August 2011

Moonlight masks shooting stars of August

This is the time of year when we notice that the days are shorter than just a few short weeks ago. For many this can be disappointing as they enjoy the extra daylight for outdoor activities. In Thunder Bay we have lost just over an hour of sunlight from the beginning of July to the start of August. By September the night has gained 2 hours and 37 minutes. However, for us stargazers, this decrease in daylight means that we can get out under a dark sky a little earlier in the evening.

Mention “August” to even the casual stargazer and the falling stars of the Perseid meteor shower come to mind. This is one of four major (and at least 12 minor) showers of the year. This year the Perseids are predicted to peak the night of August 12-13.

Normally it is possible to see up to 90 Perseid meteors per hour under a moonless and dark sky location. Unfortunately this year the full moon at that time will brighten the night sky to the point that it washes out all but the brightest of the meteors.

As comets orbit the sun, they shed an icy, dusty debris trail along their orbit. If the Earth travels through this stream, we will see a meteor shower. Depending on where the Earth and the stream meet, meteors appear to fall from a particular place in the sky within the neighborhood of a constellation. Meteor showers are named by the constellation from which meteors appear to fall, a spot in the sky called the radiant. The Perseid meteor shower is so named because meteors appear to fall, or radiate, from a point in the constellation Perseus.

“Shooting stars” and “falling stars” are both names that people have used for hundreds of years to describe meteors as they streak through the darkness of the night sky. Stars do not “shoot” or “fall”, if they did there would be none left to see. Meteors are those intense streaks of light across the night sky caused by small bits of interplanetary rock and debris called meteoroids running into and burning high up in the Earth’s upper atmosphere. Most last for an instant but some larger particles can leave a train (a persistent smoke-like trail of glowing, hot ionized gas) lasting several minutes. Traveling at roughly 60 kilometers per second these meteoroids quickly heat, glow white hot and vapourize due to friction of the upper atmosphere (50 to 150 kilometers above the ground). Almost all are destroyed in this process. The very rare few that survive and reach the ground are called meteorites. The comet responsible for producing the Perseid meteors is the debris left behind by comet 109P/Swift-Tuttle during its 133 year trip around the sun.

Jupiter’s bright yellow glow at magnitude -2.4 makes it easy to identify. It can be seen rising an hour after midnight above the eastern horizon at the beginning of August. Its bright yellow glow is easily identified amongst the much fainter stars of the constellation of Aries.

The Red Planet Mars rises in the east around 3:30 a.m. at the beginning of August. It will spend the month moving through the constellation of Gemini. Its brightness of magnitude 1.4 will compete with the two bright stars of Gemini, Castor (slightly fainter than Mars) and Pollux (noticeably brighter than Mars).

Comet C/2009 P1 (Garradd) is still quite faint at magnitude 8.0. Moving through the constellations of Pegasus, Delphinus and Sagitta it continues to brighten throughout August. If current predictions hold it could reach unaided eye visibility (magnitude 6) early in 2012.

For interested stargazers who happen to be at Dawson Trail Campgrounds (Quetico Provincial Park) on August 26 and 27 the Thunder Bay Centre of the Astronomy Society will be holding their 25th Annual Stargazing Weekend there. There will be an astronomy presentation each of these evenings. The first talk will be “Astronomy Myths and Misconceptions Explained”. The next evening’s presentation will be “Exploring the Universe: From Suns to Black Holes”. If the weather permits, we will do a sky tour pointing out various constellations and brighter stars after these presentations.

Clear Skies

Ted Bronson