Northern Rockies Skies for August

July 30, 2010

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis Idaho Observatory; and Daryl Macomb, Boise State University

August has always been a wonderful time to view the spectacular wonders in our Milky Way galaxy, see a meteor shower in the constellation Perseus, and observe the interesting passage of the planets.

This August provides another spectacular opportunity -- a major conjunction of Mars, Venus and Saturn with the crescent moon as a backdrop. Get out your binoculars and your star charts.

Our Milky Way -- this subtle, hazy band of stars -- rises high in the southern sky. After sunset start looking directly south into the constellation Scorpius -- the scorpion with its brilliant red star Antares -- and Sagittarius, the teapot.

Directly overhead the bright star Vega (constellation Lyra) and the northern cross (or Cygnus the swan) dominate the evening skies. They also mark the location of the Milky Way. Here is where you will see many star clusters with binoculars. Toward the northeast horizon Cassiopeia, the big "W" in the sky, marks the northern exposure to the Milky Way.

This month's planetary treat happens on Aug. 13 when Mars, Venus and Saturn come closest together with the waxing crescent moon slightly to the east of this grand grouping.

Finally, another solar system spectacular, the Perseid meteor showers, will last up to four days centered around its peak date, Aug. 13. You may see up to 50 meteors per hour. Perseus can be located in the northeastern night sky after midnight. You will have to see the meteor shower after midnight, right after the late night TV shows.

August 2010 Northern Rockies Skies Interest: Comets II

There are two groups of comets, long period and short period comets. Short period comets lie near Neptune's orbit. There may be as many as a trillion long-period comets in the Oort Cloud repository, which lies more that 50,000 A.U. (one A.U. equals the Earth-sun distance) from the sun (slightly more than one light-year in distance from the sun).

The gravitational influences of the nearby stars occasionally perturb the motion of an Oort-cloud comet so that it begins a long journey toward the inner solar system -- inexorably attracted to the sun -- a journey that can last thousands to millions of years. Once the comet nears the inner solar system, Jupiter and the other giant planets may sufficiently perturb the comet's orbit, capturing and adding it to the population of short-period comets. The period of the comet is then reduced to tens or a few hundreds of years.

The most recent observations of comets suggest that they are made of solid, rocky materials -- like small asteriods -- with frozen gases imbedded in sub-surface layers. Comets may make several orbits close to the sun, which by its radiation drives gases and lighter dust particles to heat up and erupt from below the comet's surface. These materials form the coma -- the head of the comet -- and are driven by the solar radiation away from the sun, forming the comet's tail.

After many passes near the sun, the comet's volatile materials are exhausted and we no longer see the comet glow. An occasional comet may suffer the ultimate "death" by plunging into the sun. In the spectacular case of Shoemaker-Levy 9, the comet ended its life breaking apart in the gravitational field of Jupiter, with the pieces colliding with the giant planet.

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