Sky & Telescope - August 2024 - 31

PORTRAIT OF WHEWELL: ERNEST EDWARDS / MITCHELL LIBRARY, STATE
LIBRARY OF NEW SOUTH WALES / WIKIMEDIA COMMONS / PUBLIC DOMAIN;
HUYGENS DRAWING OF PLANETS AND SUN: ETH LIBRARY / PUBLIC DOMAIN
does every single one of them outstrip the sun
in diameter. " Stars had to be huge to both be as
distant as Copernicus required (to explain the
absence of parallax) and at the same time appear
1
/30 the diameter of the Moon. In 1604, Kepler
calculated that Sirius was bigger than the orbit
of Saturn. Every single star visible to the eye had
to be bigger than the orbit of Earth.
Kepler's basic science showed that stars were
dim, and stars were huge. Bruno was wrong -
stars were not suns. And so, Jovians aside, stars could not
be other suns circled by other inhabited earths. Any skilled
astronomer could make some basic measurements, reproduce
Kepler's results, and conclude that the Sun was a unique body
in the universe, and therefore its planets were also unique.
What Kepler Got Wrong
Given what was known in Kepler's time, his case against
Bruno seems solid. When we look at the planets and stars,
our eyes see dots of different sizes - similar to the way stars
and planets are rendered on star maps and in planetarium
apps. Simply put, brighter equals bigger to the unaided eye.
And that didn't change with the advent of the telescope, at
least not initially.
The first telescopes often had apertures of only 10 to 20
mm - much smaller than even the smallest backyard telescopes
today. In the early 17th century, astronomers didn't
understand the relationship between resolving power and
aperture. In fact, they often deliberately reduced the apertures
of their telescopes with a mask to isolate the best part
of the imperfectly made lenses, or to reduce the glare (what
Cassini called " la vivacité de la lumière " or " the vivacity of
the light " ) from small, bright objects.
But small apertures have the unfortunate effect of making
stars look like distinct disks, much like the naked eye does.
Constrict the aperture of your scope to 10 or 20 mm and you
can see the effect for yourself. John Flamsteed (1646-1719),
England's first Astronomer Royal, argued that with a (smallaperture)
telescope he could make out the disk of Sirius better
than he could make out the disk of Mercury. Both appeared
about the same size, he noted.
Flamsteed didn't know that this was just a function of the
limited resolving power of his telescope and revealed nothing
about the true size of Sirius. But taken at face value, his
observation would mean that Sirius would be as many times
larger than Mercury as it is vastly more distant. Throw in a
few rough numbers and you quickly arrive at the conclusion
that Sirius must make the Sun look like a mere speck!
Many Different Types of Stars
In the early 18th century, Cassini measured the size of Sirius
using a telescope with its aperture masked down (to reduce
the " vivacity of the light " ) and concluded that the star had
a diameter equivalent to Earth's orbit. While Fontenelle had
said stars were other suns, Cassini presumed that they were
THE SCALE OF THINGS It was 17th-century Dutch astronomer
Christiaan Huygens who correctly determined the relative sizes of the
planets through micrometer measurements, one of his many notable
accomplishments. Huygens is perhaps best known to today's amateur
astronomers as the discoverer of Saturn's biggest moon, Titan, and for
the eyepiece design that bears his name.
sk yand tele scope .o r g * AUGUST 2024 31
ADVOCATE FOR PLANETARY DIVERSITY In the 19th
century, William Whewell used science to argue against
what astronomers for centuries had simply assumed to
be true - that planets everywhere would be home to life,
including intelligent life, just like on Earth.
other Siriuses. Stars differed from one another
in appearance, Cassini reasoned, because of
differing distances. Based on the sizes of stars
as seen in a telescope at 200× magnification, he
calculated that the most distant telescopically
visible stars were 1,200 times farther than Sirius. Astronomers
agreed with Cassini's measurements, which they found
quite reproducible.
Cassini's numbers were still being cited at the end of the
18th century, but by then observers were beginning to understand
that telescopes can't resolve stellar disks. The decisive
blow came in the early 19th century when George Biddell
Airy explained the relationship between telescope aperture
and resolving power in terms of the wave nature of light
(S&T: July 2024, p. 75). But as astronomers began to understand
how truly different stars can be from one another,
another challenge to the idea that the universe was full of
earthlike worlds was rising.
Agnes Mary Clerke, who was a multilingual, one-woman
hub for astronomical communication and knowledge in the
late 19th century, described how this understanding came
about. In her 1885 book A Popular History of Astronomy During
the Nineteenth Century, she wrote that at the beginning of
the century, stars were merely the background against which
astronomers measured the motions of the planets. William
Herschel, she noted, operated on the assumption that " the
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