Sky & Telescope - March 2023 - 7

of the Hayden Planetarium. Gutsch has
a way of making astronomy exciting, and
in our class he dealt with ways we measure
or estimate distances in space. One
key, as I recall, was the overlap of measurement
" sticks " to ensure accuracy.
While I am still trying to comprehend
the final page in Schilling's article, until
recently (I am now 70) I had not realized
that space itself is expanding. This
was not evident to me in George Abell's
Exploration of the Universe introductory
text or other sources, even though I have
a minor in physics, until I recently read
the intro text of The Cosmos: Astronomy
in the New Millennium by [the late] Jay
M. Pasachoff and Alex Filippenko. I had
always assumed redshift was totally due
to recessional velocities.
John M. Roberts
Richmond, Virginia
As a long-time subscriber, I very much
appreciate the outstanding articles in
the monthly editions of S&T. " Keep
Your Distance " is no exception. I found
it to be very interesting and informative
on how we determine cosmic
distances. My only comment is that
the article made no mention of space
dust and its effect on the apparent
brightness of stars and galaxies when
determining distances from us.
I believe the cumulative effect of
space dust on apparent brightness can
lead to substantial distance errors, especially
with objects millions or billions
of light-years away. Can space dust's
effect on apparent brightness affect the
determination of cosmic distances, or is
it considered negligible?
Angelo DiDonato
Macomb, Michigan
Camille Carlisle replies: You pose
a very good question. Dust in a galaxy
can and does affect the light curve of
"
75, 50 & 25 YEARS AGO by Roger W. Sinnott
1948
º March 1948
Gigantic Crater " In South Africa
there is an unusual topographical
feature whose origin seems
to have defied explanation on
the basis of orthodox geological
dynamics. Roughly circular and
about 30 miles in diameter, the formation
is known as the Vredefort
1973
ring structure . . .
" Of five geological theories,
1998
two have supposed magmatic
upthrust to be the primary cause
of the ring formation; a third,
tangential force of the kind
ordinarily applicable to mountain
building . . . Proof of the meteoric
theory . . . seems almost impossible
since the erosion of over
200 million years has destroyed
the original relief and may have
removed all meteoric remnants. "
Updates from a 2022 study: Two
billion years ago, the impact of a
15-mi-wide (25-km-wide) asteroid
created Vredefort. The crater was
originally 155 mi (250 km) across.
a supernova or Cepheid observed within
that galaxy. Astronomers correct for dust's
reddening effect when measuring cosmic
distances. But there is debate about just
how accurate these corrections are. For
example, some have suggested that dust
calibration is throwing off the Cepheidbased
measurements of the universe's
current expansion rate and causing the
tension between different results (S&T:
Mar. 2022, p. 14). Other methods, such as
using " tip of the red-giant branch " stars,
might avoid this problem, because those
stars tend to be in less dusty environs.
An exceptionally dusty galaxy can also
look so red that it appears to be a galaxy
at a much larger distance (https://is.gd/
WebbDistance). We can solve this ambiguity
with careful spectral measurements.
This is one of the reasons why it's so
important to have multiple ways of measuring
distances.
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.
º March 1973
Orange Soil " One of the many stories
still unfolding [from Apollo 17]
is that of the so-called orange soil
discovered by geologist-astronaut
Harrison H. Schmitt . . . As soon as
the orange material got to Houston,
it was given special attention at the
Lunar Receiving Laboratory. . . .
" At a press conference, William
Phinney, chief of the geology
branch at the Manned
Spacecraft Center, described its
appearance under a binocular
microscope: . . . 'The material is
essentially the finest-grained soil
we have yet seen from the moon,
with a grain size of 40 microns
instead of the usual 70-80 microns.
More than 90 percent of this soil
is glass, [and much] of the glass is
orange, especially the finer grains.
There are a few percent of mineral
grains, most of them partly coated
with the same glass.' "
Decades later, analysis of this
volcanic glass brought another surprise:
The Moon's interior magma
holds as much water as Earth's.
º March 1998
Thanks, Moon " Throughout the
millenniums the Moon has inspired
and influenced art, romance, and
endeavor. Now it is becoming
apparent that life itself owes a lot to
our satellite. After Earth's surface
cooled and oceans formed, . . . the
Moon kept the Earth's rotational
axis relatively stable. Studies have
shown that without the Moon, the
tilt of Earth's axis would vary chaotically
between 0˚ and 85˚. . . .
" Darren M. Williams and James
F. Kasting (Pennsylvania State
University) modeled Earth's climate
to examine how habitable our
planet would be. [They report] that if
Earth's axis tipped completely over
so that the Earth spins 'on its side'
(much as Uranus does), the zone
we now call the tropics could have
permanent snow cover. The poles,
for their part, would endure extreme
temperature swings . . . "
Recent work confirms a stabilizing
effect of the Moon but finds that
the tilt of a moonless Earth would
not vary quite so chaotically.
sk yand tele scope .o r g * MARCH 2023
7

https://www.is.gd/webbdistance https://www.is.gd/webbdistance

Sky & Telescope - March 2023

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