#perseids2018

Diagram showing the ‘radiant’ of the Perseid shower (marked with the red cross). Graphic by the blogger.

Sunday night (12th August) marks the peak of the year’s best meteor shower, and this year the circumstances for viewing them are the best they have been in over a decade.

Each August, the Earth’s orbit carries our planet through streams of debris shed by Comet Swift-Tuttle, which last flew through the inner solar system in 1992. The orbit of this comet is arranged just right such that it intersects our orbit, although fortunately the Earth has never found itself in the same place as the comet at the same time (although that outcome is not impossible!)

Comet Swift-Tuttle, 1992. Credit: NASA

With an orbital period of 130 years, no one who saw its last apparition will likely be alive to see its next. But on each trip around the sun, Swift-Tuttle leaves a trail of dust in its wake. As the Earth plows through this part of its orbit, some of those dust grains are intercepted by our atmosphere, and we see them as swarms of bright shooting stars across our night skies.

From a dark location, up to 100 meteors per hour may be seen, with occasional outbursts to even higher hourly rates. The meteor show is known as the “Perseids” because the tracks of the meteors appear to converge toward the constellation of Perseus.

2018 is an especially good year for the Perseids because the maximum intensity of the shower occurs a little more than 24 hours after the new Moon, so that there is effectively no moonlight in the night sky for the meteors to compete against. That enables good visibility of fainter Perseids, which would otherwise be lost in the glare of the Moon in other years. That means that hourly rates reported by observers in 2018 are likely to be higher than in other years.

Like other meteors, Perseids begin as pieces of cosmic dust little bigger than sand grains. After leaving the surface of Comet Swift-Tuttle, they may orbit the sun for hundreds or thousands of years before approaching the Earth. Impacting the upper atmosphere at relative speeds approaching 60 kilometers per second (134,000 miles per hour), friction with air molecules heats the dust to temperatures of over 1,000 degrees. Elements in the disintegrating dust grain, such as iron, magnesium, and sodium, are energized by the heating and radiate away some of the energy as light of particular colors like green and yellow. Some 4.5 billion years after their creation, these grains end their existence in mere seconds.

The best views of the Perseids come after local midnight, when the direction of the Earth’s rotation takes us toward the forward direction of the Earth’s orbit around the Sun; whereas earlier in the evening, meteors must catch up to Earth’s motion to reach the atmosphere, after midnight the Earth is more directly moving through the stream of dust particles.

To see the Perseids, find an outdoor space with a relatively clear horizon toward the northeast. Activity will begin to pick up by about 10pm local time, and steadily increase until after midnight. Here are some tips to improve the experience:

Safety first! Carry a flashlight to help navigate in the dark, especially in unfamiliar places. Red or brown paper placed over the front lens of the flashlight will help preserve your eyes’ sensitivity to faint light.

Use a reclining lawn chair or chaise lounge to relieve fatigue while looking up at the sky for long periods of time.

Bring plenty of water, too, because it’s possible to dehydrate in warm temperatures on summer nights.

If you’re observing after midnight, remember to dress warmly as the night may become quite cool in the pre-dawn hours.