Sky & Telescope - November 2019 - 52

Neptune as imaged by
the Voyager 2 spacecraft
during its monumental
ﬂyby in 1989. The Great
Dark Spot is visible on
the right limb.

Monitoring the
Sun with Neptune
A long campaign to measure the variability of our
star produced unexpected results.

O

n the night of September 23, 1846,
German astronomer Johann Gottfried Galle called out the conﬁgurations
of stars in the ﬁeld 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 midnight, 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 telescopes. It takes a magniﬁcation of 200×
or so to clearly resolve its pale blue disk.

52

N OV E M B E R 2 019 * SK Y & TELESCOPE

The smallness of its disk, so discouraging 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 variability must affect the climate; he even
attempted to correlate wheat prices with
his own historical data on sunspots. A
more empirical approach to the problem

began with the invention of the ﬁrst
solar radiometers in the 1840s, and the
idea of using the brightness of planets
and satellites to monitor solar brightness seems to have ﬁrst occurred to
Gustav Müller, a German astronomer at
the Potsdam Observatory, in 1897.
Like all the other planets, Neptune
shines by reﬂecting 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 reﬂected in the corresponding 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 Laboratories. Measurements were taken with
a manually operated 21-inch reﬂecting
telescope at Lowell Observatory ﬁtted 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 situated 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 astronomers 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 a

N ASA / JPL

NOVEMBER 2019 OBSERVING
Exploring the Solar System by William Sheehan
Sky & Telescope - November 2019

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