Aldebaran’s Disappearing Act on March 4th

On the night of March 4th, all you'll need are your eyes to watch the Moon occult Aldebaran. Better yet, place yourself on the graze line.

It can't get much better than this: an occultation of the brightest star the Moon ever crosses, visible across most of the U.S. on a Saturday evening, with the Moon barely first-quarter (46% sunlit), and the star's dark-side disappearance visible without optical aid.

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

Moreover, along a narrow path skirting the U.S.-Canadian border, passing the Toronto area and then the U.S. Northeast, a spectacular grazing occultation will occur that, at least in the populous eastern part of the path, will be nearly as good as last July's well-observed Aldebaran graze in the southern plains.

The graze provides another opportunity to detect the angular size of a star other than the Sun. But this time, the Moon will be brighter with the graze events occurring closer to the northern cusp, so binoculars are recommended to see the gradual and partial occultation events that will reveal the star's angular size.

The occultation will also be visible at night throughout Mexico, most of Central America, the western Caribbean Sea, and Bermuda. The occultation will occur in daylight in Hawai'i (disappearance at 3:33 p.m. HST in Honolulu).

The southern-limit graze will occur on the Moon's sunlit limb 6° inside the southern cusp, as seen from a narrow path crossing Costa Rica, Panama, and Colombia. A telescope is needed to see or record the star's reappearance even at night, because this will occur on the Moon's sunlit edge.

Event times for selected cities are below. More precise times, and additional details including the altitudes of the Sun and the Moon, are listed for over 1,000 cities and towns on this predictions page.

Los Angeles, disappearance 7:08 p.m. PST, reappearance 8:27 p.m. PST; Seattle, d. 7:21 p.m., r. 7:50 p.m. PST; Denver, d. 8:33 p.m., r. 9:33 p.m. MST; Chicago, d. 9:57 p.m., r. 10:33 p.m. CST; Austin, d. 9:44 p.m., r. 10:52 p.m. CST; Atlanta, d. 10:56 p.m., r. 11:52 p.m. EST; Miami, d. 11:03 p.m., r. 12:02 a.m. EST; Pittsburgh, d. 11:03 p.m., r. 11:36 p.m. EST; Washington, DC, d. 11:04 p.m., r. 11:39 p.m. EST; New York, d. 11:10 p.m. EST, r. 11:31 p.m. EST; Miami, d. 11:03 p.m., r. 12:02 a.m.

Aldebaran occultation March 2017 map

The first-quarter Moon will occult Aldebaran on the night of March 4th for most of the U. S., Central America, and the Caribbean. Along the graze line the Moon’s northern limb will barely skim the star, likely producing several gradual disappearances and reappearances. The entire event occurs on the night of March 4th local time — except for locations in the extreme southeastern U.S. from which reappearance occurs just after midnight on March 5th.
Sky & Telescope / Lean Tiscione; source: IOTA

The Graze

The most interesting views will be from a strip of land only a few hundred yards wide along the occultation's northern limit. You can examine the precise graze path to street-level accuracy using the interactive Google Maps links in the special page the International Occultation Timing Association (IOTA) has set up for this event, also given at the end of the article.

Viewed from this narrow zone, the giant star should disappear and reappear multiple times as hills and valleys along the Moon's northern limb cover and expose it. Most of these events will appear non-instantaneous, even taking up to a full second, due to Aldebaran's large angular diameter: 20 milliarcseconds, or 40 meters at the Moon's distance.

We can now predict these narrow zones very accurately, thanks to the laser altimeters on recent lunar orbiting spacecraft. These mapped all of the Moon's topography to high accuracy. So now, you can choose your viewing location to within a few tens of meters in order to maximize the number of contact events you can see or record.

The angle of "central graze" changes along the path, causing the lunar profile to also change. South of Vancouver, where the graze happens only a mile north of the U.S. border, the star skims the sunlit limb. In eastern North Dakota most of the graze will be on the Moon's dark side edge although the high mountains just after central graze will likely be sunlit. As far east as Rhode Island, the Moon will be only 13° above the western horizon.

Occultation timings are not as compelling scientifically as they used to be. However, video recordings of grazes are so precise that small corrections to the spacecraft lunar-profile data, and to star positions, are still being found. The greatest value of lunar occultations is now in discovering and resolving very close double stars. But we know that Aldebaran has no close companion.

Information about IOTA expeditions to observe the graze will be posted on the graze web page noted below. Plans will change in response to weather updates.

Hyades Stars

Four hours before Aldebaran disappears behind the Moon's dark limb, the 5.0-magnitude Hyades star 75 Tauri will do the same for Atlantic States south of southern Pennsylvania.
Two hours before Aldebaran the Moon will occult some brighter Hyades stars, including 4.8-magnitude ZC 677 and the pair Theta1 and Theta2 Tauri, magnitudes 3.8 and 3.4, respectively. Detailed predictions for them are in the predictions link below.

More Information

• We at IOTA have set up a special webpage for the Aldebaran graze of March 4-5 (March 5th UT), with interactive Google Maps and much other information: occultations.org/aldebaran/2017march.

Detailed predictions are available for more than 1,000 locations. Note that the page displays three long tables — for the disappearance, the reappearance, and the locations of cities — with less-than-obvious divisions between them.

10 thoughts on “Aldebaran’s Disappearing Act on March 4th

  1. Frank-ReedNavigation.comFrank-ReedNavigation.com

    It’s likely to be bitter cold in the northeast for this occultation, but this is something that can be observed quickly. Five minutes outside is all it takes! For observers in Rhode Island, where the Moon will be low in the west only about 13 degrees high, there are a couple of convenient public parking lots right along the graze band: the lot of the movie theater off route 1 in South Kingstown and the parking lot of the Stop & Shop near the interchange of route 138 and I-95. Both are easy access and safe. Be ready with binoculars at 11:18. Observe the flickering star for five minutes. Then thaw out!

  2. John-Allen

    Observed it from CT! My home site was 17km south of the nominal graze path, but I located a parking lot 300m south of the nominal graze line. The temperature was 14 deg F, with wind gusts at 14 mph, wind chill deg 2 F. Positioned my car so I could use 10×50 binoculars supported against the window frame. The event was spectacular. Aldebaran is very bright compared to the illuminated surface of the moon, and the color contrast is beautiful. I witnessed 4 separate dis- and re-appearances over the course of only a couple minutes. The occultations were not immediate, they took about a second, which is easily visible. The seeing wasn’t good, but I think I saw a couple partial fadings, which means Aldebaran was partially eclipsed! It was most definitely worth the effort to see it, more than I expected, despite a couple of totally numb fingers for about half an hour. Thanks for the great info for locating a good observing site.

  3. Eduardo Gutierrez

    Hi. I live in Costa Rica and, fortunately, almost exactly in the southern graze path. I watched the star disappear, appear again for just about half a second, and disappear again. My location is 9,9215°N and 83,8991°W. Regards to everyone

  4. Frank-ReedNavigation.comFrank-ReedNavigation.com

    We had great views of the grazing occultation from southern Rhode Island. I posted a detailed message early yesterday, but it did not appear.

    1. Frank-ReedNavigation.comFrank-ReedNavigation.com

      Apparently links in a message lead to “your comment is awaiting moderation”. Let’s see if I can post masked links…
      Here is a variant of my “detailed message” posted on the NavList message boards:
      fer3[dot]com/arc/m2.aspx/g38566

      And here’s CWB’s wonderful video of the occultation:
      youtube[dot]com/watch?v=ObszAwMfqEg

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