The Shropshire Sky At Night – August
With the moon dust settling as the excitement of the Apollo moon landing anniversary wanes, the Shropshire Astronomical Society returns to searching the summer skies for interesting objects.
Dark skies suitable for observing the night sky lie between astronomical dusk and astronomical dawn, which occurs when the sun is 18 degrees below the horizon.
At this time of year in early August this gives an observing window of approximately two and half hours starting just before midnight.
As we have passed the summer solstice this window will expand, but it is still very much antisocial hours for casual stargazers. However there are two sights worth searching out for – namely the Perseid meteor shower and Noctilucent clouds.
Between now and August 20 the Earth is passing through the tail of the comet Swift-Tuttle, a periodic solar system object 26km in diameter that orbits the sun every 133 years. Very much like a dirty snowball, Swift-Tuttle, made of ice and dust, was was last seen in 1992.
As it travels through the solar system it leaves a trail of particles in its wake. Every year the Earth passes through this trail of microscopic debris, most no bigger than grains of sand. Astronomers refer to these particles as meteoroids.
If the conditions are right some of these meteoroids try and penetrate the Earth's atmosphere producing streaks of light across the night sky. These meteors or shooting stars can be as many as 100 per hour under perfect conditions on August 11, 12, and 13. Just lie back in your sun lounger and look up at an angle of 45 per cent in a north easterly direction and enjoy the show.
The best time is after midnight but with the moon in sight all night it will hide all but the brightest meteors. Keep your eyes peeled though, as any that do fall to Earth are called meteorites and are prized by collectors.
Alternatively, looking skywards 60 to 90 minutes after the sun has set and you might be lucky enough to observe the spectacular neon blue streaks referred to as Noctilucent, or night shinning, clouds, high in the sky. Like ordinary clouds they form due to water vapour. The very cold temperatures above 80km produce the reflective ice crystals which contain dust from space and meteors, although particles from volcanoes and pollutants may also make a contribution.
The light from the sun reflects off the ice particles producing the effect.
About the same time in the darkening skies the brighter stars start to emerge. The first, almost directly above, will be Vega in the constellation of Lyra. It is soon followed by Deneb and Altair in the constellations of Cygnus and Aqulia, respectively, to form the huge asterism known as the summer triangle.
Astronomers use such asterisms as signposts to other interesting objects such as the Andromeda galaxy, the most distant object that can be seen with the naked eye at 2.5million light years.
Many star maps are available in books or downloadable, but learning the night sky is best done in groups.
The Shropshire Astronomical Society meets on the third Saturday of each month at Rodington Village Hall and always welcomes experienced or novice stargazers. For further information visit www.shrophire-astro.uk
Steve Szwajkun, Shropshire Astronomical Society