Sky & Telescope - February 2023 - 7

" It's the squeaky wheel that gets
the grease " is another expression that
applies. Planetary defense will remain a
concept until those of us riding Spaceship
Earth speak out!
Don A. Nelson
Concerned American Aerospace Engineers
Alvin, Texas
One Man's Trash . . .
I agree with the points raised by David
Grinspoon in his essay " Crash as Trash "
(S&T: Sept. 2022, p. 12). We need to
clean up after ourselves. However, the
historical significance of such artifacts
shouldn't be overlooked. Otherwise,
someday we may read, " Accepting bids
on a lot of three slightly used lunar rovers,
a bit dusty, but very low mileage. " Ad
astra cum lucrum?
Greg Konesky
Hampton Bays, New York
I read David Grinspoon's " Crash as
Trash " column in the September issue.
We should continue to explore Mars, but
we should also consider our impact on
Mars from the beginning. The presence
of our discards on the Martian surface
will affect future science results. As we
explore and use other worlds, we should
develop the habit of taking better care of
them than we have of Earth. We should
also use these new habits to greatly
improve our care of Earth as well!
James W. Scott
Vernon, New Jersey
A Solid Magazine
As a subscriber to S&T and an owner
of various telescopes and binoculars, I
certainly enjoy the magazine's various
feature articles. It covers everything a
beginner needs to enjoy the hobby of
astronomy and observing. I especially
like the pull-out section, which includes
a planisphere for the month and the
Binocular Highlight column. I also enjoy
the various articles covering the Moon,
planets, stars, and constellations. And
I'm particularly fond of the 75, 50 & 25
Years Ago column by Roger W. Sinnott
and the S&T Test Reports. All in all, S&T
is a must-have for anyone interested in
the hobby of astronomy.
Roger M. Gould
Great Neck, New York
FOR THE RECORD
* The team of Brazilian observers mentioned
in " Understanding Lunar Eclipses "
(S&T: Nov. 2022, p. 34) is the Rede de Astronomia
Observacional (Observational Astronomy
Network), which has been led for many
years by Hélio C. Vital (rea-brasil.org).
* The graph on page 52 of the September
2022 issue contains errors in the vertical
axis. A corrected version appears at
https://is.gd/errata2022.
SUBMISSIONS: Write to Sky & Telescope, 1374 Massachusetts Ave., 4th Floor, Cambridge, MA 02138, 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
1948
1973
º February 1948
Startling Sight " Astronomical
history tells of many 'great comets'
which have appeared to our
parents and grandparents. But not
since 1910, when Comet 1910a
and Halley's blazed across the
sky, have any spectacular comets
been seen - comets which could
attract the gaze of the man in the
street without any encouragement
or advice from his astronomical
friends. Now the Southern Hemisphere
and the tropics have been
treated to the sight of what surely
deserves to be called the Great
Comet of 1947, although its pres1998
ent
designation is Comet 1947n.
" This spectacular object was
first seen in South Africa on the
evening of December 8, 1947, by
practically everyone who happened
to be out in the open under a clear
sky [writes J. S. Paraskevopoulos
of Harvard's Boyden Station] . . .
[My] staff and I agreed that the tail
was not less than 25 degrees. "
º February 1973
Star Diameters " When a bright
star of late spectral type is occulted
by the moon, the duration of the
star's fading at disappearance or
of brightening at reappearance
provides an accurate measure of its
angular diameter. The high-speed
photometric technique needed for
such observations was developed
some years ago by David S. Evans
at the Royal Cape Observatory . . .
" [A] remarkable observation has
been made of the Mira-type variable
star R Leonis . . . '[G. Neugebauer
and coworkers] made observations
with three telescopes at Mount Wilson
at wavelengths of 3.5, 4.8, and
11 microns. Observations at the two
shorter wavelengths are consistent
with a disk diameter of 0.05 arc sec,
as predicted from [its] temperature
and luminosity . . . At 11 microns,
the occultation measurements show
that the radiation originates from an
area 0.3 arc sec in diameter, thus
supporting the concept of thermal
emission from a cloud of dust particles
surrounding the star.' "
The early 1970s saw two pioneering
techniques just coming into
their own: high-speed photometry
and observations in the infrared.
º February 1998
Comet Origins " Recent ultraviolet
observations of Hale-Bopp suggest
that the comet formed relatively
close to the Sun before being
tossed out into the Oort Cloud -
where it remained for ages before
returning . . . in 1995 . . . More evidence
for a solar-system origin now
comes from measurements at much
longer wavelengths. Millimeterwave
spectra from the James Clerk
Maxwell Telescope have established
isotope ratios for the comet's
carbon, nitrogen, and sulfur. From
these ratios David C. Jewitt (University
of Hawaii) and his colleagues
infer that 'the cometary volatiles . . .
show no evidence for an interstellar
component.' This contrasts with
Hale-Bopp's brilliant precursor,
Comet Hyakutake (C/1996 B2),
whose properties are suggestive of
the interstellar medium. "
sk yand tele scope .o r g * FEBRUARY 2023
7

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Sky & Telescope - February 2023

Table of Contents for the Digital Edition of Sky & Telescope - February 2023