It’s time again for the annual Perseid meteor shower. This year, the best time to observe will be the early morning hours of Aug. 13. Start observing after 11 p.m. on Aug. 12 (Monday night), and stick it out until the sun rises Tuesday morning.
If the weather forecast looks cloudy Monday night, consider going out the night before or the night after if the weather looks better. Perhaps you won’t see as many meteors, but some is always better than none.
A first-quarter moon won’t set until 11:20 and will spoil the view of some of the fainter meteors until then.
So, what are you seeing?
Meteors happen when hunks of space debris slam into Earth’s atmosphere. As they burn up from friction with the air, they ignite the air around them and leave the familiar streaks of light. Those streaks are technically named “meteors” but are commonly called “shootin’ stars.”
Of course, if a star, a ferocious hydrogen bomb much larger than planet Earth, actually fell from the sky, the view would be pretty darned spectacular before Earth was fried like a mosquito in a giant bug zapper.
Every clear, moonless night is a great night to go meteor observing. Since as many as a trillion particles enter Earth’s atmosphere daily, you’ll see about five meteors every hour if you observe on a moonless night from a dark, rural site.
Meteor showers like the Perseids happen when the Earth passes through a trail of space debris left by a passing comet. The particles, most of which are no bigger than a grain of sand, smack into Earth’s atmosphere at blazing speeds of up to 50,000 miles per hour.
Thus, meteor showers happen yearly on about the same date as Earth passes through the same cloud of comet debris. In this case, the culprit is Comet Swift-Tuttle, which last passed through our neck of the cosmic woods in 1992 on its 133-year journey around the sun. It left a fresh trail of debris, making the Perseids memorable ever since.
That happy cosmic coincidence makes the Perseids one of the most spectacular showers of the year during any given year.
Generally speaking, meteor showers, the Perseids included, are best observed after local midnight, especially during the last two pre-dawn.
Why? Because Earth orbits the sun and rotates on its axis, of course. Before local midnight, our spot on the planet is pointed away from the direction of Earth’s orbit. After midnight, your spot on Earth turns into the direction of Earth’s orbit, which happens to be toward the constellation Perseus.
This situation is similar to being a passenger facing toward the rear windshield of a car. A few bugs might hit the windshield, but they’ll have to fly faster than the car.
After midnight, you are facing forward into Earth’s orbit. The bugs don’t have to play catch-up with Earth’s velocity, and you’ll see many more of them splattering against Earth’s front windshield.
As a result, the display increases in intensity until just before morning twilight, when you might see an average of one streak of light per minute. Don’t give up until morning twilight blocks the view of the sky.
Please note that local midnight doesn’t happen at 12:00 AM. Daylight saving time puts it off until 1 p.m. Also, we’re about 35 minutes into our time zone, putting local midnight at 1:35 a.m.
Given that morning twilight will begin to brighten the sky by 4:45, that leaves only three precious hours of top-flight meteor observing. Best to make the most of them.
You would think that astronomers have known about the periodicity of meteor showers for a long time, but they haven’t. They were not particularly interested in meteors until the great Leonid meteor shower in November 1837. Thousands of meteors blazed across the sky one fine morning.
The hunt was on for annual meteor showers. By the middle of the 19th century, one astronomer after another claimed to have discovered the yearly Perseid meteor shower.
They were all wrong.
More on that next week, plus a few practical meteor-observing hints.
Tom Burns is the former director of the Perkins Observatory in Delaware.
