeast at around 2 am in mid-July, and is roughly the same brightness and color as the star Aldebaran, which lies a few degrees below it toward dawn.

Venus continues its dominance among both stars and planets in the pre-dawn sky, shining like a brilliant yellow beacon in the east-northeast.  Venus rises around 3 am, or nearly three hours before the Sun, for much of July.  Venus is paired with much fainter Mars at the beginning of July, but the two rapidly separate as the month progresses.  Mercury reached its greatest morning elongation in mid-June, and may still be visible as a yellow star low above the eastern horizon a half-hour or so before sunrise on the 1st.  However, Mercury rapidly sinks into the morning twilight during early July and reaches superior conjunction with the Sun on the 13th.

Earth reaches aphelion, or farthest distance in its elliptical orbit from the Sun, on July 3rd.  However, it is the tilt of the Earth's axis, not the orbital eccentricity, which causes seasons.

For more information on astronomy and weather, visit the Widener University Public Viewing Website at 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/

Some content for this article has been obtained from US Naval Observatory Data Services

Times given apply for observers near to the latitude and longitude of Philadelphia, USA: 40 degrees North latitude, 75 degrees West longitude.