Moon Watch

The moon is in a waning (growing smaller) gibbous (more than half lit) phase going to last quarter on Monday. Today is the last day of February, which is an anomalous month with only 28 days most of the time and 29 days every fourth year on leap year. It is the only month that can occur without a full moon. The last time that happened was in 1999, and the next time it will happen is 2018.

The waning moon rises later and later each evening for the next several days, giving us a dark sky after astronomical twilight ends a little before 8 p.m. An enjoyable observing project is to look for the zodiacal light.

From the somewhat lower latitude of Tucson you often can see it throughout the year. The zodiacal light is large pyramidal glow in the western sky after sunset or in the eastern sky before sunrise.

Now until April is a good time to look for it in the west on moonless nights 90 minutes to two hours after sunset. It is surprisingly bright and quite large, extending at least half-way up the sky from the horizon. I can see it sometimes at my house in the Foothills, though it is much more impressive from out of town.

The first time you see the zodiacal light you might mistake it for the glow of city lights. The zodiacal light is the glow of microscopic dust particles originating from comets and asteroids. These innumerable fine grains lie along the orbital plane of the solar system. The sun's light reflects off them, producing a distinct glow best seen during those times when the plane of the solar system is near perpendicular to the eastern or western horizon.

It is spectacular from a dark site. Give it a try.

If you go

Friday's meeting of the Tucson Amateur Astronomy Association will start with a lecture by John Kalas on space stamps. From Sputnik through the Apollo moon landings, space conquests have been commemorated on postage stamps across the globe.

Next, Carmen Austin, a University of Arizona astronomy student, will talk about "Teach Astronomy," the website developed by the night's main speaker, Chris Impey, and his students.

Impey, a distinguished professor at the UA and deputy head of the astronomy department, will discuss the future of amateur-professional astronomy collaboration, with research areas that extend from exoplanets to supermassive black holes.

The free event starts at 6:30 p.m. at the Lunar & Planetary Lab lecture hall, in the Kuiper Building, 1629 E. University Blvd.

Contact Tim Hunter at