The Night Sky in August
Comets bright enough to be seen with the naked eye are pretty rare, so the appearance of bright comet C/2020 F3 Neowise, which graced Northern Hemisphere skies for much of July, was very welcome if a little unexpected.
There was always a risk with this comet. Had it kept to predictions, its re-emergence from the closest part of its 6,800-year orbit to the Sun on July 3 would have left it difficult to see in the bright summer twilight. However, as Neowise swung around the Sun it brightened significantly, emerging from the solar glare as a naked eye object, bright enough to withstand the dawn twilight.
Throughout much of July, Neowise stole the limelight. As it made its closest approach to Earth on July 23, passing our planet by a mere 64 million miles, its increasing distance from the Sun meant its activity was waning and appearance dimming. As we head into August, Neowise continues dim and now requires binoculars or a telescope to see. Its track across the sky now takes it from the constellation of Coma Berenices (Berenices’ Hair) to reach a point 12 degrees below the bright star Arcturus on the nights of Aug 11/12 and 12/13, incidentally the peak dates for the annual Perseid meteor shower.
The Perseid meteor shower peak is expected between 2-5pm BST on Aug 12, during daylight. This means a watch in the early hours of Aug 12 and the night of Aug 12/13 should produce the best views. Activity gets a natural boost after 1am as our side of Earth turns to face into the oncoming meteor stream. A late phase Moon interferes with the early morning show, so position yourself somewhere dark, with the Moon hidden behind a building or tree.
Observing a meteor shower is something anyone can do. The basic advice is to find somewhere dark and get comfortable using a garden chair or recliner. Look up at an altitude of around 60 degrees – two thirds up the sky – in any direction away from moonlight. Give yourself at least 20 minutes in darkness to become properly dark adapted.
Meteor showers occur when comet dust enters Earth’s atmosphere. In the case of the Perseids, the comet is 109P/ Swift-tuttle. This has a 133-year period and despite not being due to return to the inner Solar System until 2126, Swift-tuttle’s dust has spread around its entire orbit. Every year when our planet passes through this dust stream, we experience
Perseid meteors. Activity starts as Earth enters the outer region of the stream in the last week in July, activity being low at this time.
All of the dust particles are effectively on parallel orbits when they enter our atmosphere. Visually, perspective makes the resulting meteors appear to emanate from a single point in the sky known as the shower radiant. Late July the Perseid radiant is in Cassiopeia the Seated Queen, tracking slowly east.
By the time Earth reaches the densest part of the stream, around Aug 12, the radiant is in Perseus the Greek Hero, hence the shower’s name. At peak the shower’s zenithal hourly rate (ZHR) approaches 120 meteors per hour which typically translates to a visual rate – i.e. what you’re actually likely to see – around 20-30 meteors per hour. Activity then tails off through to the shower’s end around three weeks into August.
Turn to look east as darkness falls and you’ll see the intensifying brilliance of Mars rising. Its gorgeous salmon pink colour is now extremely obvious. This planet will become the headline act as we approach autumn.