Up in the Air: See the moon eclipse a star

The moon is giving us a guided tour this month, with close passes to planets and bright stars. Having passed within a couple of degrees of Venus about dawn on Nov. 6, it was 1 to 2 degrees from Mars and Venus the next morning. The waning crescent moon will be lost in the glare of the Sun at dawn Nov. 11.

Emerging in the evening sky, the very thin crescent moon will be a few degrees from Saturn, very low in the dusk sky on Nov. 12. Our moon will brighten up to be full over the following couple of weeks and will take aim at the brightest star it can ever sidle up to.

In fact, it will be a direct hit.

Astronomers have studied such eclipses of other stars – called occultations – to learn about both the star being eclipsed and the moon itself. I cut my teeth in astronomical research as an amateur in high school, timing occultations from a rooftop observatory my dad built for me. I got my predictions, custom-computed for my location, from occultation expert David Dunham. In the late 1960s, NASA was very much interested in such data to track changes in the moon’s orbit. After all, they were headed there!

In graduate school, a fellow student developed a system to record the light of stars as they were occulted. While to the eye the event seems instantaneous, it actually takes some tens of milliseconds to dim, and it goes down with a few bumps due to diffraction (put your finger close to your eye and you will see it not as a sharp edge, but fuzzy for the same reason). Measurements can reveal the size of some nearby, large stars.

Decades later, I am doing “occult science” again, but now timing the occultation of stars by asteroids. These observations yield the size of those rocks, most of which we have not been able to image directly. (And who still collaborates on this? Yep – David Dunham!)

This month’s occultation will occur pre-dawn on Thanksgiving morning, when the moon will block the bright star Aldebaran, a red giant star.

You can try to observe the event, using a small scope or binoculars to see – barely – the star near the bright moon. The star’s orange color will help out a bit. Since we are near the southern limit for the event, the timing strongly depends on your location. You would do best to start watching the star about 5:30 in the morning. It will reappear 20 to 30 minutes after it disappears.

So, get up early on Thanksgiving and, before doing some food science on that turkey, do some occult science!

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: More on this month’s column: