The peak of this year’s Perseid meteor shower occurs on the night of 12-13 August. The shower occurs every year in August as our planet passes through debris left behind by Comet 109P/Swift-Tuttle. The Perseid meteor shower gets its name from the fact that its radiant is the constellation of Perseus.
If you can locate Perseus, look away from the radiant rather than directly at it, and you should be able to spot meteors flying in the opposite direction.
Many factors affect the visibility
of meteor showers. The two most important of these are radiant altitude and sky clarity.
The latter is complex, being affected by weather and light pollution. The most obvious natural light polluter is the Moon and this will be present during the peak of this year’s Perseid display.
The peak occurs on the night of 12-13 August when the Perseids typically deliver a maximum zenithal hourly rate (ZHR) of around 110 meteors per hour.
ZHR is a misleading value that’s often misquoted. It represents the number of meteors
you could expect to see under perfect conditions with the radiant overhead while looking at the entire sky.
In reality few of these conditions are achieved and the visual rate – that’s the number of meteors you are likely to see – will be lower.
The biggest factor affecting the Perseid peak’s visual rate is the naked eye limiting magnitude (NELM); the faintest star
you can see with just your eyes.
ZHR calculation assumes a NELM of +6.5 which few of us are lucky enough to achieve.
From the centre of the UK, the Perseid radiant manages to achieve an altitude
of 58˚. Under perfect skies with a NELM
of +6.5, a peak visual rate of 47 meteors per hour could be expected.
The effect of moonlight is complicated, affecting areas of the sky closer to the Moon more than those in the opposite direction.
Movement of the Perseid radiant during July and August. The brighter portion of the track indicates when the Moon will interfere most. Credit: Pete Lawrence
You can’t eliminate the effect of the Moon but you can reduce it. At this time of year, the fuller phases of the Moon have low altitudes, so find a dark area away from stray artificial light and locate yourself so the Moon is hidden behind
a building or fence.
Look at the patch
of sky with the best NELM – typically opposite the Moon. A NELM of +5.0 should deliver a peak visual rate of 14 meteors per hour, +4.5 reduces this to
10 meteors per hour while a +4.0 NELM brings the figure down to just 7 meteors per hour.
Cameras should fare better. Here,
the technique is to set the camera’s sensitivity high, point to a part of the
sky where the Moon isn’t in frame and experiment with the settings to achieve
a sky which isn’t over-exposed.
Typically, adjust your camera’s ISO so that a 10–15 second exposure doesn’t overexpose the shot. The lens aperture should be fairly wide open too (using a low f/number).
If this can be achieved, set the camera
to continuous shooting mode and repeat exposures using a remote shutter release with the shutter button locked down.
Gallery: readers’ images of the Perseid meteor shower
This guide originally appeared in the August 2019 issue of BBC Sky at Night Magazine. Pete Lawrence is an experienced astronomer and a presenter on The Sky at Night.