Wyoming Skies for July

June 29, 2010

A monthly look at the night skies of Wyoming, written by Ron Canterna, professor in the University of Wyoming Department of Physics and Astronomy.

Comet McNaught, discovered in 2009, will make its closest approach to the sun July 2. It can be seen in the early morning the first two days of July, but later this month will be seen right after sunset. It is the 51st comet discovered by Robert McNaught of Australia.

If you do not catch the comet our July night skies offer other spectacular views. On the southern horizon the constellation Scorpius can be seen. Look for a red supergiant (Antares) and above Scorpius Ophiuchus, "the serpent-bearer." It accompanies the fainter and smaller constellation Serpens, "the snake."

Nearly overhead, one hour after sunset, is the constellation Hercules, "the strongman." Toward the east, Cygnus, "the swan," guides the eye through a major section of the plane of our Milky Way galaxy. Midway between Cygnus and Hercules is the brilliant bluish-white star Vega. If you follow Cygnus toward the north on the northern horizon you will find Cassiopeia, appearing as a great "W" in the sky. The Milky Way is brilliantly displayed through the constellations Scorpius-Cygnus-Cassiopeia.

Planet alert: Keep you eye on the evening star Venus, Mars and Saturn on the western horizon during the July sunsets.

July 2010 Interest: Comets I (By guest writer Jay Norris, Idaho)

Comets have been witnessed and their passages recorded by ancient civilizations for millennia, but only understood since the time of Isaac Newton to be celestial objects that belong to our solar system.

In the late 1700s Edmund Halley visited Newton to discuss the nascent theory of orbital mechanics, a theory that Newton claimed to have developed but "misplaced" in his papers. The periodic comet under discussion came to bear Halley's name, and their collaboration resulted in Halley assisting in the publication of Newton's general theory of mechanics, the Principia -- the mechanical system of the world. This theory quickly became physics gospel for more than two centuries until the scope of mechanics was expanded and refined by Einstein's special and general theories of relativity.

Having monitored comet orbits for the intervening years, astronomers now understand that they fall largely into two groups: Short-period orbits that take these visitors near and past the orbit of Neptune, and long-period orbits that arrive from the Oort Cloud, the hypothetical repository of pristine comets that have rarely or never visited the inner solar system.

The distance scales of the two comet groups are vastly different. Neptune lies about 20 astronomical units from the sun (one A.U. equals the Earth-sun distance). Whereas, the Oort cloud is reckoned to be 50,000 A.U. from the sun, slightly more than one light-year in distance. This is approximately the limit of the sun's gravitational "sphere of influence." Comets lying beyond this region would migrate between our sun's domain and that of other nearby stars.

Next month Wyoming Skies will expand its geographical audience to the states of Idaho and Montana. Our new column, to be written by me, Dr. Jay Norris, and Dr. Daryl Macomb, Boise State University, will be named Northern Rocky Skies. I look forward to continue to offer my fellow Wyoming citizens a grand view of the night skies.

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