Sky & Telescope - March 2022 - 52

MARCH 2022 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
tVenus in false color as seen by NASA's Mariner
10 spacecraft in 1974
Hermann Carl Vogel and Wilhelm
Oswald Lohse at Bothkamp Observatory,
Étienne Trouvelot at Harvard
College Observatory, and Camille
Flammarion at the Juvisy-sur-Orge
Observatory. Some observers even
reported dusky collars surrounding the
bright spots, further mimicking the
Martian polar caps. Many suspected
that the cusp caps were snowfields
on lofty plateaus and attributed their
changing visibility and appearance to
variations in the density of the overlying
clouds and haze.
A host of skeptics regarded the bright
Venusian Cusps, Caps,
and Collars
Features near the poles of the hottest planet were once
mistaken for polar caps.
F
or telescopic observers, the dazzling
face of Venus is a bitterly disappointing
sight compared with the richly
detailed disks of Mars and Jupiter.
The planet's surface is concealed from
prying eyes by an unbroken canopy of
clouds, overlaid by a thick haze. It's not
unusual for experienced observers working
with powerful instruments to find
Venus utterly featureless. The few visible
markings are almost invariably diffuse,
ill-defined shadings of very low contrast.
The preeminent 18th-century Venus
observer Johann Hieronymus Schröter
failed to detect any markings for nine
years, until in 1788 he was finally able
to glimpse " . . . the ordinarily uniform
brightness of the planet's disk to be
marbled by a filmy streak. " In 1890,
Irish astronomer Agnes Mary Clerke
lamented " What we see is a shell of
clouds . . . The eye accordingly finds
sure anchorage nowhere. " It's no wonder
that for more than three centuries
observers reported wildly conflicting
52 MARCH 2022 * SKY & TELESCOPE
values for the planet's rotation period
and axial tilt!
Early in the 19th century, keen-eyed
German astronomer Franz von Paula
Gruithuisen reported seeing brilliant
spots visible near the horns or " cusps "
of crescent Venus, persisting well into
the planet's gibbous phase. These features
varied in brightness, size, shape,
and general outline from day to day and
even from hour to hour.
Nevertheless, Gruithuisen compared
these cusps to the polar caps of Mars,
suggesting that they mark the location
of the planet's poles of rotation. This
proved to be a very lucky guess indeed.
It was only when scientists bounced
radar waves off the planet's surface in
the 1960s that the rotational axis was
finally established as being inclined by
less than 3° from the orbital plane -
not far off from Gruithuisen's estimate.
During the 1870s, confirmations
of Gruithuisen's " cusp caps " trickled
in from prominent observers, notably
caps and dusky collars as optical illusions,
often citing Walter Augustin
Villiger's " artificial planet " experiments
conducted at Munich Observatory during
the 1890s. Villiger painted smooth,
2-inch-wide rubber balls white and had
people view them through a 5-inch
refractor while illuminating them with a
spotlight. At several hundred meters the
balls had same apparent size as Venus,
and by shifting the location of the
spotlight he could simulate the planet's
phases. Many sketches of these featureless
spheres by his test subjects bore an
uncanny resemblance to drawings of
Venus, replete with shadowy markings
near the terminator and bright cusps.
Villiger certainly demonstrated the
perils of assuming that features on
Venus are real simply because they have
been seen by independent observers. But
the cusp caps and collars portrayed by
his test subjects were never as prominent
or well-defined as those in many
actual depictions of the planet.
In 1927, Yerkes Observatory astronomer
Frank Ross dispelled any lingering
doubts about the reality of the cusp
caps when he photographed them
through ultraviolet filters with the
60- and 100-inch reflectors at Mount
Wilson Observatory. Ross wrote that
his striking images left " no doubt of the
existence of these bright areas, " which
he characterized as " undoubted facts to
be reckoned with. "
We now have a better understanding
NASA / JPL-CALTECH
Sky & Telescope - March 2022

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