Contents

The Night Sky in July 2010

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

Moon’s Phases in Month
  New Moon on the 11th                       Full "Thunder" Moon on the 25th

Stars and Constellations

July evenings present significant challenges for stargazing, not only because they are often hazy but also since you have to wait until after 9 pm for the sky to become dark enough to make out the constellation patterns.  As twilight fades into night, you should still be able to spot a few of the bright stars of spring before they vanish into the evening twilight. Blue-white Regulus in Leo is moving toward the western horizon and sets around 10 pm in mid-July, followed a couple of hours later by similarly colored Spica in Virgo. The brightest of the visible stars is orange Arcturus, in the constellation Boötes , which is high in the south during the early evening hours.  Although it is considered a star of spring, Arcturus will remain in view until early autumn.  Arcturus, Regulus, and Spica form the Spring Triangle, and July is the last month to catch a glimpse of it until it reappears in the morning sky next year.

 As this trio of spring stars is setting in the west, another trio of summer stars is rising in the northeast.  Making their appearance above the eastern horizon are Vega, Deneb, and Altair, located respectively in the constellations Lyra, Cygnus, and Aquila.  These three stars comprise the famous Summer Triangle, which, though smaller in sky area than its spring counterpart, is more famous. 

Located between the constellations Boötes and Lyra is Hercules, which is identified by its “keystone” of four stars.  Perhaps the most notable star in Hercules is Ras Algethi (Arabic for the “Kneeler’s Head”), a supergiant star with a diameter of several hundred times that of the Sun and  lying about 400 light years from the Sun.  In 1783, English astronomer William Herschel determined, from a careful study of star motions, that our entire solar system is traveling through our Galaxy, the Milky Way, and headed toward a point in Hercules known as the apex of solar motion.  Modern analyses have since shown that the apex lies closer to Vega in Lyra, but Herschel’s studies were nevertheless ground-breaking for astronomy.

Moving into view in the south-southeast in early evening is reddish Antares in the constellation Scorpius. Antares is a red supergiant and one of the largest stars known, having a diameter nearly 800 times larger than the Sun, and lying over 600 light years from our solar system.  A little later in the evening, the constellation Sagittarius, with its famous “Teapot” asterism will be rising in the southeast, to the left of Antares.

Located directly above Scorpius and below Hercules is Ophiuchus, the Serpent Bearer.  Its brightest star, Ras Alhague (translated as “Head of the Serpent Holder”) lies just a few degrees to the east of Ras Algethi in Hercules.  Curiously, though, the most celebrated star in Ophiuchus is too faint to see with the unaided eye.  That star is Barnard’s Star, a faint red dwarf discovered in 1916 by astronomer Edward Emerson Barnard  at the Yerkes Observatory in Wisconsin.  By comparing photographic plates taken decades apart, Barnard ascertained that the star  is moving rapidly across the sky, a sure indication that it is nearby.  Parallax studies have shown that Barnard’s star is 6 light years from our solar system, making it the second closest star after the Alpha Centauri triple system.  Barnard’s star is also much redder and fainter than the Sun, with one-fifth its diameter, or about  twice that of the planet Jupiter. Due to its swift motion across the sky, Barnard star’s will in several thousand years migrate from Ophiuchus into Hercules.

 Naked-Eye Planets In the Evening and Morning Sky

Mercury begins to appear above the western horizon during July, and sets about an hour after the Sun for most of the month, but it will be quite low and difficult to spot against the bright evening twilight. Toward the end of July, Mercury passes close to Regulus, and the pair should be easily visible in binoculars.  Much brighter Venus maintains its dominance in the evening sky, sparkling like a yellow gem in the west at dusk.  Venus has actually been gradually increasing in brightness over the past few weeks, a trend that will continue into early autumn as Venus’s orbital motion brings it closer to Earth.  But Venus will also be slowly and inexorably sinking toward the western horizon. Venus sets two and a half hours after sunset (about 11 pm EDT) at the beginning of July, but only two hours after the Sun by month’s end, at which time a telescope will reveal a nearly “quarter phase.”  During the second week in July, Venus passes just above Regulus, making for a lovely sight in binoculars.  The apparent proximity is, of course, a consequence of perspective, since Regulus is 15 million times farther away than Venus!

Mars, mimicking a bright orange star in the eastern part of Leo as July opens, continues its eastward motion, eventually migrating into Virgo and, at month’s end, meeting up with Saturn.   Mars’s brightness has now faded to about the same as that of Regulus, which it passed close to last month.  During July, Mars remains in view until late evening, setting around 11 pm at midmonth.  To the east of Mars is Saturn, which resembles a bright yellow star roughly halfway between the true stars Regulus and Spica.  Saturn’s brightness has diminished to where it is now comparable with  that of Spica, Regulus, and Mars.  At the end of July, Mars will pass just below Saturn, making for a pretty sight in binoculars or a small telescope.  Saturn remains in good position for viewing during the early evening hours of July, setting around 12:30 am on the 1st and by 10:30 am on the 31st.

As Saturn and Mars are setting in the west, Jupiter is rising in the east.  Jupiter, resembling a brilliant cream-colored star in the faint constellation Pisces, is second only to Venus in brightness among the planets.  This July, Jupiter moves into the evening sky, rising by 11:30 pm at  mid-month.

Planet Earth reaches aphelion, or its farthest distance in its elliptical orbit from the Sun, on July 6.  Note that it is the tilt of Earth’s axis, not its orbital shape, which causes the changing 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.