Amateur astronomers are looking forward to the return of one of the year’s strongest and most reliable displays of “shooting stars” – the Perseid meteor shower.
This regular display of “shooting stars” peaks in the second week of August. The build-up to this peak is quite gradual, with the first Perseids being visible in July.
Traditional guides give dates around July 17 as the start of the Perseid meteor shower, with rates a lot lower than the 80 or so an hour predicted at maximum on the night of August 12/13. The shower then tails off, and August 24 is quoted as the date for the shower’s end.
However, recent research has discovered that Perseid meteors can actually be seen for a lot longer. Cameras regularly monitoring the sky have detected meteors from the shower as early as July 1 and as late as September 3, according to NASA meteor expert Peter Jenniskens.
The shower is named after the constellation of Perseus because, when their paths are traced backwards, Perseid meteors all appear to diverge from a point, called the radiant, within it.
But the radiant itself moves against the sky during the period of activity. And at the extreme dates when meteors have been seen, it lies in two neighbouring constellations. On July 1, the radiant lies in the famous W-shaped constellation of Cassiopeia, and at the start of September it has moved into Camelopardalis.
Here is a cool, interactive visualisation of how the Earth interacts with the Perseid meteor stream every year. The meteors are produced from a dust cloud left by Comet Swift-Tuttle which has an extended orbit carrying the particles deep into the Solar System.
The maximum of the Perseid meteor shower this year will be affected by bright moonlight in the run-up to maximum as the Moon will be full on August 7, and a gibbous phase just before last quarter on the night of peak activity. The best time to observe might therefore be in the last week of July and in the days following maximum in mid August.
The theoretical figure quoted for the number of meteors seen is the Zenithal Hourly Rate (ZHR). This is calculated for ideal conditions, including a dark, moonless sky and with the shower’s radiant in the zenith, or overhead. In practice, you are unlikely to see so many, but with patience and clear skies, you should still see quite a few.
You do not need a telescope to view meteors. Find a sunlounger or comfortable deckchair, get away from streetlights and then spend some time simply looking up at the sky. You should see your first meteor within a few minutes. They appear at random, rather than equally spaced intervals, so don’t be surprised if you then see a couple within a few seconds before waiting a few minutes for the next!
You may occasionally see a starlike point moving more slowly against the heavens. It is likely to be one of the many satellites that now orbit the Earth. They can be distinguished from aircraft because they are silent and tend to show a single steady white glow whereas planes have coloured flashing lights.
Another shower that is active in July is the Delta Aquarids, which reach a peak on the night of July 28/29. The radiant is low from mid-northern latitudes, so peak rates are unlikely to exceed 10 to 15 meteors an hour under ideal conditions. You may also see meteors from the Alpha Capricornids stream. This is a weak shower but can produce bright, slow-moving “shooting stars”.
Meteor astronomer Dr Jenniskens works with the SETI Institute and NASA Ames Research Center, and his discovery of the extended displays was made thanks to the NASA-sponsored project Cameras for Allsky Meteor Surveillance (CAMS) in Northern California,