The Perseids meteor shower is now in full flow as meteoroid particles from Comet Swift-Tuttle rain into the Earth's upper atmosphere at 59 km per second. Although they are following parallel paths, perspective causes them to diverge from a radiant point in Perseus which climbs from about 30° high in Britain's NE sky at nightfall to approach the zenith by dawn. Our chart shows it at midnight, below the "W" of Cassiopeia and some 25° above-right of the conspicuous star Capella.
Also called the Tears of St Lawrence, the Perseids is one of the most prolific and reliable of our annual showers. It is also a favourite for observers since it occurs during the relatively balmy nights of August and not, like the Geminids and Quadrantids, in the depths of winter. As a bonus, there is little or no interference from moonlight this year.
Early Perseids appeared in late July, but their numbers build to a maximum which is expected to occur over several hours around 20:00 BST on Monday 12 August, before night falls for Britain. Rates then decrease and the shower is petering out by the 20th.
Under ideal conditions, a lone observer might count as many as 100 Perseids per hour at maximum, but this assumes a perfectly dark and clear sky and that the radiant point lies overhead so that we are facing directly into the oncoming meteoroids. Of course, few of us enjoy such pristine skies and the radiant is never overhead in darkness, though it does climb higher during the night.
The result is that the best Perseids displays are likely to be seen in the predawn on the 12th and over the following night, though anyone expecting to see 100 meteors per hour is likely to be disappointed. On the other hand, most of us would be happy with 20 to 40 per hour. The shower has a higher than usual proportion of bright meteors and many of these leave short-lived glowing ionisation trains in their wake.
To view Perseids, wrap up well, find a reasonably dark location and lie back in a deckchair or sunlounger. There is no need to face the radiant in the NE to see Perseids – they occur at random over the whole sky so we are as well to face a direction that gives us an open view with the least interference from artificial lighting.
Perseids were particularly numerous between 1991 and 1998, coinciding with the return of Comet Swift-Tuttle to perihelion in 1995, its closest point to the Sun just inside the Earth's orbit. The comet itself had been discovered and named one orbit earlier, in 1862, and we now realise that it had been logged under a different name in 1737 and, indeed, may have been seen as early as 69BC. It is next due at perihelion in 2126.