pWes Lockwood at the controls of the 21-inch
refl ector. He used the telescope to monitor
Neptune for nearly half a century.pThis fi nder chart plots the path of Neptune through the end of the current apparition. The mag-
nitude of stars that the planet passes near are listed to the nearest tenth of magnitude and omit
decimal points.LO
CK
WO
OD
AT
SC
OP
E:^
WI
LL
IAM
SH
EE
HA
N;^
21- IN
CHSCOPE:^
WES^ LOCKWOODroll-off-roof observatory only several
hundred yards from the famed 24-inch
Clark refractor used by Percival Lowell
to observe Mars.
It was already clear by 1965 that
the original goal had been quixotic all
along. Any actual solar variations were
much too small to be detected using
ground-based instruments. Instead of
a few percent, solar variability is now
known from satellite measurements to
vary by less than 0.1% over the Sun’s
11-year solar cycle — and so cannot
be the main driver of climate change.
However, as so often in astronomy,
the Solar Variations program produced
unexpected discoveries. Many of the
comparison stars proved to be slowly
variable, and some of the Sun-like stars
varied more than the Sun itself.
Seasonal variations in Neptune’s
brightness were also apparent in the
measurements. Thus, there was a
steady, relatively smooth seasonal rise
in brightness beginning about half-
way between the planet’s equinox and
solstice as the sub-solar point on the
planet’s disk moved southward. This
was followed in 2005 by a more delayed
drop in post-solstice brightness due to
the continued presence of long-lived
bright atmospheric features. Remark-ably, the measurements were sensitive
enough to pick up a variation in bright-
ness a few years before NASA’s Voy-
ager 2 spacecraft arrived in 1989, which
proved to be due to the Great Dark Spot
and bright companion clouds seen in
the Voyager images. A similar change in
brightness from 1972 to 1977 indicated
that a similar but much larger feature
had been present at the time.
Today, the photometer has been
replaced by near-infrared adaptive
optics and Hubble Space Telescope imag-
ing in the monitoring of the Neptunian
atmosphere. The loyally dependable and
mule-like 21-inch telescope has beenOct 1,(^2019) Nov 1 Dec 1
Pat
h (^) o
f (^) Ne
ptu
ne
Jan 1,
2020
Feb 1
Mar 1
Apr 1
May 1
June 1
July 1,
2020
AQUARIUS φ
23 h 30 m 23 h 20 m 23 h 10 m
- 4 °
- 6 °
7(^6368)
88
88
56
7
89
91
89
88
92 92 92
92
75
42
81
87
9
96
92
92
St
ar
m
ag
ni
tu
de
s
5
4
6
7
8
9
shut down, but it deserves to be remem-
bered with appreciation. As Neptune
drifts slowly among the star fi elds of
Aquarius this year, observers may follow
it with their own telescopes and use the
star chart here to estimate the bright-
ness of Neptune relative to stars along
its path. As they do so they may ponder
how, by spying on distant Neptune,
astronomers also contributed signifi -
cantly to understanding the variations
of distant Sun-like stars and the causes
of climate change on our own planet.
¢Contributing Editor WILLIAM SHEEHAN
is a researcher for Lowell Observatory.
skyandtelescope.com • NOVEMBER 2019 53
uAstronomers used the 21-inch refl ector at
Lowell Observatory (shown) to monitor Neptune.