Sky and Telescope - November 2015 - 48

OBSERVING

Exploring the Solar System

William Dawes' Jupiter
A celebrated 19th-century astronomer made remarkable
observations of Jupiter and its moons.

JOHN G. WOLBACH LIBRARY

Rising in morning twilight is the giant planet Jupiter.
It's never too early to start observing this fascinating object. Armed with just an oﬀ-the-shelf planetary
camera and a medium-size backyard telescope, you can
take stunning, high-quality images that once were only
possible with interplanetary spacecraft. But recording
details on our Sun's largest planet in the days before
photography was a slow, arduous task involving hours of
sketching at the eyepiece by dedicated observers.
The serious study of Jupiter's dynamic, swirling
atmosphere did not start until the mid-19th century.
Before then, it was a completely mysterious world - we
didn't fully know even its most basic properties (mass,
size, density). One of the ﬁrst observers to start systematic observations of Jupiter was Rev. William Rutter
Dawes (1799-1868). Today he is best remembered as a
proliﬁc double-star observer and for determining the

William Dawes twice witnessed occurrences when none of
Jupiter's four large satellites appeared alongside the planet.
This is the view he saw on the evening of August 21, 1867. Callisto is above Io on the disk (right of center), and Ganymede
sits between Callisto's shadow (center) and its own; Europa is
hidden behind Jupiter. South is up.

November 2015 sky & telescope

"Dawes' limit" of angular resolution for telescopes.
However, by the mid-19th century, "Eagle-Eyed" Dawes
was also considered one of the world's foremost planetary observers. Not only did he codiscover the Crepe (C)
ring of Saturn in 1850, but his observations and drawings of Mars and especially Jupiter were also among the
ﬁnest of the time.
One of his earliest recorded observations was on
September 28, 1843, when Jupiter appeared "without
satellites." On that particular night, Io and its shadow,
Ganymede, and Callisto were in transit across the disk,
while Europa was occulted by the planet. Dawes started
watching the event with George Bishop's 7-inch Dollond
refractor but soon returned to his own residence after
trees threatened to obscure the view. Using a 2.7-inch, f/16
Dollond refractor at 113×, he recorded the times of each
satellite's motion across the disk. He noted, "For about 35
minutes, Jupiter was visible without any satellite."
However, the relocation to his residence might have
cost him the chance to record a partial eclipse of Io by
Callisto (23:25 to 23:48 Universal Time) as both moved
across Jupiter's disk. Dawes did observe this event, but
his 2.7-inch aperture - unlike Bishop's 7-inch refractor - lacked suﬃcient resolution to separate their disks.
Many years later (1862), George Airy, the Astronomer
Royal, asked Dawes to submit these observations to the
Royal Astronomical Society (RAS), as not a single notice
of this rare event had been received by the society.
Dawes soon moved on to a superior 6.3-inch Merz
refractor and used its excellent optics to great advantage.
During 1849 and 1850, both he and William Lassell
noted a rare outbreak of "bright round spots" in the planet's South Temperate Belt and South Temperate Zone.
In the spring of 1850 Lassell submitted his observations
and sketches to the RAS. However, the white-spot activity quickly subsided, with no further observations of
them recorded until the latter part of 1857.
By then, William Dawes had become one of the
most respected observers in the world, having won the
Gold Medal of the RAS two years earlier. In September
1857, using an 8-inch Clark refractor, Dawes reported a
new outbreak of white spots. He wrote, "They were ﬁve
in number, two being nearly equal in size and almost

Sky and Telescope - November 2015

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