Sky and Telescope - September 2018 - 7

tion" sounds like something declining
downhill from the equator, but what it
measures is an increasing angle, so it
arguably could be called "inclination."
In timekeeping, "Julian Days" don't
start at midnight in Greenwich, England, but rather at local noon there.
They count Julian nights, not days. And
"magnitude" doesn't measure brightness but rather its opposite: faintness.
Just saying.
Mike Lampton
Berkeley, California

Speaking of Misnomers . . .
The sidebar to the article on Messier 27
(S&T: July 2018, p. 68) says, "'[P]lanetary nebula' isn't the only archaic and
misleading name still in common use,
so it seems destined to remain part of
astronomy's vocabulary." I wondered if
I could think of more, and it didn't take
long. "H II region" is the most obvious,
and here's how it came to be.
Early spectroscopists observed that

many elements (iron, for example)
showed two distinct spectra, according
to whether the spectrum was excited
in an arc or in a much more energetic
spark. They used Fe I to denote iron's
"ﬁrst" or "arc" spectrum, and Fe II for
the "second" or "spark" spectrum. Only
later was it understood that the ﬁrst
spectrum arose from the neutral atom
(Fe), while the second spectrum arose
from ionized iron (Fe+).
Hydrogen has only one electron
and thus only one spectrum, H I. The
strongest line of this spectrum is hydrogen alpha (Hα), which causes the red
glow that is characteristic of an "H II"
region. Ionized hydrogen is just a proton
and thus has no line spectrum, so there
is really no such thing as H II.
Yet in describing emission nebulae,
an H II region has come to mean one
where hydrogen is largely ionized, while

1968

1993

Jeremy B. Tatum
Victoria, British Columbia

FOR THE RECORD

* In "The Dark Energy Enigma" (S&T: May
2018, p.14), the distance to faraway supernovae published in 1998 was not based on
theories of how stars explode (as stated)
but rather estimated using the shapes
of the supernova light curves at different
wavelengths, based on a calibrated shapedistance relationship.

SUBMISSIONS: Write to Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, U.S.A. or email: letters@
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75, 50 & 25 YEARS AGO by Roger W. Sinnott
1943

an H I region is one where the hydrogen is largely neutral. An H II region
would - but probably never will - be
better called an "H+ region." In such
an ionized region, protons (H+) and
electrons occasionally combine to form
neutral but excited H atoms - and it's
these atoms that then radiate the H I
spectrum.

º September 1943
Parallax Pioneer "With the
death on July 10th of Dr. Frank
Schlesinger, director emeritus of
the Yale University Observatory, we
have lost one of the great astronomers in the field of astrometry. . . .
When Frank Schlesinger began his
investigations, scarcely 100 parallaxes (or distances) were known to a
fair degree of accuracy. Now, mainly
as a consequence of his standardization of the methods, the number
has grown to a few thousand."
The tally of good star distances
hardly changed over the next
50 years. The ﬂoodgates ﬁnally
opened with the Hipparcos spacecraft's survey in the 1990s, and
they're widening more with today's
Gaia mission.

fainter red companion 3.3 seconds of arc away. Last December,
Thomas A. Matthews (University of
Maryland) reported . . . that these
two components were linked by a
nebulous bridge, which became
much brighter in 1966. . . .
"This change, and even the
existence of the bridge, have now
been denied by J. Kristian and P. V.
Peach, Mount Wilson and Palomar
Observatories. . . . None of four
200-inch plates taken in 1964,
1967, and 1968 shows any trace of
nebulosity . . .
"The issue has far-reaching
implications. Proof that a quasar
can undergo observable structural
changes would have meant that
it is a small object inside our own
galaxy, rather than at the enormous
cosmic distance corresponding to
its spectral red shift."

º September 1968
Quasar Conundrum "The quasar
3C-287 has been identified on
photographs as an 18th-magnitude
blue starlike object, with a much

º September 1993
Alternate Universe "With the
enormous successes of Big Bang
cosmology and inflationaryuniverse theory, the 'standard

model' of the universe goes almost
unquestioned by cosmologists
these days. In this picture . . . the
Big Bang happened around 10 or
15 billion years ago, and, very early
in the Big Bang, inflation gave the
cosmos precisely the right density
to balance it forever between rapid
expansion and recollapse. . . .
"Or maybe not. As an exercise,
cosmologist Edward Harrison
(University of Massachusetts,
Amherst) tried to devise a radically
different universe [and] succeeded
surprisingly well. In Harrison's most
extreme model . . . the expansion
is very slow [and] the density of the
universe today is about 10 times
greater than needed to halt the
expansion. . . . Harrison's universe
is 35 billion years old. . . . The
universe will cease expanding in
another 22 billion years, then begin
recollapsing toward a Big Crunch
79 billion years in the future."
Harrison's cooked-up model
passed all the major observational
tests - without the need to invoke
unknown types of dark matter.

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Sky and Telescope - September 2018

Table of Contents for the Digital Edition of Sky and Telescope - September 2018