Sky and Telescope - July 2017 - 8

Earth Is Not Alone
In Shannon Hall's article on superEarths (S&T: March 2017, p. 22), the
graph at the lower right on p. 27 seems
to show our Earth as an oddball, with
nearly all the discovered planets of our
size having orbital periods of less than
50 days. So does this mean we are truly
an outlier - or is it just an indication
that our detection techniques have low
sensitivity to planets in this region of
size-vs.-orbit parameter space?
Dan Kuchta
Hilton, New York

that the stars themselves turned out to be
less stable - and their light noisier - than
had been expected.

Some Credit Due
In the last paragraph of his article about
Supernova 1987A (S&T: Feb. 2017, p.
36), Robert Kirshner notes, "Tycho had
his supernova, Kepler had his." Perhaps
it would have been appropriate to add a
mention of my fellow countryman Ian
Shelton as SN 1987A's discoverer.
Pat Browne
Mississippi Mills, Ontario

Camille Carlisle replies: Astronomers call it Sagittarius A*. This was the
name of the compact radio source detected
at the black hole's position; the * emphasizes
its unusual nature. Astronomers struggled to
ﬁnd a good term for the object from its discovery in 1974 until 1982, when, inspired by
atomic physicists' nomenclature for excited
atoms (He*, Fe*, etc.), Robert Brown coined
Sgr A* - he was thinking of it as the "exciting
source" of nearby HII clouds. Astronomers
have since come to use the name generally
for the Milky Way's supermassive black hole.

FOR THE RECORD

Kelly Beatty replies: The clumping

of Earth-size planets toward shorter
orbital periods is due to the Kepler spacecraft's observational bias. The mission's scientists required three solid transit detections
to assign an object "candidate" status, and
those bodies with long orbital periods didn't
necessarily reach that detection threshold
before the planet-hunting portion of its mission ended. A big part of the problem was

Milky Way's
Supermassive Black Hole
Does the black hole at the center of our
galaxy have a proper name?
Gordon Nanninga
Covington, Washington

* The Umbrella Galaxy (S&T: April 2017,
p. 29) is NGC 4651, not NGC 4641 as noted
in the caption.

* The load-carrying capacity of the Vixen
Polarie Star Tracker (S&T: May 2017, p. 70)
is 7 pounds.

LETTERS TO THE EDITOR: We love to hear from our readers! Write to: Sky & Telescope, 90 Sherman

St., Cambridge, MA 02140-3264 or email: letters@skyandtelescope.com. Please limit comments to
250 words.

75, 50 & 25 YEARS AGO by Roger W. Sinnott
1942

1967

1992

º July 1942
Scutum Cloud "This shieldshaped mass of stars is one of the
most interesting 'clouds' of the
Milky Way.
"In the center of the field is
the cluster, Messier 11. . . . Until
recently the search for variable
stars in the Scutum cloud and its
vicinity has yielded few variables.
. . . But the open scale of plates
made with the Bruce 24-inch doublet at Harvard's South African station is revealing an as yet unlimited
number of new variables. . . .
"It is hoped, from this mass
of variables, that periods can be
obtained for a sufficient number of
classical and cluster-type Cepheids to reveal their distances, and
consequently, to allow us to decide
whether we are dealing with a
galactic cloud or a galactic window.
In the first case, the Scutum cloud
will really be what its name implies;
in the second, it will be merely a

J U L Y 2 0 1 7 * SK Y & TELESCOPE

portion of the Milky Way which is
not obscured by dark nebulosity."
When she wrote these words,
Margaret Harwood was midway
through her 40-year career as
director of Maria Mitchell Observatory, much of it tutoring young
women in variable-star work.
º July 1967
Weapons from Space "The Freer
Gallery in Washington, D. C.,
contains a magnificent collection of
ancient Chinese art. Two of the oldest objects are a ceremonial ax and
a dagger. . . . These Chou dynasty
weapons are puzzling, for stylistic
evidence dates them as 10th-8th
century B.C., which is before the
manufacture of iron in Honan province, where they were found. . . .
"Roy S. Clarke, Jr., of the
Smithsonian Institution's division of meteoritics . . . performed
mineralogical, X-ray, chemical, and
spectrographic tests that detected
the presence of nickel. . . . Furthermore, optical examination revealed

taenite and kamacite . . . characteristic of iron meteorites."
º July 1992
A Spinning Success "April 3, 1992,
4:18 p.m. MST, air temperature
1,160° Celsius and falling. Roger
Angel steps out of the control room
and crosses the laboratory floor. . . .
The glass master's grin is as bright
as the crescent Moon. He catches
my eye and gives 'thumbs up' with
panache. . . .
"A 6.5-meter mirror - having 65
percent more area than Palomar's
and, more importantly, being 40
percent lighter - has just been
born in a spinning furnace at the
University of Arizona's Steward
Observatory Mirror Laboratory."
Editor Leif Robinson was on
hand for spin-casting of the large
mirror that replaced six smaller
ones in the Multiple Mirror Telescope on Mount Hopkins. Since
1992, the world-renowned mirror
lab has cast four more 6.5-m and
seven 8.4-m mirrors.

Sky and Telescope - July 2017

Table of Contents for the Digital Edition of Sky and Telescope - July 2017