Sky and Telescope - January 2017 - 52

JANUARY 2017 OBSERVING
Exploring the Solar System by William Sheehan

The Schröter Effect: A Retrospective
O

p Johann Hieronymus Schröter in 1791. This
modern painting by Heinrich Wolfgang VogtVilseck is based on a rendering by George Tischbein that appears in Schröter's famous book on
the Moon, Selenotopographische Fragmente
(Volume 1).

Venus at
Greatest
Elongation

Venus

Sun

90°

Earth

p When the Sun-Venus-Earth angle is 90°, we
should see the planet's disk exactly half lit. But
when Venus is in the evening sky, as it will be
this month, telescopically this dichotomy actually seems to occur several days earlier.

52

J A N U A R Y 2 0 1 7 * SK Y & TELESCOPE

n January 12th, Venus reaches its
greatest elongation east of the Sun,
appearing brilliant and well placed for
observers in the evening sky. Theoretically, as the Sun, Venus, and Earth
will then form a right triangle, Venus
should appear exactly 50% sunlit in a
small telescope, a geometry known as
dichotomy.
However, as first noted in 1793 by
German astronomer Johann Schröter,
Venus looks exactly half lit on a date
that does not coincide with the theoretical one. On January 12th, for example,
Venus will instead appear to have
already passed the point of dichotomy,
instead showing a phase that is slightly
concave. At western elongations, when
Venus appears in the morning sky, the
effect is reversed, and the observed
dichotomy occurs later than predicted.
Schröter, who died 200 years ago last
August 29th, is no longer a household
name, but he was once referred to as
the "Herschel of Germany." William
Herschel, indeed, was his inspiration.
Born in Erfurt in 1745 and trained as a
lawyer, Schröter had a lifelong interested in astronomy and obtained a
2.25-inch Dollond refractor in 1779.
However, not until after Herschel's
discovery of Uranus in March 1781 did
Schröter make a dramatic change in his
life. He switched jobs to become chief
magistrate of Lilienthal, a sleepy hamlet
in northern Germany. His official
duties must have been light, because
thereafter he devoted most of his time
to astronomy.
Beginning with the small Dollond
refractor, which he placed in a small
structure in the town hall's garden, he
added new telescopes on a regular basis,

including reflectors with 4- and 7-foot
focal lengths built by Herschel himself
- the latter an almost exact copy of the
telescope used to discover Uranus.
Although Schröter always will be
remembered best for his pioneering
observations of the Moon, Venus was
actually the first object to which he
devoted serious study. He searched
for surface features with the Dollond
refractor beginning in 1779 and for a
long time reported only negative results.
He wrote of his years-long quest: "I
perceived neither spots, nor any other
remarkable appearance, except the
unusually quick decrease of light toward
the boundary of illumination" - a summation with which telescopic observers
of the dazzling planet ever since can
commiserate.
The decrease of light seen near the
terminator was, as Schröter was well
aware, evidence of an atmosphere. An
even more definitive demonstration
came as Venus neared inferior conjunction in March 1790. Using the 7-foot
reflector, he saw the planet's delicate
cusps extended into thin arcs of light
reaching a good way around the dark
hemisphere. He concluded, correctly,
that he was seeing a twilight arc, produced by the diffusion of sunlight by
the planet's atmosphere. Earlier, during
the transits of Venus of 1761 and 1769,
observers had noted an arc or aureole
around the planet and suspected that
Venus had an atmosphere. Schröter had
found definitive proof.

A Geometric Mismatch
Thus he'd already made significant
progress in studying Venus when he
discovered the discrepancy between the

V ENUS IM AG E: SE A N WA LK ER; SK Y & TELESCOPE ILLUSTR ATIONS: LE AH TISCIONE

Thanks to tricks of light in its atmosphere, Venus isn't always quite what it seems.



Sky and Telescope - January 2017

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