Sky and Telescope - August 2016 - 51

David Dunham, a founder of IOTA, has led the coordination of occultation
timings since 1962. Write him at dunham@starpower.net.

David Dunham

Occultation of Aldebaran
July 29, 2016

The Graze
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S&T: LEAH TISCIONE, DATA: IOTA

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 link in the special page we've 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 angular size: 40 meters wide at the Moon's
distance. Sometimes a sideways-speeding
hill may cover only a part of the star's face,
causing an incomplete partial occultation.
We can now predict these narrow zones
very accurately, thanks to the laser altimeters on Japan's Kaguya and NASA's Lunar
Reconnaissance Orbiter spacecraft. These
mapped all of the Moon's topography to
high accuracy. So now, you can choose
your viewing location to within just a few
meters in order to maximize the number
of contact events you can view or record.
See the web page for the event.
From the mid-1960s into the 1980s,
observers with the International Occultation Timing Association (IOTA) recorded
hundreds of grazes, mostly visually, to
reﬁne the lunar proﬁle and star positions. Now that we know both of these
parameters very well, such observations
are less valuable to astronomers. So since
the early 1990s, IOTA has concentrated its
eﬀorts more on asteroid occultations.
But lunar occultations do remain valuable to discover and resolve close double
stars, especially when recorded with
video. The Kepler-2 Project is currently
looking for exoplanet transits of stars
near the ecliptic. The Kepler team is very
interested in which of their program stars
might be close doubles, because stellar
duplicity throws oﬀ the analysis of any
sign of an exoplanet. Aldebaran isn't suitable for this particular work, but some

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Observers in the southern and eastern US, the Caribbean, and most of Mexico can watch
the Moon occult Aldebaran on the morning of July 29th. Along the graze line, you're likely
to see gradual events due to the star's relatively large angular size. The graze occurs during night for Texas, dawn from Oklahoma to Lake Erie, and daytime northeast from there.

A Blue-Sky Aldebaran Occultation on August 25th
One lunar month after the July 29th
occultation, the Moon crosses Aldebaran
again: during daytime on August 25th for
much of the southwestern U.S.
This time the Moon will be last quarter.
Once again, because the Moon is waning,
the disappearance happens on the bright

of the Hyades stars to be occulted on the
same night are. Again, see the web page.
Even though we don't expect to learn
much new from it, the July 29th Aldebaran graze is probably the best of the
current series for populous parts of North
America. For that reason, IOTA will hold
its annual meeting in nearby Stillwater,
Oklahoma, the weekend after the Fridaynight graze; details are in the link. Most
attendees will try to observe the graze
from northeast of Edmond, Oklahoma;
some will be ready to travel if clouds
threaten. You're welcome to join our expedition, the IOTA meeting, or both.

Hyades Stars
About two hours before Aldebaran reappears from behind the Moon's dark limb,
the 4.8-magnitude Hyades star ZC 677
will do the same for much of the eastern
US. Its graze line will run about 100
miles south of Aldebaran's and pass over

limb and the reappearance on the
dark limb. Some times: at Kansas City,
disappearance 12:56, reappearance 1:11
p.m. CDT; Denver, d. 11:43, r. 12:14 p.m.
MDT; Los Angeles, d. 10:21, r. 11:26 a.m.
PDT; Berkeley, d. 10:17, r. 11:15 a.m. PDT;
Honolulu, d. 5:52, r. 7:17 a.m. HAST.

Cincinnati, Ohio, and Ithaca, New York.
Details are in the event link.
Also to be occulted for various parts
of North America are several other stars,
including the pair Theta1 and Theta2
Tauri, magnitudes 3.8 and 3.4, respectively. Detailed predictions for those two
are in the predictions link.

For More Information
* We at IOTA have set up a special web
page for the July 29th event, with an
interactive Google Map and much other
information: occultations.org/Aldebaran.
* Detailed predictions for most of the
events mentioned here are listed for more
than 1,000 locations at lunar-occultations.
com/iota/bstar/bstar.htm. Note that the
page for each star displays three long
tables with less-than-obvious divides: the
disappearance, the reappearance, and the
locations of cities. ✦
- David Dunham
Sk yandTelescope.com August 2016

http://www.occultations.org/Aldebaran http://www.lunar-occultations.com/iotat/bstar/bstar.htm http://www.lunar-occultations.com/iotat/bstar/bstar.htm http://www.SkyandTelescope.com

Sky and Telescope - August 2016

Table of Contents for the Digital Edition of Sky and Telescope - August 2016