Sky & Telescope - January 2023 - 6

SATURN: NASA / ESA / A. SIMON (GODDARD SPACE FLIGHT CENTER) / M.H. WONG (UNIVERSITY OF
CALIFORNIA, BERKELEY) / THE OPAL TEAM; NEPTUNE: NASA / JPL-CALTECH
FROM OUR READERS
What's In a Discovery?
The caption at the bottom of page
29 in " Seeing Saturn's Ring Spokes "
(S&T: Aug. 2022, p. 28) states, " Galileo
Galilei was the first to see the rings [of
Saturn], in 1610 . . . "
Galileo certainly saw something
unusual about the appearance of Saturn,
but I don't believe his telescope
was good enough for him to recognize
what he saw as rings or a ring. At first,
he thought there were two stationary
spherical bodies, one on either side
of Saturn. A few years later, the rings
appeared to him as two handles or
" ansae " attached to Saturn.
pEvery year, astronomers use the Hubble
Space Telescope to image Saturn in order to
monitor the gas giant and its rings. This image
is from 2019.
It wasn't until nearly half a century
later that Christiaan Huygens clearly
saw, recognized, and described, in
Latin, what he saw as a ring around
Saturn: " Annulo cingitur, tenui, plano,
nusquam cohærente, ad eclipticam
inclinato " ( " It [Saturn] is surrounded
by a thin, flat ring, nowhere touching,
inclined to the ecliptic " ).
Jeremy Tatum * Victoria, British Columbia
In " Discovering Neptune, " Trudy Bell
overlooks an important point. Both
Urbain Jean Joseph Le Verrier and John
Couch Adams assumed that the mean
distance of Neptune from the Sun
was about 38.8 astronomical units,
whereas it is, in fact, about 30.1 a.u.
This is because they assumed the correctness
of Bode's Law (actually due
to J. D. Titius), which approximated
the distances from the Sun of all the
planets from Mercury to Saturn and
had been confirmed by the discoveries
of Uranus and the first four asteroids.
It fails for Neptune.
Lisa Budd
London, England
Neptune Takes
the Stage
Thank you for Trudy
Bell's article " Discovering
Neptune: What Really
Happened? " (S&T: Sept.
2022, p. 32). I have been
doing 18th-century reenacting
for some years.
Living in the Brandywine
Valley in Pennsylvania, I volunteer at
several 18th-century sites. Some years
ago, Caroline Herschel became my
18th-century persona.
Bell perfectly put in words the
role of a reenactor: " [T]he practice of
historical research continued to evolve.
Instead of judgmentally viewing events
in hindsight, the role of the historian
came to be understood as (in the words
of the historian Angus Macintyre)
'seek[ing] an empathy with the actors
of the past . . . by diligently finding all
6
JANUARY 2023 * SKY & TELESCOPE
September 23, 2021, was
the 175th anniversary of
the discovery of Neptune.
Voyager 2 captured this image
nearly one month before the
discovery's 143rd anniversary
in 1989.
surviving evidence, then
immersing ourselves in
their milieu, their circumstances.'
The object
is to understand events as the people
then would have experienced them [Bell's
emphasis], without foreknowledge of
the future, as well as to consider the
social context in which the people lived
and the obstacles they faced. "
We have been saying this for years.
We ask people to leave their 21st-century
brains and come back with us to the
18th century. I am passing on the above
quote to all my reenacting friends.
K. Lynn King
Wilmington, Delaware
Trudy E. Bell replies: You are
correct in pointing out that Neptune,
at 30.1 a.u. from the Sun, turned
out to be far closer than the 38.8 a.u. astronomers
expected from Bode's Law. In
fact, there is a lot to this, and the details
are described in Neptune: From Grand
Discovery to a World Revealed, edited by
William Sheehan et al. The complicated
problem of inverse perturbations, which
Adams and Le Verrier set out to solve,
would have been utterly intractable were
not some simplifications introduced at
the outset. Thus, Adams assumed (just
to get started) the Bode's Law distance,
circular orbit, and no inclination to the
plane of the ecliptic in his first very rough
calculation, and by the time he completed
what were actually his fourth and
fifth calculations, though the Bode's Law
distance was retained, he was working
with a quite high eccentricity. This eccentricity
is much greater than the actual low
eccentricity of Neptune, but it is clear in
retrospect that the too great Bode's Law
distance and the excessive eccentricity
balanced each other out, and so the
planet's calculated longitude, which was
what was needed to reveal the position
of the planet, was close enough to the
truth at the time to allow discovery.
Adams himself wasn't satisfied with
the large eccentricity and recalculated
everything again, so that his final result
the summer before the planet was found
at Berlin on September 23rd had a mean
distance of 37.25 a.u.
"
https://www.nxtbook.com/nxtbooks/aas/st_202208/index.php#/p/28
Sky & Telescope - January 2023

Table of Contents for the Digital Edition of Sky & Telescope - January 2023
