Sky and Telescope - December 2016 - 6

The Show Must Go On
I enjoyed Eden Orion's "Making It Special" (S&T: July 2016, p. 84). If you do public outreach, sooner or later you're going
to have to improvise. When the weather,
the venue, or the planets don't cooperate,
and you already have people lined up for
viewing through your telescope, it's just
really hard to pack up and go.
Here at the Desert Foothills Astronomy Club, we've done those one-star solo
performances. We've also used a single
bright star at low elevation to show twinkling and dispersion eﬀects. And we've
alternated between stars (like Betelgeuse
and Rigel) to emphasize color diﬀerences
- people are surprised by the contrast.
Dan Heim
New River, Arizona

The Power of Totality
William Sheehan's article, "The Great
American Eclipse of the 19th Century"
(S&T: Aug. 2016, p. 36) calls to mind a
solar eclipse of even longer duration
- nearly 5 minutes - whose path of
totality swept across North America
from Baja California to Massachusetts
on June 16, 1806. Eclipse timings in
New England were collected and analyzed by Nathaniel Bowditch, the nation's
foremost mathematician and expert on
marine navigation.
The eclipse was also witnessed from
upstate New York by 16-year-old James
Fenimore Cooper, a decade before he
penned The Last of the Mohicans. Long
afterward, Cooper recalled the sublime
experience in a magazine essay: "There
ﬂoated the moon, distinctly apparent, to
a degree that was almost fearful. . . . It
looked grand, dark, majestic, and mighty."
Perhaps the most enduring impact
of the 1806 eclipse on then-ﬂedgling
American science was to inspire a young
clockmaker's apprentice, William Cranch
Bond, to pursue a career in astronomy. In
1839, Bond became the founding director
of Harvard College Observatory.
Alan Hirshfeld
North Dartmouth, Massachusetts
6

December 2016 sky & telescope

Clear

0.1D

0.2D

0.3D

0.4D

0.5D

As this simulation shows, increasing the diameter of a telescope's central obstruction as a fraction
of its aperture (D) causes noticeable changes in the resulting Airy disk, which is the diﬀraction pattern of a point source as seen through telescopic optics.

A Note About Airy Disks
I enjoyed "Find your Dawes Limit" (S&T:
Sept. 2016, p. 30) so much. Let me add
one minor point, however, regarding
Phillip Kane's comment, "A scope with a
central obstruction shows a slightly larger
Airy disk . . . than an unobstructed aperture." Doesn't an obstruction make the
center of the Airy disk slightly smaller?
Nozomu Muto
Sinjuku (Tokyo), Japan
Phillip Kane replies: You are correct - as
the obstruction increases, fewer photons end
up in the Airy disk's central peak and more
end up in the outer rings. And you're in good
company to notice this. The great J. B. Sidgwick (Amateur Astronomer's Handbook,
4th edition, 1980) notes: "For work requiring
maximum resolving power, increasing the
diameter of the central obstruction might
proﬁtably be tried. . . . Sir John Herschel
long ago reported an improvement in a
telescope's resolving power when used with a
D/5 to D/6 central stop, despite the brightening of the rings." One more thing: my big
table of double stars didn't include their SAO
identiﬁcations. Because these are so widely
used by Go To telescopes, you can download
a version of the table that includes them at
https://is.gd/Dawes_Limit_star_list (or I
can send you this version if you'll just email
me at icycomet1944@gmail.com).

Two Amazing Observations
Is it possible for Mars to get bright
enough at opposition to cast a shadow?
I've never read about this anywhere, and
I'd never noticed one, though I've seen
both Venus and Jupiter cast shadows. So I
decided to look for it on some good, dark
nights last May, while standing outside
my observatory with no artiﬁcial lighting

around. By slowly rocking back and forth,
I could watch my shadow jump across the
observatory's outer wall. Actually I found
it fairly easy to see.
Mark Hays
Newbury Park, California
I am in my 32nd season as a U.S. Forest
Service ﬁre lookout. During August's
Perseid meteor shower, I had a remarkable "ﬁrst." Many of its shooting stars
coursed across the heavens, and at one
magical moment two meteor trails actually crossed. We all know that "X marks
the spot," so I'm trying to ﬁgure out how
to get up there. I've got a call in to NASA
- I'll let you know how it goes.
Don Evans
Prineville, Oregon

Measuring Bent Starlight
I wish Donald Bruns the best of luck and
skill for his attempt to duplicate Arthur
Eddington's 1919 test of Einstein's general
theory of relativity during next year's total
solar eclipse (S&T: Aug. 2016, p. 32). Bruns
mentions the inability of a 1973 eclipse
expedition to measure the deﬂection of
starlight by the Sun's gravity. However,
let me note that a dust storm at its site in
Mauritania brought transparency down to
10%, a major reason for the failure.
A decade ago, Carl Pennypacker
(Lawrence Berkeley Laboratory) and I,
with students, tried to use computerized
data reduction to help Jean-Luc Dighaye
pull such a stellar-deﬂection measureWrite to Letters to the Editor, Sky & Telescope,
90 Sherman St., Cambridge, MA 02140-3264,
or send e-mail to letters@SkyandTelescope.com.
Please limit your comments to 250 words.

VLADIMIR SACEK /
TELESCOPE-OPTICS.NET

Letters

http://www.TELESCOPE-OPTICS.NET https://www.is.gd/Dawes_Limit_star_list

Sky and Telescope - December 2016

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