Every year in mid-August the Earth intersects the orbit of the Swift-Tuttle comet. Lewis Swift and Horace Tuttle independently discovered this comet in 1862.
Tsuruhiko Kiuchi saw the same comet in 1992, meaning that it is a periodic comet with an orbital period of approximately 130 years. It is expected to return again around 2122.
Night Sky Festival
The Night Sky Festival, a public astronomy event for all ages, will be from 8 p.m.-1 a.m. Aug. 10 in Richland at LIGO Hanford Observatory, 127124 N. Route 10.
Festival-goers can watch Perseid meteors and use hosted telescopes to glimpse planets, stars, star clusters and galaxies.
At 8 p.m. Fred Raab will present “The Sounds of the Future: Listening to the Birth Cries of Black Holes with LIGO,” which will describe LIGO listens as gravitational waves announce the birth of black holes.
For driving directions and more information, email outreach@ligo-wa.... or call 509-372-8181.
Swift-Tuttle is a fairly large comet, with a nucleus about 16 miles across. A comet is often referred to as a dirty snowball made mostly of ice, with small pieces of rock and dust mixed in.
In the part of its solar orbit when it is closest to the Sun, the comet starts to melt and releases some of the rocks and dust that were trapped in the ice. Over time, most of this material spreads out over the orbital path and creates a debris field.
When the Earth intersects the comet’s orbit, it enters this debris field and we see a meteor shower. Although the Swift-Tuttle comet can’t be seen in our lifetimes, it is the parent body to the Perseid meteor shower that we can see every August.
The meteors in a shower all appear to come from the same point in the sky — the point the Earth is traveling toward at the time of the shower. This point is the shower’s radiant.
Because we always intersect the comet’s path at the same place in the Earth’s orbit, the radiant is located in the same place in the sky each year. The shower takes the name of the constellation where the radiant is located; the Perseid shower has a radiant in the constellation Perseus.
We entered the debris field on July 17 and will leave the field on Aug. 24. Perseid meteors can be seen any night during this period.
Because of the direction of the Earth’s rotation and orbit, the best time to look for meteors is after midnight, with the very best time being one to two hours before dawn.
At that time you are facing the direction of the Earth’s motion and hitting the particles head-on, just as your car’s windshield hits more bugs than its rear window.
By the first week in August, you may see as many as six to eight meteors per hour. This rate will increase, with a predicted peak between the mornings of Aug. 12 and 13. On these mornings, rates may be as high as 100 per hour.
To see this many, you will have to be at a dark site with a full view of the sky. A few days before and after Aug. 12 will also be good times to view.
This year the moon should not interfere with viewing. On Aug. 11 there will be a waxing crescent moon that sets at 10:37 pm PDT in Walla Walla. This means that the morning hours of Aug. 12 will be moonless, so we should have a dark sky for viewing. The constellation Perseus will be in the northeast sky and the Perseid meteors will come from that direction, but you could see them anywhere in the sky.
In the morning hours of August you can also hunt for the planets Jupiter, Mars and Mercury.
If you look east on Aug. 3 about 45 minutes before sunrise, you will see a waning crescent moon next to Jupiter in the constellation Gemini.
Jupiter is the very bright object just north of the moon.
Just below Jupiter and a little more north, you will find the red planet Mars. Extending the line of Jupiter and Mars toward the horizon, you will find the planet Mercury just above the horizon.
At the beginning of the month, Jupiter and Mars are just 5 degrees apart, but by the end of the month the distance will have increased to about 20 degrees.
Jupiter will rise earlier each morning during the month. On Aug. 1 it rises at 3:30 am, about 20 minutes before Mars.
On Aug. 12, Jupiter rises at 2:20 am, followed by Mars at a little after 3 am. By the end of the month, Jupiter will rise at 1:30 am.
The earlier the rising times, the higher in the sky these planets will be just before dawn. (All times are approximate and may vary depending on your location.)
So during the month of August, go to bed a little earlier and get up in the early hours of the morning to enjoy the wonders of the night sky.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at email@example.com.