Sen— Amateur astronomers in the northern hemisphere are taking advantage of the warm summer nights to watch the build-up of the year’s best known meteor shower, the Perseids.
Though this shower of “shooting stars” does not peak until 12 August, activity is already underway. And it is a great year to observe the Perseids because moonlight will not interfere to drown them out.
As with all meteor showers, the meteors are produced when the Earth ploughs through a stream of dust left by a comet orbiting the Sun and they flare in our atmosphere.
They are called Perseids because, if you trace their streaks backwards, they appear to radiate from a point in the constellation of Perseus. This is an effect of perspective, like straight rail tracks converging to a distant point, and in fact the meteors are travelling on parallel paths.
The comet responsible for the Perseids was Swift-Tuttle. The dust particles in space, called meteoroids, and about the size of grains of sand, are now spread right along the swarm’s orbit. This means we see Perseids every August. They are a reliable shower, always rich in activity, although the numbers vary according to how dense the orbiting stream is at the point where we intersect.
The stream is broad too, with the first Perseids appearing in our skies in late July and the final stragglers falling well into the second half of August. They are an excellent project for amateur stargazers and in fact it is the work of amateurs over many decades that has helped professional scientists learn more about such streams’ orbits.
Though professional techniques have helped them to compile pretty accurate maps of the orbiting dust, amateur observations are still considered very useful - as well as being fun to do.
The sight of a bright meteor is an awe-inspiring one and the Perseids are rich in such natural fireballs, according to Dr Bill Cooke, head of NASA’s Meteoroid Environment Office. This is because the head, or nucleus, of Comet Swift-Tuttle is particularly large for a comet, at around 26 km.
A number of astronomical societies around the world encourage and collate observations of meteors by amateurs. A dedicated group, made up of a mix of serious amateurs and professionals, is the International Meteor Organization.
In the UK, the Society for Popular Astronomy has a Meteor Section which is aimed particularly at helping beginners of all ages while also collecting useful results to help meteor research. Its director is Tony Markham.
Sen asked Tony how a newbie might go about watching for Perseids. He told us: “Find as dark an observing site as possible with an unobstructed view of the sky and watch an area of sky that is around 30 degrees away from the constellation of Perseus and about 50 degrees above the horizon and wait.
“If you are lucky you will see a meteor within the first few minutes but you may have to wait longer, especially if your eyes are still adjusting to the dark.
A NASA video helps explain what you might see of the Perseids. Credit: NASA
“The best Perseid rates will be seen during the nights of Aug 11-12 (Sun-Mon) and Aug 12-13 (Mon-Tue), but good rates will also be seen during Aug 10-11 and Aug 13-14. More patience is required if you observe on earlier nights, but several Perseids may still be seen each hour and if you are lucky a bright fireball might appear.”
So how many meteors might one expect to see? Tony said: “Although peak rates of 80 to 100 meteors per hour are often quoted for the Perseids, these would only be seen in perfect observing conditions with no haze and no sky glare from streetlighting.
“From a reasonably dark observing site you can expect, on the night of Perseid maximum, to see about a dozen Perseids per hour early in the night, rising to more than double this late in the night.”
While you sit watching, in your comfortable garden chair, you might well see other meteors that don’t belong to the Perseid shower. Tony said: “The paths of Perseid meteors when traced backwards show them to have originated from the constellation of Perseus. There are however a number of minor meteor showers that are also active.
“These produce meteors that originate from Aquarius and Capricornus and collectively provide a few meteors per hour. In addition half a dozen or so meteors per hour can be seen appearing at random which we term sporadics.”