Sky & Telescope - July 2022 - 7

subscription base. The February issue
contains a stellar (pun intended)
example, " Ultra-Deep Imaging " by Rolf
Wahl Olsen (S&T: Feb. 2022, p. 60).
The New Zealand author provides a
unique perspective on how and, more
importantly, why to " go deeper " in
astronomical imaging. I had recently
launched my own deep-dive campaign,
but this entertaining, informative, and
inspirational piece just moved me into
hyper overdrive.
No, I am not taking credit for this
editorial supercharge, but it is nice to
know the voice of the reader is alive and
well at S&T. The S&T staff seem to make
it happen issue after issue!
Frank P. Puzycki
Long Valley, New Jersey
Mason and Dixon's Great
Adventure
I am grateful for Ted Rafferty's " Mason
and Dixon's Great Venus Adventure "
(S&T: Mar. 2022, p. 28). The extent
of my knowledge concerning Charles
Mason and Jeremiah Dixon was a
vague memory of that appropriately
named boundary line from an American
history class years ago - and more
recently, and more memorably, the
great song by Mark Knopfler, " Sailing
to Philadelphia, " which always piques
my interest to learn more about them.
I will order the book mentioned in
the article, Edwin Danson's Drawing
the Line. Thank you for your periodic
inclusion of historical subjects.
Nancy Huff
Beverly Hills, Florida
In a caption on page 30 of the March
issue, Ted Rafferty describes two Gregorian
reflectors made by James Short
as having " a focal length of two feet
(61 cm) and speculum metal primary
mirrors that were likely 41/2 inches
(114 mm) in diameter. " These dimensions
give a focal ratio of f/5.3, which
is far too fast for a Gregorian design.
The primary mirrors' focal length
probably is around 2 feet, but in the
Gregorian design, the effective focal
length (EFL) of the entire system is
amplified by the concave secondary
mirror. Therefore, the EFL of a typical
James Short Gregorian of this aperture
is about 12.75 feet (3.9 meters),
according to an article by Robert Royce
at rfroyce.com/short.
This would make these telescopes
f/34 systems.
Tim Black
Edmonton, Alberta
FOR THE RECORD
* The image of NGC 4725 on page 30
of the May issue was taken with a 4-inch
f/6.5 refractor.
SUBMISSIONS: Write to Sky & Telescope, One Alewife Center, Suite 300B, Cambridge, MA 02140, USA or email:
letters@skyandtelescope.org. Please limit your comments to 250 words; letters may be edited for brevity and clarity.
75, 50 & 25 YEARS AGO by Roger W. Sinnott
1947
1972
º July 1947
Oops " During the shower of Draconid
meteors on October 9th last
year, many amateurs obtained [photos]
. . . At Harvard College Observatory,
Carl Bauer [studied a photo]
bearing a meteor trail with conspicuous
sinuous curves . . . taken
by the members of the Jacksonville
Amateur Astronomers Club. . . .
" Bauer . . . calculates that the
1997
lateral acceleration required to
produce the observed trail is about
10,000 times the acceleration of
gravity and about 100 times the
measured decelerations of meteors
. . . due to the resistance of the
earth's atmosphere. "
Bauer concluded the trail's waviness
had a mundane cause: a rapidly
quivering camera, accidentally
bumped during the exposure.
º July 1972
Wandering Sun " The analemma is
the closed curve, resembling a fatbottomed
figure 8, that is placed in
the tropical zone of many terrestrial
globes . . . Since most teachers
don't know what the analemma's
function is, modern school globes
ordinarily omit it. But in my youth it
was always present, though many
years elapsed before I learned the
true significance of this mysterious
symbol. [If] the analemma were
placed on the Greenwich meridian
(longitude 0°), it would show the
geographical latitude and longitude
of the subsolar point at noon
Greenwich mean time, for each day
of the year. . . .
" The very slow precession
of the earth's axis causes the
equinoxes to slip westward along
the ecliptic [while other effects]
cause the earth's perihelion point
to shift gradually eastward along
the ecliptic. [As a result,] some
10,000 years from now . . . the
analemma's northern loop [will be]
broader than the southern, in contrast
to the present situation. "
Bernard M. Oliver's classic
article has inspired sundial makers
and creative photographers alike.
A true polymath, Oliver was a key
figure at Hewlett-Packard Company
and promoted development of the
first HP pocket calculators.
º July 1997
Fleeting Globulars " Favorite
observing targets for amateur
astronomers are the giant, spherical
collections of stars called globular
clusters. These ancient star systems
orbit around the center of our galaxy
in an enormous halo. But they
won't be around forever. . . .
" Oleg Y. Gnedin and Jeremiah
P. Ostriker (Princeton University)
cataloged 119 of the Milky
Way's globulars and investigated
how quickly the clusters would
disperse. [Such clusters] feel the
effects of various gravitational
tuggings when they cross the disk
of the galaxy, pass close to its
central bulge, or skirt giant clouds
of gas. [During] the next 10 billion
years, they say, more than half of
all the clusters - perhaps up to 90
percent - will be disrupted. "
sk yand tele scope .o r g * JULY 2022
7

http://www.rfroyce.com/short http://www.skyandtelescope.com

Sky & Telescope - July 2022

Table of Contents for the Digital Edition of Sky & Telescope - July 2022