Don’t Miss Aldebaran’s Last Occultation Till 2033

Aldebaran joins the crescent Moon at dawn for its last occultation in 15 years — it's sure to be a beautiful sight.

Aldebaran occultation animation

On July 29, 2016, the waning crescent Moon grazed Aldebaran for those positioned in just the right locations. These images were acquired 0.1 second apart; south is up.
Bob Sandy / IOTA

Forty-nine consecutive months. That's how many times the Moon will have covered the bright star Aldebaran  when the current series of occultations — begun on January 29, 2015 — ends on September 3rd. If you haven't seen one of these stunning stellar cover-ups, Tuesday morning, July 10th, will be your last chance until the next series begins in 2033.

During its 18.6 year nodal cycle, the Moon weaves between 5.1° north and south of the ecliptic because of its inclined orbit, putting it within reach of four first magnitude stars: Spica, Antares, Aldebaran and Regulus. Of these, Aldebaran is the brightest at magnitude 0.9. Because Aldebaran lies 5.5° south of the ecliptic, the Moon should theoretically pass nearly ½° north of the star every go-round. It doesn't, though, thanks to parallax.

Beauty at dawn

For many locations in the Western Great Lakes, the occultation will resemble this view from Duluth with a thin crescent low in the east during at dawn.
Stellarium

Because the Moon is relatively close to Earth, it shifts position against the background stars depending on one's location. For instance, if viewed from diametrically opposite ends of Earth, say the poles, its position varies by 1.54° or three full moon diameters. From equator to pole, it's still about ¾° or 1.5 moon diameters — plenty of reach to cloak Aldebaran.

What makes Tuesday's event even more special is that none of four bright stars will be occulted again until the next Antares series begins on August 25, 2023, five years from now! What a bummer. All the more reason to set the alarm Tuesday, rub the sleep from your eyes and watch the 11% waning crescent rise next to the bright pink star.

Narrow viewing window

This map shows where the July 10th Aldebaran occultation will be visible. Cyan = occultation at moonrise/moonset; red dotted = daytime occultation; blue = twilight occultation; and white = nighttime occultation. Observers living in the Great Lakes region have the best seats. From all locations, the Moon will be low in the northeastern sky during morning twilight. Click the map for more information. 
Occult V4.5.9

Now for a bit of bad news. This last accessible Aldebaran occultation — the remaining two occur across the Arctic wilderness — will only be visible for observers in the Western Great Lakes region from Ontario as far west as North Dakota, Iowa, and southern Manitoba. At the western end, the Moon rises with Aldebaran already occulted, so those places will only witness the star's reappearance, which is the best part anyway.

Since the Moon will be just 2.5 days before new, it will be low in the sky for everyone — only 3° high for Minneapolis and 7° for Mackinaw City, Mich. at reappearance — so find a location with an unobstructed eastern horizon. You'll find a list of cities and times of the star's disappearance and reappearance here. Times are Universal Time, so remember to subtract 4 hours for Eastern, 5 for Central and so on.

Mighty Moon, tiny star

Even a basic mobile phone can do a decent job of recording an occultation. I took this picture with an older iPhone through a 10-inch scope on Oct. 2, 2015, shortly before the Moon occulted Aldebaran an hour after sunrise.
Bob King

The good news is that even if you aren't in the path, you can still see a close conjunction of star and Moon. In New Orleans the two will be only 20′ apart shortly after local moonrise, 1° in San Francisco and a scant 4′ apart in New York when closest at 4:28 a.m. in morning twilight. 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. Be sure to bring a camera to capture this beautiful pairing.

An occultation of a bright star is a wonderful thing to witness in binoculars or a telescope. As the Moon moves eastward in its orbit, it edges ever closer to the star. Moments before disappearance, the star hovers at the limb of the Moon for what seems like forever. Then all at once, it vanishes in a flash. Minutes later, we're treated to an equally sudden "star rise" along the Moon's darkened, earth-lit limb. An incredible sight.

Catch a Grazing Occultation

Aldebaran Grazing Occultation March 5, 2017 Combined Videos from Andreas Gada on Vimeo.

You want an even more amazing sight? Travel to the graze line, where Aldebaran will scrape along the Moon's southern limb, flashing in and out of view as it alternately disappears behind mountain peaks and reappears as they travel past. Depending on your location, you may witness several to more than a dozen flashes.

Keen-eyed skywatchers may notice that Aldebaran — unlike lots of other occulted stars — won't blink out in an instant. With a radius 43 times that of the Sun, it has a true angular size, it will take a moment for the star to disappear and reappear. David Dunham, a founder of the International Occultation Timing Association (IOTA), described Aldebaran's return to view in a 1962 occultation as "appearing like a drop of water coming out of a faucet."

