Sky & Telescope - November 2021 - 52

NOVEMBER 2021 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
The Ever-changing Great Red Spot
Jupiter's famous storm is the focus of a rich tradition of discovery by amateur astronomers.
T
he first unambiguous record of
the Great Red Spot (GRS) dates
from 1831, when German astronomer
Samuel Heinrich Schwabe glimpsed a
portion of its outline as a pale " bay "
indenting the southern edge of the
dusky South Equatorial Belt. This muted
appearance attracted scant attention
until 1878, when the feature suddenly
took the form of a vivid brick red ellipse
measuring 40,000 km long by 12,000
km wide (25,000 by 7,500 mi). It dominated
the face of the planet until 1882
when it abruptly faded.
Although it has slowly but steadily
diminished in size and its ruddy color
has waxed and waned many times during
the past 140 years, the GRS remains
Jupiter's most iconic feature and the
subject of intensive study. Two exceptionally
talented amateur astronomers
spearheaded efforts to understand its
nature and evolution.
The first was Percival Braybrooke
Molesworth (1867-1908), an officer
in the British Army's Corps of Royal
Engineers posted to the harbor city of
Trincomalee in the colony of Ceylon
(known today as Sri Lanka). While
there, he installed a superb 12.5-inch
52 NOVEMBER 2021 * SKY & TELESCOPE
Newtonian reflector made by one of
the era's foremost telescope makers,
George Calver, on the front lawn of
his bungalow on a high bluff overlooking
the Indian Ocean. From the site's
latitude of only 8.6° north, the planets
ride high in the sky, and the laminar
coastal airflow frequently results in
superb seeing conditions.
Molesworth soon emerged as the
leading Jupiter observer of the era. He
employed a powerful technique recently
devised by another British amateur,
Arthur Stanley Williams. Williams
recorded the precise times that welldefined
features in Jupiter's turbulent
and seemingly chaotic atmosphere
passed through the central meridian,
an imaginary line running from pole to
pole, bisecting the planet's disc. By referring
to an ephemeris, he determined the
longitude of features from their central
meridian transit times. Equipped with
an accurate timepiece, a practiced eye
can achieve an accuracy of one or two
minutes, corresponding to about 1° of
Jovian longitude. Williams discerned the
presence of nine discrete, persistent currents
at various latitudes on Jupiter, all
running parallel to the equator.
pThe gradually diminishing length of the Great
Red Spot is striking when comparing these images
sent back by NASA's Voyager I spacecraft
in 1979 (left), Galileo in 1996 (center), and Juno
in 2020 (right).
Possessing keen eyesight and seemingly
boundless energy, Molesworth
recorded a whopping 6,758 central
meridian transit timings during the
1900 apparition alone! Not only did he
accumulate this vast amount of observational
data in neat ledgers supplemented
by beautifully tinted watercolor renderings
of the planet, but he also performed
the tedious manual calculations
required to make sense of it. By 1901
he was able to identify all the principal
currents that control large-scale circulations
in the Jovian atmosphere.
The GRS is sandwiched between a
westward jet stream located along the
southern edge of the South Equatorial
Belt (SEBs) and an eastward jet stream
that runs along the northern edge of
the South Temperate Belt (STBn). It
rolls counterclockwise between these
opposing atmospheric currents like an
enormous ball bearing.
In 1905 Molesworth reported his
strong suspicion that the GRS was
VOYAGER: NASA / JPL, GALILEO: NASA / JPL / CORNELL UNIVERSITY,
JUNO: NASA / JPL-CALTECH / SWRI / MSSS

Sky & Telescope - November 2021

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