The August night sky

Graphic: Finbarr Sheehy

The Perseids meteor shower is a reliable highlight of our August nights, and there are grounds for hoping that we might enjoy a bonanza this year.

Rich in bright meteors, it began more than a week ago and builds to a peak predicted for about 13:00 BST on 12 August before petering out after the 20th. Expect the highest meteor rates on the nights of the 11/12th and 12/13th.

Perseids appear in all parts of the sky, their paths pointing back to a radiant point in Perseus – hence the shower’s name. That radiant is climbing through the middle of Britain’s NE sky at nightfall to stand just E of overhead before dawn. Meteor rates increase with the radiant’s altitude, so the morning hours are favoured, particularly since the Moon sets in the middle of the night on the critical dates.

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An observer with the radiant overhead in a fully dark moonless sky might count 80 or more Perseids per hour at the maximum. There are claims that the rates could be higher this year, even twice as high, because of a periodic gravitational pull of Jupiter on the stream of Perseids meteoroids that follows the orbit of its parent comet, Swift-Tuttle.

Our chart shows the Summer Triangle of Vega, Deneb and Altair high on the meridian as the Square of Pegasus climbs in the E. The only bright planet on the chart, Saturn at mag 0.4, is sinking in the SW but stands some 15° high in the S as darkness falls at present. It hardly moves this month, being 6° above Scorpius’ star Antares.

Brighter, but fading from mag –0.8 to –0.3, is Mars which stands 11° below and right of Saturn on the 1st, and tracks E to pass between Saturn and Antares, and 1.8° above the star, on the 24th. Although it sets before our map times, Mars is the most obvious object very low in our SSW to SW sky at nightfall where it lies below the Moon on the 11th.

Brighter still, and conspicuous in the W at nightfall for more southerly observers, are Venus and Jupiter, with Mercury currently between them. All lie deep in Britain’s bright evening twilight where Venus blazes at mag –3.9 but is only 5° high at sunset and sets itself 40 minutes later. Jupiter, mag –1.7, is a little higher to its left and, while easier to spy at present, will certainly require binoculars by the 27th when we see it only 8 arcmin below-left of Venus.