One of the sky’s brightest stars will be blanked out by the waning crescent moon in a dramatic morning occultation this Sunday, Oct. 15. The event happens in a dark sky before and at dawn for all of the U.S. and Central America. There are only four bright stars that are ever occulted by the moon: Aldebaran, Spica, Antares and Regulus. Because the moon’s path across the sky varies cyclically over the years, this will be one of the last bright stars it will cover for U.S. skywatchers for the next several years. Reason enough to get up early and see it.
Fortunately, it’s a Sunday and not a work day, making it a little easier to lose the sleep. The moon’s bright limb (the eastern side as seen in the sky) will cover up the star around 5:30 a.m. Eastern Time (4:30 Central, 3:30 Mountain and 2:30 Pacific). You can watch as the moon, which travels eastward in its orbit around Earth, slowly approaches Regulus. Just before the star is covered, it will appear to hover right next to the edge for what seems like a long time. Almost as if time stops. Then all at once it disappears in a flash!
There’s no gradual transition because at its distance of 77.6 light years, the star is virtually a pinpoint and the moon’s has no atmosphere that would otherwise cause Regulus to fade gradually instead of blink out. If you live in the northern U.S., the star will disappear for only a few minutes because it nearly grazes the moon’s northern limb. Matter of fact, along a line from western Oregon through Montana, North Dakota, and northern Minnesota observers will see a grazing occultation, with Regulus flickering in and out of view as it alternately disappears behind mountain peaks and reappears from valleys between them.
I think the best part of this event happens right at the end, when the star will dramatically reappear at the moon’s dark, earthlit edge. With only modest glare from the sunlit part of the moon, we should be able to see Regulus jump back into view without any optical aid at all.
It’s not often you can see either a disappearance or reappearance of a star with just the naked eye, so if you’re in an experimental mood, give it a try. Binoculars or a small telescope will guarantee the sight. Just click on this link , and you’ll be whisked to a site with a list of more than 700 cities with times of disappearance and reappearance. Since you’ll see the moon approach Regulus and can anticipate its disappearance, that half of the occultation will be no problem to follow. The reappearance time is more important because it’s harder to anticipate exactly where and when Regulus will pop back into view along the opposite lunar limb. Times are in UT, so remember to subtract 4 hours for Eastern Daylight, 5 for Central, 6 for Mountain, and 7 for Pacific.
Given that October is the month with closest ties to the occult, I’m tickled at the timing of this occultation. It would be even better if it happened on Halloween.