Sky and Telescope - June 2015 - 10

Letters
As I began reading Grinspoon's article,
I started to fear that S&T's new management team had chosen to subtly enter
the politically charged debate on manmade global warming. Then I reached
the sentence referring to cyanobacteria
"ﬂooding Earth's atmosphere with poisonous oxygen" and realized that I had
been snookered into taking the article
literally. It was only then that I noticed
the column's title, "Cosmic Relief," and
concluded that Grinspoon was writing in
the vein of Jonathan Swift's famous satire
"A Modest Proposal." Well done!
The only thing he left out was a suggestion that, at some time in the distant
future, our descendants will ﬁgure out a
way to replenish the Sun's core with fresh
hydrogen at just the right rate to prevent
Old Sol from ever needing to initiate
helium fusion. That would avoid the
troublesome consequences of Earth being
swallowed by the solar system's newly

75, 50 & 25 Years Ago
May-June 1940
No Moving Cluster
"When not properly
accounted for, the eﬀect
of the solar motion may
lead to a false conception of the true character
of stellar motions. Such
a misconception was
recently brought to light by Dr. W. M. Smart of
the Glasgow Observatory . . . who has convincingly disproved the notion that the stars of
the Scorpius-Centaurus group form a moving
cluster [like that] in Ursa Major, which includes
ﬁve Dipper stars. . . . The apparent similarity
in [the Sco-Cen stars'] velocities arises from a
fortuitous reﬂection of the solar motion."
While moving at random and not gravitationally bound, the hot, young stars of spectral type
O and B in Scorpius, Centaurus, and Crux have
astrophysical importance as the nearest OB
association to our Sun.
June 1965
Mercury's Day "Radar study of the planets has
been an important activity of the Arecibo Ionospheric Observatory in Puerto Rico ever since
its 1,000-foot radio-radar telescope went into
operation in late 1963. . . . Recent results were

June 2015 sky & telescope

born red giant star. Of course, that would
also deprive our galaxy of a potentially
beautiful new planetary nebula - but,
hey, you can't have everything.
Edward Novak
Waynesville, Ohio

scopes, used with the Quark's amplifying
optics and 21-mm back ﬁlter, permit a
full-disk solar view. The TV-60 (360-mm
focal length) might perform even better,
since it yields a 0.8° ﬁeld of view.
°
Al Nagler
Chester, New York

Solar-Filter Clariﬁcations
Sean Walker's review of DayStar's new
Quark hydrogen-alpha solar ﬁlter (S&T:
Nov. 2014, p. 38) states, "Optical designs
with rear elements, such as the Petzval,
and any oil-spaced objectives require a
full-aperture energy-rejection ﬁlter for safe
use." However, some Petzval telescopes,
such as those made by Tele Vue, have rear
elements 10 inches from the focal plane
and therefore do not require such a ﬁlter.
Nor are ours oil-spaced or cemented.
Also it's not the focal ratio of a fast
refractor that permits "wide-ﬁeld views
of the Sun" but rather its focal length.
This is why the 480-mm Pronto or TV-76

Editor's note: Nagler is founder and CEO
of Tele Vue Optics. Walker notes that the
warning about Petzval designs was part of
the manufacturer's literature.

For the Record
✹The current orbital position angle of
Sirius B (S&T: Mar. 2015, p. 50) is 78°
(measured eastward, or counterclockwise,
from north), not 282°.
✹When viewing a nearly edge-on galaxy
(S&T: Mar. 2015, p. 20), it's the increased
surface brightness (not optical depth) that
renders them easier to see than those tipped
toward us with more open angles.

Roger W. Sinnott
reported . . . by Gordon
H. Pettengill and Rolf B.
Dyce. . . .
"The most startling
result from this surveillance was a preliminary
ﬁnding that the length
of Mercury's day is 59
± 5 of our days, and
that the sense of its rotation is direct - from
west to east like the earth's. Hitherto, most
astronomers had accepted G. V. Schiaparelli's
conclusion of 80 years ago that Mercury's day
was equal in length to its year, that is, 88 Earth
days. . . .
"One serious contradiction is raised by the
radar rotation period. To experienced visual
observers with suitable telescopes, Mercury has
fairly conspicuous dusky markings, and a number of rather accordant maps of its surface have
been drawn [assuming an 88-day rotation]. . . ."
The radar result proved the old maps were
bogus. Mercury rotates once every 58.646 days.
June 1990
Galaxy Merger "The true nature of Arp 220,
a peculiar 14th-magnitude galaxy in Serpens
Caput, may ﬁnally be coming to light. For years
astronomers have debated whether the system

is a Seyfert galaxy, as
its bright, active core
suggests, or a starburst
galaxy, as its very strong
infrared emission would
indicate. . . .
"James R. Graham
(California Institute of
Technology) and colleagues made a very sharp 2.2-micron image
using an infrared detector array on the 5-meter
reﬂector atop Palomar Mountain. A special
f/415 secondary mirror yielded an eﬀective
focal length of 2 kilometers. . . .
"The resulting image . . . shows two distinct sources barely 1 arc second apart [that]
coincide with radio sources detected earlier
at the Very Large Array. . . . This 'establishes
beyond doubt that Arp 220 is the remnant of
the merger of two galaxies,' says Graham, 'an
event which triggered the ultraluminous phase
now being witnessed.'"
The gravitational turmoil of this galactic
merger seems responsible for the vigorous starforming activity being observed there, and it
might in some way explain a surprise ﬁnding in
2011, namely, that Arp 220 spawns a supernova
every few months.

Sky and Telescope - June 2015

Table of Contents for the Digital Edition of Sky and Telescope - June 2015