O
n the night of September 23, 1846,
German astronomer Johann Gott-
fried Galle called out the confi gurations
of stars in the fi eld of view of the 9-inch
refractor at the Berlin Observatory, while
an assistant, Heinrich Louis d’Arrest,
checked them on a map. Just after mid-
night, Galle called out that a star of 8th
magnitude was in a particular location.
D’Arrest immediately exclaimed: “That
star is not on the map!”
That “star,” of course, was a new
planet, Neptune. Although a giant
planet with an equatorial diameter
almost four times larger than Earth’s, it
appears almost starlike in small tele-
scopes. It takes a magnifi cation of 200×
or so to clearly resolve its pale blue disk.The smallness of its disk, so discour-
aging to the casual viewer, proved an
asset to a very important investigation,
in which Neptune wasn’t the actual
target of study but just an innocent
bystander. The real target was the Sun.
The question astronomers hoped
to use Neptune for was, is the Sun a
variable star? Determining whether or
not the solar constant might, in fact,
vary has been one of the holy grails
of solar physics, going back at least to
William Herschel. He thought it did
vary in brightness, and that its vari-
ability must affect the climate; he even
attempted to correlate wheat prices with
his own historical data on sunspots. A
more empirical approach to the problemMonitoring the
Sun with Neptune
A long campaign to measure the variability of our
star produced unexpected results.NASA/^ JPL52 NOVEMBER 2019 • SKY & TELESCOPE
began with the invention of the fi rst
solar radiometers in the 1840s, and the
idea of using the brightness of planets
and satellites to monitor solar bright-
ness seems to have fi rst occurred to
Gustav Müller, a German astronomer at
the Potsdam Observatory, in 1897.
Like all the other planets, Neptune
shines by refl ecting sunlight in optical
wavelengths, and so by measuring the
planet’s brightness with a photometer it
should, at least in principle, be possible
to measure the intensity of sunlight
indirectly. The Sun’s brightness variation
will then be refl ected in the correspond-
ing variation of the sunlight scattered
from Neptune’s atmosphere. Most
importantly, such measurements are
technically far easier to carry out on the
minute disks of Uranus and Neptune
than directly on the Sun itself, a huge,
boiling, far-from-homogeneous disk.
Photometric monitoring of Neptune
began in 1953 at Lowell Observatory as
part of the Solar Variations program,
one component of the Project for the
Study of Planetary Atmospheres funded
by Air Force Cambridge Research Labo-
ratories. Measurements were taken with
a manually operated 21-inch refl ecting
telescope at Lowell Observatory fi t-
ted with a photometer resembling an
old-fashioned radio tube. In those days,
data were recorded as squiggly traces on
a moving strip of chart paper, similar to
the readout on a seismometer used to
detect earthquakes.
The precise magnitude of Neptune
in the blue region of the spectrum was
determined relative to the magnitudes
and colors of comparison stars situ-
ated along the planet’s path as it slowly
crossed the sky. The observers were
hoping to detect solar variation of a
few percent, if it indeed occurred. The
project involved several distinguished
observers, including the Polish astrono-
mers Krzysztof Serkowski and Mikolaj
Jerzykiewicz. Operations ceased in 1966
due to a lack of funding, but in 1972
Wes Lockwood revived the project with
the help of associate Don Thompson.
Over a period of 43 years they spent
thousands of nights at the controls
of the 21-inch telescope, located in aNOVEMBER 2019 OBSERVING
Exploring the Solar System by William SheehanNeptune as imaged by
the Voyager 2 spacecraft
during its monumental
fl yby in 1989. The Great
Dark Spot is visible on
the right limb.