The meteors are coming! Three annual meteor showers are already active and guaranteed to spark up your summer nights.
Open the gate. Here they come! It's time for the annual trifecta of late July-early August meteor showers beginning with the Delta Aquariids which peak the night of July 28–29. The last meteor shower of note occurred in early May when the Eta Aquariids sprinkled a modest few meteors across the dawn sky. Yes, it's been a long time.
The Delta Aquariid meteor shower takes its name from Delta Aquarii, a 3rd-magnitude star in the constellation Aquarius, the Water Carrier. Shower meteors fan out across the sky, but all appear to streak away from a point in central Aquarius called the radiant.
A sure way to know if you've spotted a Delta Aquariid is to trace its path backwards. If it returns you to the vicinity of Delta Aquarii, you've got a keeper.
The radiant is a perspective effect caused by meteors arriving from the same direction at similar speeds. If you've ever driven through a heavy rain or snowstorm at night, you've probably noticed that precipitation caught in your headlight beams appears to "radiate" from a point in the distance. The illusion is identical to the that of railroad tracks converging in the distance. Still, despite appearances, we know the tracks remain parallel just the way meteors do when they spear through the atmosphere.
Some meteor showers have sharp peaks, others are "soft", with broadly spread-out activity. The Delta Aquariids typically fire off 15 to 20 meteors per hour before dawn begins, when Aquarius is highest in the sky — its broad peak is centered on Friday morning. Since most shower members are rather faint, plan to catch the show from a reasonably dark sky.
What's nice about these more diffuse showers is that they keep putting out meteors a week or more before and after maximum. If the weather doesn't cooperate one night, you can look the next. Or the next.
You can start watching as "early" as midnight when Aquarius pokes its head up in the southeastern sky, but 2 a.m. local time might be better since that's when the radiant crosses the meridian and stands highest in the south at 3:15 a.m.. Dawn begins around 4 a.m. for observers at mid-northern latitudes.
Because the radiant is relatively low in the southern sky, skywatchers in tropical and southern latitudes will see more meteors than those farther north. Why? Meteors that flash a significant distance south of the radiant get cut off by the southern horizon.
Whatever time you choose to begin your vigil, find a location with a minimum of light pollution and set up a comfy reclining lawn chair facing east or south for the best view. Remember to bring a blanket to ward off the damp chill of a humid night. Coffee and tea are welcome and even a little music for atmosphere. Or you can soak in the soft stridulations of crickets and katydids.
The Moon hugely affects how many meteors you might see. Lucky for meteor lovers, it will be a 23% waning crescent considerably farther east in Taurus on Friday morning and should make only a small dent in meteor counts.
But hold on — the Moon has its own agenda. That very same morning, it will either just miss or occult the bright star Aldebaran shortly after 10:00 UT (5 a.m. CDT).
If you live south of a line that crosses from Toledo, Ohio, through St. Louis, Missouri; Tulsa, Oklahoma; and El Paso, Texas, the Moon will cover the star. North of the line, it will glide just beneath it. Fortunate observers living within a half-mile either side of the line will witness a spectacular grazing occultation as Aldebaran repeatedly disappears and reappears behind the Moon's north polar peaks. Rarely has there been a better incentive to get up early for a meteor shower! Check here for full details.
The Origin of the Delta Aquariids
Each Delta Aquariid that flashes into view represents a crumb of debris sloughed off by comet 96P/Machholz discovered in 1986 by American amateur astronomer Don Machholz. Solar heating vaporizes ice and loosens dust and small rocks from the comet. Some of the material falls back and coats the surface, and some gets pushed away by sunlight to form a trail of debris in the comet's wake. Every July, Earth sprints through 96P's trail; as the material strikes the atmosphere at 93,000 miles per hour (42 km/sec) and vaporizes, we get treated to glittery streaks of light.
The Alpha Capricornids, which originate from Comet 169P/NEAT, are active at the same time as the Delta Aquariids and radiate from northwestern Capricornus. They stand out for being unusually slow-moving and often bright. You may see five of these per hour in the last week of July under ideal conditions radiating from the northwestern corner of Capricornus. But in the far future, this shower should become intense!
Meteor-stream calculators Peter Jenniskens and Jérémie Vaubaillon find that only the outer fringe of the stream currently crosses Earth’s orbit, and that the stream should have a rich inner core. This core should start intersecting Earth’s path in the 24th century. They predict that the Alpha Capricornids will be a major sky event every year from about 2200 to 2400 A.D., “stronger than any current shower,” according to S&T Senior Editor Alan MacRobert.
Perseid Meteor Shower Completes the Trifecta
Earlier I mentioned a trifecta of meteor showers. You can probably guess the third offering — the Perseids! Although the shower peaks on the night of August 12-13 with numbers in the neighborhood of 100 per hour, it's been active since mid-July, so don't be surprised if an outing over the next few nights nets a few Perseids, too.
On the night the shower peaks, the Moon will be two days past first quarter in Ophiuchus and compromise the view a bit until it sets around 1:30 a.m. From then till dawn, we're likely to see the shower at its best.
Perseids originate from 109P/Swift-Tuttle and slash the sky with their swiftness. Fireballs as well as meteors leaving persistent, smoke-like trains are common.
With all three showers, keep an eye out for earthgrazers, meteoroids that arrive at Earth at a very low angle. Instead of following a more typical, steeply-slanted path downward, they skim the upper air, often taking many seconds to finally fizzle out. Earthgrazers are more common when a shower’s radiant is either just below or a short distance above the horizon.
Let the fireworks begin!
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