The July Blue Moon means good news for meteor observers. Since new moons occur about two weeks before and after the full, the dark of the moon will occur around the middle of the months, coinciding with the three best-known, dependable, and occasionally spectacular meteor showers. The Perseids peak on Wednesday-Thursday 11-12 August, the Leonids on 17 November, and the Geminids on 13 December. On these dates the moon will be crescents (waning within four days of new moon for the Perseids, waxing for the other showers) and thus interfere little (if at all) with dark skies during the favorable early morning hours.

These meteor showers are named for the constellation from which the meteors appear to come. For example, tracing backwards, Perseid meteor path appear to converge near the Double Cluster in Perseus. Leonids radiate from the sickle of Leo, and Geminids from near the bright star Castor. A radiant is an optical illusion, a result of perspective. The meteor paths through space are nearly parallel (like rows of soybeans or railroad tracks) and only appear to converge. Perseids meteoroids are leftover dust from Comet Swift-Tuttle. Leonids and Geminids originate from Comet Temple-Tuttle and asteroid (possibly defunct comet) 3200 Phaethon, respectively. The geometry of the parent comet's orbits explains the location of the radiants, the speed of the meteors, even the multiple peaks or storms that are sometimes predicted. Explore the geometry and physics for yourself using the interactive NEO orbital plots linked above. Set the date for the day of the shower, zoom in centered on Earth and note how closely Earth's orbit intersects the blue orbit of the parent body. The angle of intersection determines meteor radiant and speed.

Following on impressive performances in the early 1990's, anticipation has now turned to this month’s Perseid shower (S&T August 2004, p. 78). The shower peaks on the night of 11-12 August 2004, at 11 UT or 7 am local time favoring the western US. Typically, up to 1 to 2 meteors per minute may be seen. Another, earlier peak may may favor Europe and Asia. We're in between, and the crescent moon will rise here about 2:30 (twilight begins about 4:30). Should clouds interfere on peak night, Perseids will be seen at half to one-quarter maximum rates a day or two before and after.

Perseids are an annual favorite with vacationers and inquisitive neighbors because summertime meteor watching couldn’t be easier. Choose a dark location with an open sky, a comfortable, reclining chair or sleeping bag, and look up toward your darkest region of sky. There will be little light from the Moon to dim viewing during any of these showers. Counts of meteors can be made using standardized procedures. A tripod-mounted camera suitable for time exposures, fast film and a bit of luck will photograph shooting stars (S&T August 1993, p. 97). Low light surveillance video cameras can record meteors on video tape for later viewing. Whether your goal is to contribute scientific results, tinker with new astro-toys or just spend an evening under the stars, there are many ways to enjoy the show. Mark your calendar, go out, and keep looking up!

Other useful web sites include the North American Meteor Network, International Meteor Organization, ALPO Meteors Section and the Americam Meteor Society. Moondark is written by Doug Miller, published on the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 25 July 2004. Text and images copyright © 2004 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.