When the moon covers a bright star it’s called an occultation. There have been 49 occultations of Aldebaran in a row every month since Jan. 29, 2015. Many of them have occurred in daylight or over distant lands and oceans, so the actual number visible from any one location is much lower. Add in bad weather and the number drops even lower. That’s why I’m hoping you’ll make the effort to see the final one visible this Tuesday morning (July 10), when a sliver of a crescent moon covers and releases the star at dawn.
The event happens in twilight’s rosy glow for locations in the Western Great Lakes region including parts of Ontario, Michigan, Wisconsin, Minnesota, North Dakota and Iowa. In areas further south, the moon misses the star but the two will be in close conjunction. From New York City for instance, skywatchers will see the star pass just 0.06° south of the moon’s southern horn. That increases to 0.3° in New Orleans and 1° (two moon diameters) in San Francisco.
Key to seeing the event will be finding a place with a wide-open view to the east-northeast, where the moon will be rising. From Duluth, Minn., the moon will be just 3° high in the east when it first covers Aldebaran at 3:30 a.m. and only have climbed to 5° when the star reappears 28 minutes later. You can easily find out exactly where to look and how close the two will be for your location by downloading an app for your iPhone or Android. I use Star Chart (free) and the recently introduced free version of Sky Safari. Just Google ’em up. Or you can download a free PC or Mac version of Stellarium for your laptop.
That’s how occultations work. The moon orbits the Earth, slowly pushing east across the sky at the rate of about one outstretched fist each day. As it approaches Aldebaran, the star will appear to get closer and closer to the bright, sunlit crescent until all it once, it vanishes behind its bright edge. Minutes later, Aldebaran will pop back into view along the “dark” edge. It’s not really dark but lit up by a faint bluish, gray light called Earth-light — sunlight reflected by the Earth out to the moon, which reflects a portion back to Earth.
Along its path, the moon can cover up to four bright, first magnitude stars: Aldebaran, Spica, Antares and Regulus. Aldebaran is the brightest of them. Occultations can happen during any phase of the moon. Because this one involves the thin, waning crescent, it will be fairly easy to see (no glare from a big moon!) and especially beautiful. Add in the dawn colors, and wow — not to miss despite the arduous hour. Don’t forget a tripod and camera to record the scene.
To see a list of cities where the occultation occurs, click here or on the map for the times when Aldebaran will disappear and then reappear. Times are given in UT or Universal Time. To convert these to your time zone, subtract 4 hours for Eastern, 5 for Central, 6 for Mountain and 7 for Pacific. For example, if the time reads 8:47 UT and you live in Minnesota, subtract 5 hours, making it 3:47 a.m. Central Time.
Aldebaran Grazing Occultation March 5, 2017 Combined Videos from Andreas Gada on Vimeo.
Those skywatchers fortunate enough to live along or who can otherwise get to the graze line will see something even more amazing. Along this line, Aldebaran will appear to scrape along the southern edge of the moon, flashing in an out of view as it’s alternatively hidden by individual mountain peaks and then exposed in low-lying lunar valleys. Depending on your exact location, you might see a couple flashes or more than a dozen. The graze line passes over Mackinaw City, Mich. and Appleton and Green Bay, Wis. To see it in greater detail, click here for individual maps and times, so you can plan to be in just the right spot.
I’d travel to the graze line in Green Bay if I had the day off, but I don’t. The view from home should be great with the occultation visible from start to finish. Aldebaran should be bright enough to see with the naked eye near the moon once it clears the horizon haze, but the best views will be in binoculars and small telescopes. If you are using a telescope and have a mobile phone, it’s easy to get a picture of the two by hovering your phone’s camera directly over the eyepiece until the moon shows on your touchscreen. Tap the moon lightly to ensure a crisp focus then snap a photo. Lots of them! Then send me an e-mail at firstname.lastname@example.org with your images, and I’ll post them later Tuesday.
A final thought. Like the other three bright stars, Aldebaran occultations occur in series. Tuesday’s will be the last one visible over populated areas (the final two occur in the Arctic) until the year 2033. No other first magnitude star will be occulted by the moon until August 25, 2023, when a new series begins with Antares in Scorpius. That more than 5 years away! Before the drought begins, take a deep drink at dawn.