As a follow-up to my previous post about watching golfballs hit the moon at 100,000 miles an hour, here’s a guide to the Perseids and how to enjoy them.
- What are the Perseids? They are particles shed by Comet 109P/Swift-Tuttle, a periodic comet that last passed near Earth in 1992. Composed of mostly silicate minerals, the typical meteor particle, or meteoroid, is about the size of a grain of sand. The motion of the Earth through its orbit carries us toward “streams” of these particles left behind in the comet’s wake; our relative motion toward them makes the particles encounter the Earth at a relative velocity of tens to hundreds of thousands of miles per hour. They meet the upper atmosphere and vaporize as they reach temperatures of thousands of degrees for a few brief seconds; we see their remnants as bright streaks of light commonly known as “shooting stars”. The Perseid shower is one of the year’s more notable events, leading sometimes to hundreds of meteors per hour being seen.
- How often do they happen? The Perseids are an annual event each August, and has been happening at least since antiquity. Last seen exactly 150 years ago during the Civil War, Swift-Tuttle was somewhat considered “lost” because its observations in 1862 were not sufficiently precise to nail down its orbit. The comet “announced” its coming return to the inner solar system during the 1991 Perseid shower with a huge burst of activity during the early morning hours over East Asia. Speculation as to whether this feature represented material recently shed by the comet was well founded; the comet re-appeared in Earth’s skies about a year later. Each time the comet returns, a new stream of particles is shed and the shower is replenished.
- Why are meteors important? There’s a lot of science involved. So many comets have come and gone over the history of the solar system that the inner part is chock full of dust particles. These particles constantly rain down on the Earth and other terrestrial planets, bringing many tons of cosmic material per year; early in the solar system’s history, this may have included the essential components needed in the chemistry of life. The particles are thought to be relatively pristine, in the sense that they avoided the thermal shocks and chemical mixing that occurred in the solar nebula. So a meteor seen streaking across our skies is a glowing messenger from the earliest times of our planetary system, telling us what conditions were like then.
- Do I need a telescope to observe the Perseids? Nope! Meteor showers are a phenomenon requiring no special equipment at all; in fact, binoculars and telescopes allow views of such relatively small parts of the sky that they’re mostly ineffective for meteor observing. Rather, showers like the Perseids contain many larger particles, which make for brighter meteors, and the best instruments to observe them are unaided human eyeballs. Our eyes are pretty sensitive once dark adaptation has occurred, and we can see a very large area of sky at a time with them, increasing the chances of seeing meteors.
- When should I look for Perseid meteors? The shower peaks in intensity around August 11-13, so it’s going on right now — even during daylight hours. However, for nighttime observing, the hours between midnight and dawn are best, because of the relative speed difference between the meteors and the Earth. Before midnight, we’re facing away from the direction the Earth moves in its orbit around the Sun, so to be seen the meteors have to “catch up” to us. After midnight, we’re facing the direction of motion and the relative speeds are highest. Speed correlates with meteor brightness (for a given particle size), so you’re likely after midnight to see fainter meteors as a result.
- What part of the sky should I look toward? The Perseids can be seen in any part of the sky, but they appear to “radiate” from a point in the constellation Perseus. In August, that constellation is rising in the northeastern sky toward midnight in the Northern Hemisphere. So facing northeast aligns the observer with the direction from which the meteors appear to originate and increases the chances they’ll be seen before burning out. Also, choice of seating is important, and a reclining lawn or lounge chair is highly recommended. Staring upwards for hours at a time is otherwise rough on the neck muscles!
- What can I expect to see? In an average year, observers can typically expect about one meteor per minute during the shower’s peak. The rate is somewhat variable, and at times several will be seen per minute. The Perseid shower is known for lots of bright, slow-moving fireballs, some of which explode at the end of their travel in a “terminal burst”. At times these fireballs are bright enough to cast shadows on the ground. There is rarely obvious color in meteors, but the Perseids tend toward a pale yellow, possibly due to emission of light from ionized sodium atoms. The Perseids, one of the most reliable meteor showers of the year, rarely fail to impress!
- Will the Moon be a problem? Yes and no. This year’s shower peaks a few days after the Moon is at last quarter, meaning two things: (1) The early part of the night will be Moon-free, and (2) After the Moon rises, it will present some interference. Moonlight adds to the sky background, making the faintest Perseids difficult to see. But don’t worry — the brightest meteors will easily outshine this light. The main tip here is to not look in the direction of the Moon, to keep its light from ruining one’s dark adaptation. One of the best tricks is to keep the Moon behind some obstruction, like a building or a tree, while maintaining a clear view to the northeast. Remaining in shadow will minimize the interference.
- Are my observations useful? Yes! Meteor watching is a great citizen-science opportunity. Meteor science still relies on boots-on-the-ground observations from people, many of whom are amateur astronomers and other interested skywatchers. Ways to participate can be found here, here and here. Having multiple people at one observing site helps in collecting observations, since several individuals can cover more of the sky at once than one person. But one person can still see a lot of sky, and solo observations are quite useful.
- Finally, stay warm, as the predawn hours in August can be cold in many parts of the Northern Hemisphere. My meteor watching inevitably involves a thermos of hot coffee to help ward of the chill and drowsiness.