Sky & Telescope - December 2019 - 64

p BAD BREAKOUT Flammarion regularly included dramatic sketches
of large sunspots in L'Astronomie, including this one of the great spot of
November 1893 as sketched by Théodore Moreaux of Bourges.

Jupiter: A Small Sun?
Jupiter's cloud features and its prominent satellites naturally
received ample attention at the dawn of the 20th century, but
only Newcomb showed much insight into the planet's inner
workings, drawing comparisons between it and the Sun. He
came to the conclusion that Jupiter could be a small Sun
based on simple calculations that showed the planet's density
must be much less than Earth's, in fact, "only about one third
greater than the density of water," while its surface gravity
was "between two and three times that at the surface of the
earth." That is, greater compression due to gravity resulted in
greater density. From this he concluded that Jupiter's "outer
portions at least were composed of aeriform matter." Combining his argument on density with evidence of rotation,
Newcomb suggested that Jupiter had "a greater or less resemblance to the Sun in its physical constitution." On the whole,
he viewed Jupiter as "a small sun of which the surface has
cooled off till it no longer emits light." Newcomb would have
prized brown dwarfs!

Is the Milky Way All There Is?
To illustrate the then-known extent of the universe, Newcomb took the reader on an imaginary voyage from Earth to

u AIMING FOR VERACITY James Nasmyth took up astronomy
after a successful career as a mechanical engineer. In 1874, he
co-wrote The Moon: Considered as a Planet, a World, and a Satellite with the professional astronomer James Carpenter. The pair
illustrated their discussions with photographs of elaborate plasterof-Paris models based on drawings that "were again and again
repeated, revised, and compared with the actual objects" in an
attempt to represent the lunar surface as accurately as possible.

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D EC E M B E R 2 019 * SK Y & TELESCOPE

the "inconceivable" distance of 100,000 light-years. "So far as
we know," he revealed, "we should at this point ﬁnd ourselves
in utter darkness, a black and starless sky surrounding us on
all sides. But, in one direction, we should see a large patch of
feeble light spreading over a considerable part of the heavens
like a faint cloud or the ﬁrst glimpse of dawn. Possibly, there
might be other such patches in different directions, but of
those we know nothing." Newcomb imagined the observer
ﬂying toward one of those light patches, watching as it spread
over more and more of the sky with each passing moment.
Interestingly, Newcomb's concept of the cosmos didn't
extend much beyond William Herschel's 1785 depiction of
the Milky Way, which was based on star counts and positional measurements of clusters and nebulae, and Herschel's
proposed position of the Sun. Oddly as well, Newcomb made
no mention of "island universes" (galaxies), an idea proposed in the 18th century by philosopher Immanuel Kant,
and instead described the Milky Way as ". . . a girdle apparently spanning the sky and perhaps, in reality, spanning the
entire universe of stars, uniting them, as it were, into a single
system, one stupendous whole." This collection of "millions
of stars," some of which had bodies (planets) associated with
them, comprised the knowable universe.
In contrast to that of Newcomb, Todd's perspective was
signiﬁcantly more developed, probably reﬂecting the difference in their ages. Newcomb was retired and approaching
70 at the turn of the 20th century while Todd was still in
his mid-40s and an active scientist when he wrote A New
Astronomy. Consequently, he used the term "galaxy" in
reference to the Milky Way and clearly hinted that the Great
Nebula in Andromeda was different from many of the other
nebulae that had been cataloged by then. "Its spectrum shows
that it is not gaseous," he wrote, yet "still no telescope has yet
proved competent to resolve it."
Keeping in mind that the term "universe" at the time
referred only to what had been mapped of the Milky Way,
which also was thought to encompass all stars and assorted

SUNSPOTS: L'ASTR ONOMIE (1894) / PUBLIC DO M AIN; LUN A R M ODEL: RIJKSM USEU M / PURCHASED WITH THE SUPPORT
OF THE M ONDRIA A N STICH TING, THE PRINS BER NH A RD CULT U URFONDS, THE VSBFONDS, THE PAUL HUF FONDS/
RIJKSM USEU M FONDS A ND THE EG BERT K UNSTFONDS / PUBLIC DO M AIN

What We Knew Then
Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019
