FROM OUR READERS
8 FEBRUARY 2020 • SKY & TELESCOPE
Eager Reader
I would like to thank everyone at Sky &
Telescope for publishing such a won-
derful magazine for both experienced
astronomers and beginners. I love how
the articles are not too detailed yet
still include up-to-date research in an
easy-to-read style. Reading S&T has
helped me learn more about astronomy
and also inspired me to get deeper into
observing the night sky.
I have always liked watching the night
sky, especially the Moon. I can’t afford
to buy a telescope, but I truly enjoy read-
ing about different telescopes in S&T.
Also, the monthly observing charts for
stars, planets, and the Moon’s phases
are extremely helpful. I’m always excited
about getting the next month’s issue!
Alexandra Zovi
Menomonee Falls, WisconsinThe Eyes Have It
Don Ferguson’s article about double
stars (S&T: June 2019, p. 36) reminded
me of an interesting phenomenon: Our
eyes have better horizontal resolution
than vertical resolution. This distinc-
tion only applies when you observe close
to the resolution limit of your telescope.
Here’s a case in point: Epsilon Lyrae
consists of two star pairs oriented 90°
with respect to each other. If observed
with one pair separated horizontally
near the threshold of resolution, it will
be split — but the second pair won’t.
Rotating your head 90° resolves the
second pair but not the fi rst.
I suspect this is due to our greater
need, as hunter-gatherers, to resolve8 FEBRUARY 202 0 • SKY & TELESCOPE
ºFebruary 1945
Flower Refractor “W. H. Pickering
has argued that in 1924 better views
of Mars were obtained with small
apertures than with ones exceeding
20 inches. [W. F.] Denning [believed]
the optimum aperture for planetary
work is about 15 inches...
“From 1941 to the present, I
have made some such observa-
tions with the 18-inch Brashear
refractor at the Flower Observatory... in suburban Philadelphia....
“Encke’s division [in Saturn’s
rings] has often escaped skilled
observers using good telescopes,
but the Flower refractor usually
reveals it clearly.... The close
‘canals’ on the northwest inner wall
of the lunar crater Aristillus have
been divided when conditions were
favorable.”
Author Walter H. Haas went on
to found the Association of Lunar
and Planetary Observers in 1947.
T he Flower 18-inch objective, after
being mothballed since 1954, has
been fully restored as part of a new
observatory dedicated in 2019 near
Lake Tekapo in New Zealand.
ºFebruary 1970
Bad-Seeing Fix “Most astrono-
mers at one time or another have
wished for a way to sharpen a
blurred photograph, that is, to undo
the results of imperfect focus, trail-
ing, optical aberrations, and atmo-
spheric turbulence. Two ways for
doing just that were described by
George W. Stroke, State University
of New York at Stony Brook....
“Dr. Stroke uses a variant of the
aperture-synthesis techniques of
radio astronomers. They employ
high-speed computers to combine
mathematically the signals from
many small antennas into one
signal that has the same resolu-
tion as a big antenna fi lling all of
the space between the smaller
ones. But because of the much
greater amount of information
contained in an optical image, one
such aperture synthesis at optical
wavelengths could take months of
computer time.”
Astronomers were starting to
foresee today’s many techniques
for sharpening their astronomical
images.ºFebruary 1995
Drinking Gourd “In 1912 an ama-
teur folklorist named H. B. Parks
accidentally overheard an African-
American singing a folksong in
North Carolina that was new to
him.... The cryptic lyrics, Parks
learned, described a sky and land
map directing slaves [northward]
out of the South and toward free-
dom.... The term ‘Drinking Gourd’
is masked language for the Big
Dipper....
“Escaping slaves... concealed
their awareness of the sky. As a
result,... African-American songs
and stories on celestial topics are
scarce. ‘Follow the Drinking Gourd’
survives as a beautiful and rare
exception.”
In 1928 Parks published a few
stanzas of the song as he knew it,
based on a sketchy oral tradition.
The song quickly became a folk
standard, thanks to the Weavers
and other artists at the height of
the civil-rights movement. We may
never know whether, as author
Gloria D. Rall contends, it actually
guided fl eeing slaves northward.19451970199575, 50 & 25 YEARS AGO by Roger W. SinnottSUBMISSIONS:Write to Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, USA or email: letters@
skyandtelescope.com. Please limit your comments to 250 words; letters may be edited for brevity and clarity.detail around the horizon and a lesser
need for gauging the heights of trees.
Robert Dick
Ottawa, OntarioFOR THE RECORD- When Venus was seen and sketched
with spots on April 17, 1940 (S&T: Sept.
2019, p. 53), it was near greatest eastern
elongation, not superior conjunction. - Regarding the rate of growth of human
knowledge (S&T: Dec. 2019, p. 60), a
doubling every fi ve years is considered
exponential growth. - Date labels in the light curve of T Ursae
Majoris (S&T: Dec. 2019, p. 10) aren’t quite
correct. See https://is.gd/T_UMinoris for
a revised graph.