Sky and Telescope - December 2016 - 7

ment from his observations taken in
Africa before and during the 2006 total
solar eclipse. The attempt failed, probably
because the camera's pixels were too large
- a problem overcome in Bruns's plans.
One suggestion: After completing his
observations next August, Bruns might
try using a comparison exposure of his
eclipse-centered star ﬁeld without the Sun
in it, instead of using the Gaia catalog.
Jay Pasachoﬀ
Williamstown, Massachusetts
Editor's note: A veteran of 33 total solar
eclipses and 31 more annulars and partials,
Pasachoﬀ chairs the International Astronomical Union's Working Group on Eclipses.

Hydrogen-Alpha by Eye?
I'm curious about why hydrogen-alpha
ﬁlters aren't mentioned in Rod Mollise's
informative article "Visual Filters for
Deep-Sky Observing" (S&T: Aug. 2016,
p. 28). His mention of H-beta ﬁ lters suggests there is some advantage to using

75, 50 & 25 Years Ago
December 1941
Cosmic Upheaval
"Twenty years ago when
the new director entered
upon his duties, what
were his surroundings? I
don't mean the Harvard
College Observatory of 20
years ago . . . I am thinking
about the universe. . . . Indeed, it wasn't the
same universe. . . .
"There are not many of you who can
remember the time when there were, so to
speak, no galaxies. I remember it very well.
I remember the day Dr. Shapley said: 'My
universe has been shattered.' . . . Our modern
world-picture, with its galaxies, its supergalaxies, its metagalaxy, was perceived - consciously perceived - all within the last 16
years. I doubt whether any other observational
science can point to a parallel expansion."
Cecilia Payne-Gaposchkin was speaking at
a staﬀ dinner celebrating Harlow Shapley's ﬁrst
20 years at Harvard. He had lost the famous
Shapley-Curtis debate of the early 1920s, conceding that most "nebulae" were stellar systems far
beyond the Milky Way.

those rather than an H-alpha ﬁ lter for
observing extended celestial objects containing atomic hydrogen. Why so?
Hal Heaton
Damascus, Maryland
Kelly Beatty replies: Although many deep-sky
objects emit plenty of hydrogen-alpha light,
the dark-adapted eye doesn't perceive it with
any kind of useful sensitivity. With a wavelength of 656 nm, it's simply too deeply red.
Conversely, hydrogen-beta is blue (486 nm)
and thus very obvious to the eye at night.

the 1950s, comes awfully close to my own
start in astronomy.
I have never been able to fully explain
how this all works. But no matter how
many spectacular Hubble images of
galaxies or incredible planetary shots by
Damian Peach we see, there is something
magical about the view in the eyepiece that
no imager can match. And that's one reason why I have remained a strictly visual
observer in this age of CCD imaging.
Vahe Sahakian
Houston, Texas

Moonstruck

For the Record

Many thanks for "The Moon Does That to
You" (S&T: Oct. 2016, p. 84). I can relate
to Mark Mathosian's experience, as I have
been involved in similar interaction with
strangers. His story also brings back the
memory of another recollection that left a
lasting and powerful impression on me,
namely "The Mysterious Coney Island
Telescope Lady" by Paul Johannsen (S&T:
Jan. 1992, p. 97). That one, dating from

✹ The time listed for full Moon (S&T: Aug.
2016, p. 42) should be 5:26 a.m. EDT.
✹ In the article "Processing with PixInsight" (S&T: Aug. 2016, p. 69), the sentence
beginning on the third line should be:
"Because atomic hydrogen [not Hα] also
emits at blue wavelengths . . ."
✹ Dan Dill's Canon EOS 5D Mark III, used
to record the transit of Mercury (S&T: Sept.
2016, p. 73), is a DSLR (not CCD) camera.

Roger W. Sinnott
December 1966
No Earth Cloud "For the
past seven years, data
have been accumulating
that suggest the earth is
surrounded by a cloud of
dust. . . . It now appears,
says Carl Nilsson of
[NASA's] Goddard Space
Flight Center, that the data gathered in early
satellite measurements should not be used
to test the hypothesis of a dust concentration
around the earth.
"Most of these satellite experiments used
piezoelectric microphones as detectors. . . .
But Dr. Nilsson has found from both satellite
and laboratory data that the microphone crystals emit noise spontaneously when subjected
to slowly varying temperatures, such as satellites traveling through the shadow of the earth
may experience. The rate of occurrence of the
noise pulses is consistent with ﬂight data that
have previously been interpreted as micrometeorite impacts."
Nilsson's study of detection methods knocked
out the underpinnings from a key "ﬁnding" of the
early Space Age.

December 1991
Halley Flare-up "Something strange happened
to Comet Halley last
February 12th. While
slowly receding from the
Sun between the orbits of
Saturn and Uranus, the
comet suddenly became
300 times brighter . . .
"The problem facing astronomers is explaining how so much energy might have been
released from Halley's nucleus so far from the
Sun's warmth. David W. Hughes (University of
Sheﬃeld) proposes that an asteroid some 2.6
to 60 meters across smacked into the comet.
"[But] Brian Marsden of the Central Bureau
for Astronomical Telegrams, for example, simply
disagrees . . . 'Comets are rather unstable
things. It doesn't take much to make them ﬂare
- a little bit of sunlight, weak though it may be,
gets into a crack and stirs up some volatiles.' . . .
"Hughes may have to wait until 2061 to
be vindicated. If he's right, spacecraft visiting
the comet during that return should detect a
fresh crater about 2.2 kilometers across on the
surface of the comet's nucleus."

Sk yandTelescope.com December 2016

http://www.SkyandTelescope.com

Sky and Telescope - December 2016

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