º June 1945
Speed of Light “‘Among the
distinguished observers who have
measured the velocity of light,
an implicit faith in its constancy
seems to have prevailed. But other
writers, comparing the published
observations, have interpreted the
apparent differences as secular
and periodic variations.’
“So comments Harold D.
Babcock [reviewing a reanalysis
of past results by] N. E. Dorsey,
formerly of the U. S. Bureau of
Standards.... [Dorsey] fi nds that
there is no indication of any pro-
gressive change in the velocity, the
best value for which he determines
as 299,773 kilometers per second
(in a vacuum) plus or minus 10
kilometers a second.”
In 1983, international agree-
ment fi xed the speed of light
at 299,792.458 kilometers per
second. So it is no longer subject
to remeasurement and in effect
defi nes the kilometer.ºJune 1970
Comet Bennett “Everyone in
mid-northern latitudes who viewed
Comet Bennett in early April agrees
that it has been the fi nest comet
for them in many years. Its great
brilliance, long tail, and convenient
location in the dark predawn sky
tempted some watchers to com-
pare it with such historic objects
as Donati’s Comet of 1858 and
Halley’s Comet of 1910....
“[Bennett’s] Type II or dust tail
was 19 degrees long to the naked
eye on April 8th, according to John
E. Bortle at Stamford Observatory
in Connecticut. That same morn-
ing, the slender Type I or gas tail
was estimated by him as eight
degrees long....
“Using a 22-inch Maksutov tele-
scope, Mr. Bortle detected spiral
jets emerging from the nucleus,
sometimes as many as fi ve at once.
During the fi rst week in April he saw
and photographed luminous hoods
around the nucleus, resembling
those in George P. Bond’s classic
drawings of Comet Donati.ӼJune 1995
Sculptor Dwarf “Painstaking
positional measurements on pho-
tographic plates spanning a half
century have produced the latest
results from a nascent astronomical
industry: assessing absolute proper
motions for [local galaxies. These
are key to] unraveling the history
of our galactic neighborhood, and
obtaining ‘test particles’ to ‘weigh’
the Milky Way’s dark halo....
“Andrea E. Schweitzer (Univer-
sity of Wisconsin), Kyle M. Cud-
worth (Yerkes Observatory), and
their colleagues [measured] stellar
positions in the Sculptor dwarf
spheroidal galaxy with respect to
six distant background galaxies.
By comparing South African plates
from the late 1940s with those
acquired in Chile throughout the last
decade, the researchers assessed
the average shift of more than 1,
stars. The Sculptor dwarf’s resultant
proper motion — 36 arc millisec-
onds eastward and 43 northward
per century — is the most precisely
determined by this method to date.”19451970199575, 50 & 25 YEARS AGO by Roger W. Sinnott
A Twist of Terminology
As a physics major in college, I learned
that the small wooden circle mounted
on legs with two telescopes at different
angles and a prism at the center is a
spectroscope, an instrument that records
is a spectrograph, and a spectrometer
measures a wavelength.
Jerry Oltion (S&T: Feb. 2020, p. 32)
omits the spectrometer, but identifi es the
instruments in the article with the termi-
nology from the standpoint of a physicist.
Through an apparent accident of
history, astronomers do not use the
standard physics terminology. Instead,
what Jerry and I call a spectrograph, they
call a spectrometer. This is one of many
instances where one fi eld of science uses
different terminology than another.
I wonder when the usage of the words
for spectrometer and spectrograph were
confused, and what other astronomy
terms also experienced this mix-up.
Mark Trueblood
Sonoita, Arizona
skyandtelescope.org• JUNE 2020 7SUBMISSIONS:Write to Sky & Telescope, One Alewife Center, Cambridge, MA 02140, USA or email: letters@
skyandtelescope.org. Please limit your comments to 250 words; letters may be edited for brevity and clarity.FOR THE RECORD
We published a photo essay on micrometeorites by Ted Kinsman (S&T: Feb.
2020, p. 14), which included discussion of his collection methods and scan-
ning electron microscope images of micrometeorites he had gathered. Before
we published the photo essay, a NASA meteorite scientist carefully reviewed
the text and images for accuracy, and we published only the images he
thought clearly showed micrometeorites.
However, since publication we have received skeptical responses from
readers familiar with this fi eld. Subsequently, we contacted two additional
micrometeorite experts for their opinion. Both inform us that, although the
author’s collection method has merit for fi nding micrometeorites melted by
their passage through the atmosphere (called cosmic spherules), almost all
the samples shown in the images are likely terrestrial contaminants.
The takeaway, every source tells us, is that sorting out imposters from true
micrometeorites is immensely challenging. Despite the author’s efforts to
remove anything of terrestrial origin, the specialists we contacted don’t think
he was successful. Therefore, we the editors are fl agging Kinsman’s article
as likely inaccurate. We will consider publishing a separate micrometeorite
article informed by these lessons at a later date.