Sky & Telescope - October 2021 - 35

GEGENSCHEIN FROM LAS CAMPANAS: YURI BELETSKY;
ILLUSTRATION: COURTESY DON OLSON
Gegenschein?
T
hree related celestial glows - the zodiacal light, the
zodiacal band, and gegenschein - all arise from
sunlight reflected and scattered by dust in or near the
ecliptic, the plane defined by Earth's orbit around the Sun.
Along the ecliptic is where you'll also find the Sun, the Moon,
the planets, and the stars of the zodiacal constellations.
The zodiacal light appears with the shape of a tilted
pyramid with its base at the Sun, below the horizon. Due to
seasonal variations in the angle of the ecliptic relative to the
horizon, observers in the Northern Hemisphere find that the
zodiacal light is best seen in the west after sunset in February
and March, and in the east before sunrise in September
and October. This magazine's Sky at a Glance (on page 41)
lists each season's prime, Moon-free observing windows for
observing the zodiacal light.
The zodiacal band is a dim and narrow glow that extends
from the zodiacal light pyramid and continues along the
ecliptic across the entire night sky through the constellations
of the zodiac.
The gegenschein (German for " counterglow " ) appears as a
diffuse brightening of the zodiacal band at the anti-Sun (or
anti-solar point) - the spot 180° opposite the Sun's position
on the celestial sphere.
Ghosts in the Night
Detecting these faint phenomena - especially the zodiacal
band and gegenschein - requires a moonless night and a
site with dark and transparent skies. Moreover, the overwhelmingly
bright glow of our home galaxy washes away
the gegenschein during months when the anti-Sun lies in or
near the Milky Way. The gegenschein is positioned relatively
far from the Milky Way during favorable viewing months of
(late) September, October, and November, and then again in
February, March, and April.
The best time of night to search for the gegenschein is
near local midnight, when it appears as an oval patch in the
zodiacal constellation highest in the sky and far from the
atmospheric absorption that prevails near the horizon. Sky &
Telescope columnist and experienced observer Bob King noted
that " you can see it an hour or two earlier or later, but it's
highest and easiest to spot during the midnight hour. "
Harlan Smith, director of McDonald Observatory for
many years, related similar stories of viewing the gegenschein
from the dark skies of West Texas. He told me he always
found the time to step outside the telescope dome and carefully
note the position of the gegenschein among the stars.
It's time to set the record
straight and finally give
credit where credit is due.
If you've been fortunate enough to see any of these faint
zodiacal glows, you may wonder who was the first to observe
and describe the gegenschein. The answer may surprise you.
A Popular Choice
According to the Wikipedia entry for " Gegenschein, " five
different astronomers independently discovered the phenomenon,
with the first sighting credited to Esprit Pezenas:
The gegenschein was first described by the French Jesuit
astronomer and professor Esprit Pézenas (1692-1776) in
1730. Further observations were made by the German
explorer Alexander von Humboldt during his South
American journey from 1799 to 1803. It was also
Humboldt who gave the phenomenon its German name
Gegenschein.
GHOSTLY PYRAMID In this illustration, the luminous cone of the
zodiacal light extends obliquely into the sky on the evening of February
20, 1876, with the Pleiades star cluster just to the north (to the right)
of the tip of the triangular glow. Étienne Léopold Trouvelot created
this chromolithograph as part of The Trouvelot Astronomical Drawings
Manual published in 1882.
sk yand tele scope .o r g * OCTOBER 2021 35
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Sky & Telescope - October 2021

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