Sky & Telescope - June 2019 - 53

developments that Jupiter observers may
witness in coming months:

JUPITER: JOHN BROWNING / WIKIMEDIA COMMONS / PUBLIC DOMAIN;
TELESCOPE: JOHN BROWNING / A PLE A FOR R EFLECTORS 1876

The color typically develops over a
few months, simultaneously at all
longitudes . . . Sometimes the color
begins as a pure yellow, but evolves
within a few months to a darker, less
pure color such as ochre or tawny or
merely brown. Sometimes the evolution
continues as the colored material
concentrates over a year or two into a
massive dark belt, passing from yellow/
orange through brown to grey. However,
some episodes have begun with dull grey
or brown color and only later become
more vivid. And some episodes end with
a gradual brightening back through
yellow to white.

While the colors may be more pronounced at some longitudes, they never
show any distinct structure. Rogers
notes that the fading of the adjacent
South Equatorial Belt usually accompanies EZ coloration events. Conversely,
the intensity of the North Equatorial
Belt seldom diminishes and sometimes
even increases.
About 1/3 of EZ coloration events
produce intense displays of color. The
spectacular EZ coloration event of
1961-65 was unusually prolonged and
arguably the darkest on record. In 1964
Sergei Vsekhsviatskii, then director of
Kiev Observatory, reported that Jupiter's
overall brightness in visible light had
decreased by almost 15%. This estimate
was based on careful visual comparisons with naked-eye reference stars at
the same apparent sky elevation.
The ﬁrst well-documented EZ
coloration event occurred during the
years 1869-73. In October 1869 John
Browning, a prominent English maker
of scientiﬁc and medical instruments,
reported that Jupiter's EZ had taken on
an extraordinarily deep-yellow tint. The
vivid descriptions of these intensifying
hues of the EZ by Browning and a host
of observers during the following years
included amber, orange, ochre, salmon,
ruddy, rosy, coppery, faint chocolate,
and rich dark brown.
In 1920 Arthur Stanley Williams,

a British solicitor who was the leading
Jupiter observer of his era, published
a review of the sparse observational
record prior to 1869 that uncovered
unmistakable evidence of EZ coloration
events dating back as far as the late
18th century. Four drawings of Jupiter
made by Charles Messier in 1767 depict
a single dark broad band at Jupiter's
equator. William Herschel reported that
the Equatorial Zone displayed "a yellow
cast" in 1790 and appeared "brownish
grey" in 1792. The Bavarian physician
Franz von Paula Gruithuisen recorded a
reddish-brown EZ in the spring of 1839
and a bright, colorless EZ the following
year. Williams suggested that EZ coloration events occur at 12-year intervals
(about half the frequency derived from
the recent thermal-infrared data), but
he cautioned that there were many gaps
in the record and that "the observations
are seldom sufﬁcient to ﬁ x exactly either
the beginning or end of the period of
visibility or the real time of maximum
intensity of the tawny coloration."
qJohn Browning produced many impressive
Newtonian reflectors like the instrument shown
below. He extolled the superiority of reflectors
over the era's achromatic refractors for discerning the true colors of planetary features.

p Browning's depiction of Jupiter's distinctly
yellow EZ on the evening of July 31, 1870.

Well into the 20th century many
astronomers imagined that EZ coloration events were caused by upwelling
dark material spreading over the bright
white clouds. Yet even as the EZ coloration event of 1870 was unfolding, the
English astronomer Richard Anthony
Proctor correctly interpreted them as
clearings rather than obscurations. In
his popular 1870 book Other Worlds
Than Ours he wrote:
Suppose we regard the ordinary white
light of the equatorial belt as indicative
of the existence of enormous masses
of cloud reﬂecting ordinary solar
light to us, then we should regard the
appearance of any other color over
this region as an indication that these
cloud masses had been, through some
unknown cause, either wholly or in part
swept away.

Jupiter has been called "the amateur's planet" because amateur observers discovered the planet's principal
atmospheric currents and have systematically monitored its ever-changing
appearance since the Victorian era. The
long-term data they have amassed will
continue to play a vital role in unlocking the secrets of weather patterns and
climate variations on the solar system's
largest and most dynamic planet.
¢ THOMAS A. DOBBINS observes the
denizens of the solar system from his
home in Gainesville, Florida.
sk yandtele scope.com * J U N E 2 019

http://www.skyandtelescope.com

Sky & Telescope - June 2019

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