Monthly Archives: August 2013

Greetings fellow astrophiles!

This article has been posted in preparation for our Perseid Session and International Starry Night event at Baltimore Woods this coming Monday, August 12th (with the 13th as the weather-alternate). We might even get a view or two of the Perseids at our Thursday, August 8th Beaver Lake Nature Center lecture!

The Perseid Meteor Shower is an almost perfect combination of location and timing for amateur astronomers and the general public, as the Earth grazes a rich debris field from the tail of Comet Swift-Tuttle during the peak of the Northern Summer. We’ll cover the details of this confluence below so you know what makes the Perseids the most anticipated (and observed) meteor shower of the year.

One Thousand And Thirty Words (And Two Numbers)

Comedian: “Ask me what the key to comedy is.”
Assistant: “What’s the -”
Comedian: “Timing!”

The image above shows all of the important pieces of the Perseid puzzle. We find the Earth in its orbit around the Sun as it approaches a mid-August position (the 10th to the 14th, although one may see meteors at the fringe of Perseid territory several nights before and after) that finds Earth (and us) scraping against the edge of a debris field produced by Comet Swift-Tuttle on its 133-year orbit around the Sun. Last seen in our vicinity in 1995, observers will have to wait until the 2120’s for another good view of its flaring core. Fortunately, it leaves enough tiny pieces of itself as it draws close to the Sun to provide us with a brilliant reminder of its existence every mid-August.

Unlike Halley’s Comet, which passes close to Earth’s orbit on its way toward (producing the Eta Aquariid Meteor Shower in early May) and away from (producing the Orionid Meteor Shower in late October) the Sun, Comet Swift-Tuttle’s eccentric orbit finds it passing close to Earth only at one point, like a snapshot capturing a hula-hoop (Swift-Tuttle’s orbit) as it touches the belt buckle (Earth) of a gyrating dancer whose waist is Earth’s orbit in circumfrence.

What’s In A Name?

We refer to this meteor shower as the “Perseids” because the meteors associated with Swift-Tuttle appear to streak across the sky from a point (known as a “radiant“) originating in the direction of the mythical constellation Perseus. The shower itself has nothing to do with the stars of the constellation Perseus, only the part of the sky that Perseus occupies on the late nights and early mornings in mid-August. One might even consider Perseus the beneficiary of this shower, as the constellation has taken on a new-found importance to astronomers over the last several millennia as the marker for this shower in the August skies.

It’s All Relative

Anyone caught driving late at night during a snow storm knows the sensation of making the Millenium Falcon’s “jump to lightspeed” as the snowflakes appear to shoot towards, then past or onto, your windshield. To the driver cruising at 65 mph on a highway, the snowflakes appear to have no motion but the one directly towards the windshield. If you were standing on a snowflake, you’d notice the very slow decent to the Earth’s surface, the rapidly oncoming car headlights, then the swift rush across the windshield as the aerodynamics of the windshield combined with the high speed of the car.

This same state of “relative observation” occurs during all meteor showers as the Earth revolves around the Sun. The meteors, themselves mostly no larger than grains of sand, are not moving rapidly towards the Earth’s atmosphere. They lie scattered about the path of Comet Swift-Tuttle, a result of the comet heating enough as it approaches the Sun to lose small pieces of its surface. If Swift-Tuttle were a massive gravel delivery truck (to continue the driving analogy), these small grains would be the random pieces of rock that fall to the ground as the truck bumps over uneven pieces of highway.

Clash Of The Tinys

It is the Earth, revolving around the Sun at a dizzying 110,000 km/hour (that’s 30 km/second!), that powers the meteor shower we see on the ground. As the Earth rushes through the debris field of Comet Swift-Tuttle, these tiny grains of comet come into contact with our atmosphere at speeds so great that they ignite the air around them, causing brilliant streaks of light as the tiny grains are incinerated.

The number of meteors one can observe over a Perseid session is determined by (1) your looking at the right place at the right time (no long blinks!) and (2) the density of tiny Swift-Tuttle-ettes in the comet’s orbit as Earth passes through it. There are some meteor showers where one is lucky to see a few per hour. Because the Earth passes through a generally rich part of Swift-Tuttle’s orbit, two or three per minute may not be uncommon for a “usual” Perseid session. Those outside for the 1972 Perseid Meteor Shower were treated to what many believe to be the best meteor shower in recorded history (and those outside for the 1998 Leonid Meteor Shower (a close second by all metrics) know what it’s like to see thousands per hour raining down on dark skies).

Finding Perseus

The Perseids appear to radiate from the constellation Perseus. For your best chance of seeing Perseid meteors, it is not your eyes that should be transfixed on the heart of Perseus. Instead, you should anchor the bottoms of your toes towards Perseus, then find a comfortable piece of ground (or reclining chair) that gives you a clear view of the sky right above you. Perseid meteors will then, with a thick patch of debris field and a bit of patience, appear to blaze across the night sky from your toes (Northeast) past your head (to the Southwest).

Perseus will appear to rise above the Northeast horizon after 9:00 p.m. Directly above the stars of Perseus resides Cassiopeia – a giant and prominent “W” in the night sky that, for many hours after sunset, will appear as a West-facing throne for this ancient Ethiopian queen. Those familiar with the many tricks amateur astronomers use to learn the Night Sky will simply find Polaris, perhaps using the two end stars of the bowl of the Big Dipper and an imaginary line along these stars in the direction of the bowl’s open face to pick out the dim North Star. Polaris does not shine with the brightness one might have imagined for the second most important star in the sky (after our own Sun), but it is in a piece of sky that contains few brighter stars, making it the most obvious member of a very modest piece of northern sky.

If you’re still too new to constellation hunting, the solution is simple! Grab a compass (or a compass app in your smart phone) and find Northeast the new-fashioned way. With luck, the Perseids will race to the Southwest at a rate of a few per minute, increasing in count, then decreasing, from around 10:00 p.m. to 4:00 a.m. local time. With the good fortunes of all the Olympian Gods, we’ll all be treated to many, many more.

Additional Information

The Perseid Meteor Shower

en.wikipedia.org/wiki/Perseids
earthsky.org/astronomy-essentials/everything-you-need-to-know-perseid-meteor-shower
solarsystem.nasa.gov/planets/perseids.cfm

Comet Swift-Tuttle

en.wikipedia.org/wiki/Comet_Swift%E2%80%93Tuttle
ssd.jpl.nasa.gov/sbdb.cgi?sstr=109P

Meteors And Meteor Showers

en.wikipedia.org/wiki/Meteor_shower
leonid.arc.nasa.gov/meteor.html