Sky and Telescope - April 2016 - 6

Letters

Your "75, 50 & 25 Years Ago" column
(S&T: Jan. 2016, p. 10) recalls an article
from the January 1966 issue describing the potential advantages of a lunar
observatory. While such a facility is still
in the far future, telescopic astronomical research has already been conducted
from the lunar surface. In 1972, Apollo 16
astronauts John Young and Charles Duke
deployed a far-ultraviolet camera-spectrograph at their Descartes Highlands
landing site. The 75-mm f/1.0 Schmidt
instrument took photographs at a wavelength that is inaccessible to groundbased astronomers.
While on the lunar surface, the astronauts manually aimed the telescope -
just as we amateurs did before the advent
of Go To mounts. Among the astronomical objects photographed were Earth's
upper atmosphere, nebulae, star clusters,
and the Large Magellanic Cloud. When
Young and Duke departed the Moon,
they took a ﬁlm cartridge that contained
178 exposed frames. But that ﬁrst lunar
telescope remains on the Moon to this
day - perhaps "one small step" toward
an eventual lunar observatory.
Frank Ridolfo
Bloomﬁeld, Connecticut

It's All Relative
Thank you for the excellent articles on
the intimate tie between general relativity
and modern astronomy (S&T: Dec. 2015,
p. 18 and p. 26). They inspired me to pull
out my ﬁrst S&T issue, June 1955, which
was published just after Einstein's death.
In it, George McVittie struggles to show
"Why Should an Astronomer Study Relativity?" How times have changed!
Richard Stanton
Big Bear City, California

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April 2016 sky & telescope

NASA (2)

Astronomy from the Moon

Apollo 16 astronauts set up a far-ultraviolet telescope on the Moon in 1972 (left, foreground) and
used it to take images of auroral emissions on Earth (right) and other celestial targets.

space and time. Simplicity never goes out of
fashion in science. Interferometers of today,
though complex devices operating at the
limits of quantum mechanics and the atomic
nature of matter, still allow us to measure
such phase shifts of light in a way that can
be veriﬁed simply.

Govert Schilling's wonderful article
concerning the progress in the search for
gravitational waves took me back to a tour
I had of the LIGO facility in Hanford,
Washington, in 2000. The researchers
there had just achieved "ﬁrst lock" (their
version of "ﬁrst light"), and I remember
being impressed with the engineering
feat required to create an interferometer
on such a grand scale.
But after that visit I couldn't shake a
gut feeling about it just being the wrong
instrument for the task. After all, isn't
LIGO really just a scaled-up version of the
Michelson interferometer of 1887? And
aren't all such investigations using this
same basic technique (with the exception
the Pulsar Timing Array folks)? It's like
using a gauss meter to detect radioactivity. That meter might be very good and
precise, but it's the wrong tool - you
really need a Geiger counter. Am I wrong
in thinking that researchers have yet to
build their relativistic "Geiger counter"
for gravity waves?
Jay Jiudice
Los Angeles, California

A "Quiet" Superﬂaring Star

LIGO director Frederick Raab replies:
According to general relativity, the speed of
light is constant and becomes the "ruler"
for measuring space. A gravitational wave
reveals its passing most simply by causing a
phase shift in a beam of light crossing space.
Whether it's radio emission from a pulsar or
laser light in an interferometer, we measure
the same eﬀect - just on diﬀerent scales of

As Monica Bobra points out (S&T: Nov.
2015, p. 22), a solar superﬂare could
have a dramatic impact on Earth and its
inhabitants. Some solace can be found in
the fact that, unlike our Sun, most ﬂaring
solar-type stars show strong emission
lines due to ionized calcium - and the
Sun doesn't. However, one of the ﬁrst
superﬂaring solar-type stars identiﬁed
showed no such abnormal spectral char-

Govert Schilling mentions that radio
astronomers are looking for tools to measure "longer, slower gravitational waves."
Don't all these waves move at the speed
of light?
Irene Kitzman
Tucson, Arizona
Camille Carlisle replies: Yes, gravitational
waves travel at the speed of light. "Slower"
was a reference to their frequency - because
the wavelengths are longer, a complete wave
takes more time to pass by us compared with
a shorter wave. We should have stuck with
thinking of these as radio signals and used
"lower frequency" instead of "slower" to
avoid that ambiguity.

Sky and Telescope - April 2016

Table of Contents for the Digital Edition of Sky and Telescope - April 2016