For Northern Hemisphere skywatchers, August is usually
regarded as "meteor month," with one of the best displays of the year
reaching its peak near mid-month. That display is, of course, the annual
Perseid meteor shower beloved by everyone from meteor enthusiasts to summer
campers.
This year experts predict an excellent
Perseids display, as peak activity will coincide with a new moon, meaning
dark skies that allow the meteors to shine.
Meanwhile, there are other lesser-known summer meteor
displays to check out right now.
When to watch
In general, the Earth encounters richer meteoric activity
during the second half of the year. And you're more likely to see twice as many
meteors per hour in the predawn hours as compared to the evening hours. Here's
why: During the pre-midnight hours we are on the trailing side of the Earth as
it moves through space. Any meteoric particle generally must have an orbital
velocity greater than that of the Earth to "catch" us. After
midnight, when we have rotated onto the Earth's leading side, any particle that
lies along the planet's orbital path will enter our atmosphere as a meteor.
In these head-on collisions,
meteors hit our atmosphere at speeds of 7 to 45 miles per second. Their energy
of motion rapidly dissipates in the form of heat, light, and ionization,
creating short-lived streaks of light popularly referred to as "shooting
stars."
Summertime meteors are especially noticeable between
mid-July and the third week of August. And between Aug. 3 and 15, there are six
different minor displays. When they run (and peak):
- Southern Delta Aquarids, July 12-Aug. 19 (July 28),
15 per hour, faint, medium speed.
- Alpha Capricornid, July 3-Aug. 15 (July 30), 4-5
per hour, slow, bright, a few fireballs.
- Southern Iota Aquarids, July 25-Aug. 15 (Aug. 4), 1
to 2 per hour, faint, medium speed.
- Northern Delta Aquarids, July 15-Aug. 25 (Aug. 8),
1 to 4 per hour, faint, medium speed.
- Kappa Cygnids, Aug. 3-Aug. 25 (Aug. 18), 1 to 3 per
hour, slow moving, sometimes brilliant.
- Northern Iota Aquarids, Aug. 11-31 (Aug. 20), 1 to
3 per hour, faint, medium speed.
How to watch
The only equipment you'll need is your eyes and a modest
amount of patience. Telescopes and binoculars are of no use for fast-moving
meteors.
The actual number of meteors a single observer can see in an
hour depends strongly on sky conditions. The rates above are based on a limited
star magnitude of +6.5 (a really good sky), an experienced observer, and an
assumption that the radiant is directly overhead. The radiant is the place in
the sky where the paths of shower members, if extended backward, would
intersect when plotted on a star chart.
Your clinched fist held at arm's length is equal to roughly
10-degrees on the sky. So if the radiant is 30-degrees ("three-fists")
above the horizon, the hourly rate is halved; at 15-degrees it is a third.
While the hourly rates from these other meteor streams are
but a fraction of the numbers produced by the Perseids, combined, overall they
provide a wide variety of meteors of differing colors, speeds and trajectories.
The Southern Delta Aquarids, for example, can produce faint,
medium speed meteors. The Alpha Capricornids generate slow, bright, long
trailed yellowish meteors. And the Kappa Cygnids are classified as slow-moving
and sometimes producing brilliant flaring fireballs.
Note that five of the six showers listed, come from the
region around the constellations of Aquarius and Capricornus. These
constellations are highest in the southern sky between roughly 1 and 3 a.m.
local daylight time, so that's generally the best time to watch.
And don't forget to reserve the overnight hours of Aug.
12-13 for observing
the Perseids, which under clear, dark skies will produce one or two meteors
every minute.