This week marks the peak of the Perseids, a prolific meteor shower. The Perseids are visible each year from the middle of July until late August, with their activity coming to a climax between 9 August and 14 August.
They can be seen across the night sky, but are most visible in the Northern Hemisphere. In 2007 and 2015, the dark sky of a new moon meant ideal conditions for witnessing the Perseids, but this year the moon will be three-quarters full as they reach their peak, making them more difficult to spot.
* * *
A meteor or ‘shooting star’ occurs when a meteoroid – a small rock moving through space – passes into Earth’s atmosphere at a high speed, heats up, and in the process produces a streak of light, caused by the glowing object and the glowing material it leaves in its wake.
Meteors can occur singly at random, or as showers, where streams of debris left by comets pass into Earth’s atmosphere on parallel trajectories. Meteor showers therefore appear to radiate from a single point in the sky, which is called the radiant. Meteor showers tend to be named after the constellation from which they appear to radiate.
The Perseids are debris from the comet Swift-Tuttle: a comet with an orbital period of 133 years. Their radiant lies in the constellation Perseus. The name ‘Perseids’ stems from Greek mythology, where the Perseides are descendants of the hero Perseus.
A meteorite is a piece of debris which survives impact with Earth’s surface. However most Perseids burn up in the atmosphere at heights above 80 kilometres.
* * *
The peak of a meteor shower arrives when the shower’s radiant is highest in the sky. At their annual peak, the Perseids can offer sixty or more visible meteors an hour.
The best hours for observing the Perseids are often the few prior to dawn, although the evening can offer the rare prospect of an ‘earthgrazer’, which will only appear when the radiant is still near the horizon, and a meteor crosses horizontally overhead. Likened to a skipping stone across the surface of water, an ‘earthgrazer’ passes slowly and colourfully.
Though the Perseids could be carefully traced back to the constellation Perseus, for the sake of viewing, there is no need to look towards any particular point in the sky. The Perseids will shoot across the sky in all manner of directions. Just find a dark spot and look up.
