Sky & Telescope - March 2022 - 50

MARCH 2022 OBSERVING
Celestial Calendar
Debate Over Dichotomy
AS VENUS ORBITS the Sun, it swings
from solar conjunction to greatest
elongation and back again while cycling
through Moon-like phases over a period
of 584 days. This month, the planet
reaches its greatest western elongation
on the 20th at 21:35 UT (17:35 EDT),
when it lies at a right angle to the Sun
as seen from Earth. At dichotomy - as
this arrangement is called - you'd
expect the planet to appear exactly halfilluminated.
But
you'd be wrong.
Two factors are at play. First, the
eccentricities in the orbits of Earth
and Venus can cause the theoretical
moment of dichotomy to differ by
more than two days from the time of
greatest elongation. Eccentricity variations
change the distance between the
two planets, and thereby, the orbital
position required to achieve the critical
right angle. For the current Venus
apparition, dichotomy occurs on March
21st, nearly 8½ hours after greatest
elongation.
Then there's the Schröter Effect,
first described by German astronomer
Johann Schröter in 1793. He noticed
The angle between the Earth, Venus, and
the Sun is 90° during greatest elongation. At
such time we would expect the planet to appear
exactly 50% illuminated, but telescopic
observations show otherwise.
pShahrin Ahmad of Kuala Lumpur, Malaysia,
captured this view of Venus on October 27th,
2021 UT when 50.6% of the planet was illuminated.
Greatest eastern elongation occurred
on October 29th, while dichotomy took place a
day earlier, on the 28th.
that Venus usually appeared half-lit six
days before evening elongations and six
days after morning ones. Modern observations
have shown that the deviation
is closer to four days on either side of
elongation. The cause of this difference
remains unknown.
One study has shown that when we
Sun
Greatest
eastern
elongation
Venus
90°
90°
Greatest
western
elongation
use our telescopes to view the terminator
near the Venusian poles, our line of
sight passes through a thicker cross-section
of the planet's atmosphere. Therefore,
due to light scattering, the polar
regions appear brighter compared to the
equatorial zone where the sightline is
shorter. The difference makes the terminator
appear slightly concave. While
at first blush this explanation sounds
convincing, it's problematic because
atmosphereless Mercury also exhibits
the same anomaly. Other studies have
suggested that differences in aperture,
atmospheric conditions, or even physiological
or psychological differences
among observers could play a role.
If you have a 4-inch or larger
Earth
Diagram not to scale.
telescope, try looking for the Schröter
Effect yourself this month. Time your
dichotomy observing sessions for
around 1 p.m. local daylight time, when
the planet is nearest the meridian.
That's when you should have the clearest,
sharpest views possible.
Venus reaches greatest elongation
again in June, 2023.
50 MARCH 2022 * SKY & TELESCOPE
DIAGRAM: GREGG DINDERMAN / S&T
Sky & Telescope - March 2022

Table of Contents for the Digital Edition of Sky & Telescope - March 2022
