Sky and Telescope - August 2016 - 50

OBSERVING

Celestial Calendar

Resolve a Star's Disk Naked-Eye!
"Stars are points." You learned that as
soon as you took up astronomy. They're
so far away, with apparent diameters so
tiny (not counting the Sun, of course),
that they're indistinguishable from how
mathematical point sources would look,
even in the largest telescopes.
Well, most of the time. Various
technologies and techniques can now get
around that stricture to some extent. But
for amateurs? Never!
Except in one instance: during a grazing lunar occultation of a bright red or
orange star - in particular Aldebaran,
the 1st-magnitude eye of Taurus.
Aldebaran is not only the brightest star
that the Moon can ever occult, it's also
one of the largest in apparent diameter:
20 milliarcseconds. If you're positioned to
see the Moon's edge barely skim Aldebaran, you may witness it fading and reappearing less than instantaneously. Even
with your naked eyes.
On Friday morning July 29th, observers in much of the southern U.S. as well
as Mexico and Central America will be

treated to a naked-eye reappearance of
Aldebaran from behind the dark side of the
waning crescent Moon. The crescent will
be 23% sunlit. Observers near the graze line
on the map at right have a chance to see a
gradual occultation. For observers farther
north, the Moon and Aldebaran will perform a near miss.
You'll need at least a small telescope if
you're on the eastern side of the continent,
or in southern or central Europe, because
there the occultation happens in daylight.
The farther west you are, the darker the sky
will be. Right along the graze line, the map
indicates where the graze happens in daytime, bright dawn, deep dawn, or night.
Where the occultation is total rather than
grazing, you can also catch the star's disappearance on the Moon's bright limb. For
this you'll need a telescope even at night,

because without high magniﬁcation,
the glare of the Moon's sunlit terrain
hides even a 1st-magnitude star right
next to it. Aldebaran's reappearance on
the dark limb will be much easier - if
you're watching at the right moment!
Here are some times:
At Austin, the disappearance is at
4:39 a.m. CDT, the reappearance at 5:19
a.m. CDT; Atlanta, d. 5:48 a.m., r. 6:40
a.m. EDT; Miami, d. 5:33 a.m., r. 6:43
a.m. EDT; Pittsburgh, d. 6:11 a.m., r.
6:44 a.m. EDT; Washington, DC, d.
6:05 a.m., r. 6:53 a.m. EDT; Boston, d:
6:20 a.m. EDT, r. 7:03 a.m. EDT.
More precise times, and additional
details including the altitudes of the
Sun and the Moon, are listed for over
1,000 cities and towns in the predictions link at the end of this article.

´ , Poland on April 28, 1998, Ladislav Smelcer
When the Moon's limb grazed Aldebaran over Łódz
was there videorecording with an 11-inch Schmidt-Cassegrain scope. Here is every 5th frame of
a 3-second clip from his movie. Aldebaran dims gradually, rebrightens partially, then fades out
altogether. His setup inserted the date and local time into each frame; Universal Time is two hours
earlier (so the UT minute is 18:59). Twenty-ﬁve other observers timed graze events visually, mostly
by voice calls into tape recorders. In all, 142 contacts were timed.

LADISLAV SMELCER / DAVID DUNHAM/ IOTA

August 2016 sky & telescope

Sky and Telescope - August 2016

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