The Night Sky in August
During August, our planet passes through debris strewn around the orbit of comet 109P/swift-tuttle. The particles, or meteoroids, encountered tend to be small, typically about the size of a grain of sand. Although on parallel orbits, a perspective effect makes it look like the meteor trails produced emanate from a small region of sky. This is a meteor shower and the apparent point of origin is known as the shower radiant.
This particular shower produces meteors from July 23 to August 23 from a radiant drifting slowly in position from the W-shaped constellation Cassiopeia, eastwards into Camelopardalis. The number of meteors produced is low during much of the period. However, during the nights around August 12, we pass through the densest part of the stream and the number of meteors increases. The radiant is in Perseus when this happens so this is known as the Perseid meteor shower.
A meteor shower’s activity is expressed by its zenithal hourly rate (ZHR). At its peak on August 12/13, the Perseid shower will have a ZHR of 80-100 meteors-per-hour. This normalised value is calculated on the assumption that the radiant is directly overhead, conditions are perfect and that the whole sky can be seen in one go. In reality few of these conditions are ever met and the visual rate is significantly lower than the published ZHR.
A bright Moon will interfere this year. The best viewing strategy will be to find as dark a location as possible and centre your view approximately two-thirds up. Look in any direction where the Moon cannot be seen.
This year’s Perseid maximum occurs less than two weeks before a total eclipse of the Sun set to be seen along a 71 mile (115km) wide track which crosses the US. Dubbed the “Great American Eclipse”, this event takes place on August 21 and is likely to be the most observed and photographed total solar eclipse in history. The conditions leading to a particular eclipse almost repeat 18 years 11.3 days later, a period known as the Saros cycle. The Aug 21 2017 event will be the next eclipse in Saros series 145. The last eclipse in this series crossed the Cornish peninsular in August 1999. This time the UK’S experience won’t be as dramatic. The eclipse starts around 19:40, maximum eclipse is around 20:02 and it will end at 20:26. These are for the centre of the UK and vary by location. The Sun also sets before the eclipse ends from some locations. View safely and never look at, or point any instrument at the Sun unless through a certified solar safety filter.
The Moon is new at the point of a total solar eclipse and for those more interested in the night sky, this is the ideal time to enjoy the darkness. The three bright stars that form the Summer Triangle dominate the view high to the south as night falls during August. The most southerly star in the triangle is Altair – the brightest in Aquila, the Eagle – and notable as it appears flanked by two dimmer stars; Tarazed above and Alshain below.
North of Altair is a pattern representing Sagitta, the Arrow. In mythology the immortal Titan Prometheus took it upon himself to give mankind the gift of fire without Zeus’s permission. As punishment, Prometheus was chained to a mountain where he was attacked by Aquila. The eagle tore into the Titan’s flesh and ate his liver. Being immortal, it grew back leaving Prometheus to suffer repeatedly. Release came when Hercules killed Aquila with the arrow Sagitta.