Sky & Telescope - November 2021 - 7

20th-century translator and scholar of
ancient mathematics. " MacDougal's
explanations of Mars' oppositions and
synodic period, and his descriptions of
his calculations, shed new light on what
I learned in my college course.
The one thing that puzzled me was
Kepler's drawing of the path of Mars in
the sky on page 28. The figure's caption
doesn't explain what the small dashed
circle at the center is nor the labels
A and B.
Edw. S. Ginsberg
Lexington, Massachusetts
Douglas MacDougal replies: How
lucky you were to take a course
from Otto Neugebauer! Your question
is best answered in Kepler's own words
(from William Donahue's translation of
Astronomia Nova): " Take note, too, that
since the orb of Mars requires such a vast
space, the spheres of the sun, Venus,
Mercury, the moon, fire, air, water, and
earth, have to be included in the tiny little
"
circle around the earth A, and in its little
area B. " Kepler, a devout Copernican,
used the Martian spirals diagram to make
a forceful argument for the implausibility of
Ptolemy's geocentric solar system.
Under Ptolemy's scheme, the order of
the solar system outward from the central
Earth was the Moon, Mercury, Venus, the
Sun, Mars, Jupiter, and Saturn, and each
sphere's maximum (apogee) radius touched
the minimum (perigee) radius of the next.
This meant that the Moon, Mercury, Venus,
and the Sun all had to fit within the nearest
perigee of Mars, the innermost loops of
the diagram. All four bodies thus would be
crammed tightly into the space enclosed
by the circle, A. This seemed impossible to
Kepler, in contrast to the much simpler system
of Copernicus, which entirely rid all the
planets of these extremely intricate spirals
and placed the individual planets into their
respective orbits.
Deep-Sky Delights
Ted Forte's article " Observing the
Pegasus I Galaxy Cluster " (S&T: Aug.
2021, p. 57) made me concentrate on
the first image. For a beginner like me,
the breadth of stars and galaxies in this
central image appear realistic.
The layout of the article encouraged
me to continually flip back to page 57,
forcing me to stop reading and focus on
the visual practice of locating the deepsky
objects in my 81-mm (3-inch) and
254-mm (10-inch) telescopes.
Forte's writing is nothing short of a
Bortle 1 sky. His ending lines are lovely
and romantic, encouraging readers to
attempt to find objects that must have
taken him years of training to locate.
The confident, succinct history lessons
made me read this article twice.
Julian Samuel
Toronto, Ontario
SUBMISSIONS: Write to Sky & Telescope, One Alewife Center, Suite 300B, Cambridge, MA 02140, 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
1946
1971
º November 1946
Galactic Bulge " [A] bright nucleus
or clustering of stars about the
center [of our galaxy in Sagittarius]
has never been observed, presumably
because of interstellar dust
clouds. . . . During the summers of
1945 and 1946, Drs. Joel Stebbins
and A. E. Whitford . . . made
a search in infrared light [with] the
60-inch reflector . . .
" [They found a] persistent bulge
1996
. . . outlined by an elliptical figure
some eight degrees long and
3½ degrees wide. [This region's]
extreme redness . . . indicates that
less than 1/1,000 of the ordinary
blue photographic light gets through
the obscuring interstellar dust. "
Stebbins and Whitford were on the
right track to search using infrared.
The Milky Way's compact nucleus,
now called Sagittarius A*, was first
accurately located by radio interferometry
in 1974. More recently,
infrared studies have been possible
with large telescopes, but in visible
light, interstellar dust almost completely
obscures the Milky Way's
central bulge.
º November 1971
Test Double " Just east of Gamma
Orionis is a 4th-magnitude double
star worth examining in fairly large
amateur telescopes on a winter
evening. It is 32 Orionis . . .
" Discovered by William Herschel
in 1782, this pair consists of two
stars of magnitudes 4.6 and 5.9.
According to W. van den Bos, the
stars probably do not form a binary
system, but instead are unrelated
objects that chance to lie nearly in
the same line of sight. . . .
" The closest approach of the
components came in 1906, when
the angular separation was only
0″.20. By the end of 1971, the
companion will be [northeast of the
primary], separation 0″.88. "
Today the stars' separation has
grown to 1.5 arcseconds, making
this unequal pair a good test for a
4-inch telescope.
º November 1996
Protogalaxies " The building
blocks of present-day galaxies like
the Milky Way may have been captured
in images from the Hubble
Space Telescope (HST).
" [In Hercules, a] recently discovered
cluster of galaxies . . . is
seen as it was only two or three
billion years after the Big Bang. To
seek sites of star formation at that
early epoch, Sebastian M. Pascarelle
(Arizona State University)
and his colleagues imaged the
cluster with a filter centered on the
Lyman-alpha emission line from
hydrogen gas. . . .
" As the researchers report . . .
18 amorphous objects stood out
in their filtered images. . . . The
objects typically span only 5,000
to 15,000 light-years [and] a few of
the curious objects show evidence
of merging. These observations
led Pascarelle's team to speculate
that the blobs are destined to
coalesce, eventually forming elliptical
or spiral galaxies like those
seen today. "
sk yand tele scope .o r g * NOVEMBER 2021
7

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Sky & Telescope - November 2021

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