Thursday, August 7, 2008

The Pursuit of Perseids

The Perseids (pronounced PURR-see-yidds) are a major, reliable meteor shower and one of my personal favorites. It simply wouldn’t be August if I didn’t try to spot some of those long, yellow-tailed beauties streaking across the sky.

Sometimes a big Moon will thwart or at least hamper my attempt. That’s the case this year. The peak of the meteor shower--when the number of meteors per hour is greatest--is predicted to occur in the early morning hours of Tuesday, August 12. As this is less than one week before Full Moon, the waxing (growing) Moon will be in a gibbous phase. Gibbous means the Moon is greater than 50% illuminated without being fully illuminated. In other words, a big Moon.

A big Moon tends to wash out the sky, reducing the contrast needed to see faint streaks of light. No Moon is optimum. Since the Moon doesn’t set until two to three hours after midnight (depending on location) on the morning of the 12th, you’ll have a narrow window of opportunity to spot peaking Perseids before morning twilight lightens the sky. But the reward will be great: the predicted peak rate is 100 meteors per hour!

By all means, don’t get too hung up on peak predictions. I’ve seen some awesome meteor shower displays on nights prior to and after the so-called peak. In some cases, I was pleasantly surprised to discover they bested the peak display. The Perseids are active this year through August 24. Pick any night from now until then, go out after midnight when the Moon’s not around, and you’re bound to see some dreamy Perseids.

There’s a reason why I specified “go out after midnight” above. Perseids emanate from, and are named for, the constellation Perseus (pronounced PURR-see-yuss). The show can’t begin until the radiant in Perseus rises above the northern horizon. The radiant is the point in the constellation where the Perseids appear to originate. The higher the radiant rises in the sky, the more likely you'll see meteors. The radiant is well above the horizon--and rising--from midnight through dawn.

The radiant is marked on the map below. Despite their origination point, you can see Perseids in any part of the sky, in any direction you happen to be looking. However, knowing the radiant will help you identify which “shooting stars” are Perseids and which are sporadics, meteors not associated with a particular meteor shower. If you can trace a meteor’s trajectory back to Perseus, it’s most likely a Perseid.

Chart created with Your Sky

1) Go to as dark of a site as you can, with no line-of-sight outdoor lights. Take extra layers of clothing or blankets with you. Even though it’s summer, you won’t be moving around much and could get chilled.

2) Take a lounge chair or a blanket for the ground. Reclining is really the best way to watch a meteor shower, because it allows you to see the most sky at one time. Although the meteors originate from the radiant, you can see them fizzling anywhere in the sky. I normally lie with the top of my head pointed toward the rising radiant (which will be in the northeast) so that I can be scanning the rest of the sky.

3) A meteor is the streak of light caused when a bit of interplanetary dust, comet debris, or manmade space junk slams into Earth’s atmosphere at high velocity. Typically, the meteoroid--the bit of debris--vaporizes upon impact with the atmosphere. But occasionally, a large chunk may survive its screaming 100,000-plus mph entry and hit the ground, at which point we refer to it as a meteorite.

As you watch the fireworks, keep in mind that meteors are typically no bigger than a grain of sand. The really bright ones that go off like Roman candles may be no bigger than a grain of rice. It is their high entry velocity that causes the wonderful light show.

4) A meteor shower occurs when Earth encounters a stream of debris left behind by a comet’s close approach to the Sun. Comet Swift-Tuttle takes 135 years to complete its orbit of the Sun. Each time it approaches, the Sun's heat releases dust trapped in the comet's icy nucleus or frozen center. This periodic renewal of the discarded dust stream ensures us cosmic fireworks every August when we move through it.

Meteor showers are one of the supreme naked-eye treats for lounge-chair astronomers everywhere, beginners to veterans. Do make the pursuit of Perseids part of your summer recreation. Do make a memorable evening with a friend or family member. Don’t forget to make a wish.