Celestial "Vegas Strip"

This diagram shows the general graze path across the U.S. Midwest and Ontario. Click for more detailed maps.
Google / IOTA

Check here for detailed graze paths and times for specific cities and regions including Green Bay and Appleton, Wisconsin; Mackinaw City, Michigan; and Iron Bridge, Ontario. To help plan your outing, check the latest forecast at the National Weather Service site by typing in your town's name in the local forecast box.

A delicate, Earth-lit Moon and bright orange star fused in the growing light of dawn — I think this is going to be a beautiful event. I'd love to hear about your experience, so drop by later with a comment or photo.

14 thoughts on “Don’t Miss Aldebaran’s Last Occultation Till 2033

  1. astromasterastromaster

    Hi Bob: Great article about Aldebaran grazing the Moon. I am glad you mentioned Dr. Dunham who has virtually single handedly created the IOTA movement. I was glad to see you include the New York team’s Toronto graze videos of the latest grazing occultation – their multiple teams really show what a grazing occultation looks like in action. You also might take note of my Johns Hopkins University, Applied Physics Lab. web page at: http://iota.jhuapl.edu/watts.htm for further information on Dr. Watts’ work on the Marginal Zone of the Moon and some of the strange connections with other historical events such as the Cold War. Also, I have just published a chapter on “Grazing Occultations” in the Encyclopedia for Lunar Science edited by Brian Cudnik for Springer Publishing 2018.

    1. Bob KingBob King Post author

      Thanks, astromaster and kudos on the “Grazing Occultations” chapter for the encyclopedia. That Toronto graze video is VERY exciting to watch.

      1. astromasterastromaster

        HI Bob: Yes, I would often attempt to explain to my teams what their individual observations would look like if they could see them all replayed together, but they would have to wait until I turned their observations in and obtained a Pictorial Reduction in return to show them. In the Toronto graze the New York team was able to see the topography to some extent in the first replay. With a little imagination you can use the Disappearances and Reappearances of Aldebaran to picture mountains and valleys along the Lunar Limb. You can use the Occult Software now to obtain beautiful “fly over” views of the Lunar Limb Topography known as the Watts P&D datum. In the old days the Cassini 3rd Law Regions had to be left blank in the original Watts P&D Charts, but thanks to our Grazing Occultation Expeditions in these areas along with the Japanese Kayuga and USA LRO laser altimeter missions we now have a pretty complete view that only needs some final corrections in a few places. The southern Cassini Region happens to contain the very low crater Ashbrook named after the well known S&T historical writer Joseph Ashbrook!

    1. Bob KingBob King Post author

      That’s OK, Jakob. I won’t be in the graze zone either because of other commitments, but I will be watching and photographing the whole event from my home further north.

  2. Anthony BarreiroAnthony Barreiro

    Thanks Bob. I’ve seen a few of Luna’s recent occultations of Aldebaran, but never a graze. The video is exciting! The different observers set up between 300 and 600 meters from something, but I don’t understand from what. What is the zero point or reference line?

    1. astromasterastromaster

      The zero point is known as the Limit Line as defined these days in the computer program Occult. The program will generate the line for you on Google Earth and also generate the same line on the Prediction Profile for each graze. The Prediction Profile will also show you the Lunar topography which can be either north or south of the Limit Line. You can then use both to help you decide where the best place is in your area in order to make the best observations.

        1. astromasterastromaster

          Yes Anthony, that is the basic idea. However, remember that the Lunar topography in some areas can extend outside the Limit Line where you would find the infinitesimally brief moment. We need observers to go for that brief tall peak, but we also need many others to end up getting a complete map of all the mountains, valleys and craters. Particularly, if you are alone you will want to take a careful look at the profile and find what we call the “sweet spot” – the area along the Lunar topography where the Moon will cause the star to disappear and reappear the greatest number of times so that you obtain the most data for your single site! Typical Grazing Occultations will provide the average observer with between 4 to 10 timings, but sometimes you can get a really “sweet spot” that will provide something like 20 timings. They should be video recorded with a GPS Time Inserter, since many times the timing can come very quickly and within a minute or two. When this happens the average observer simply can not accurately detail the events unless they have been properly recorded. Check the International Occultation Timing Association IOTA web site for more info.

  3. Joe StieberJoe Stieber

    Bob — another great article, thanks for the heads-up!
    Here in southern New Jersey, we’ve been blessed with three consecutive nights of cool temperatures and clear skies, most unusual for July. Although we were too far south for an occultation, the moon’s close pass by Aldebaran this morning (July 10) was a beautiful sight nevertheless.

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