Sky & Telescope - November 2020 - 49

t The gegenschein and part of the zodiacal band are visible in this photo showing the
European Southern Observatory's Very Large
Telescope facility in Chile's Atacama Desert.
Also called counterglow, the gegenschein is a
fuzzy patch of sunlit interplanetary dust.

ESO / Y URI BELE TSK Y

Midnight
Meeting
with the
Gegenschein
YEARS AGO, I THOUGHT the gegenschein was reserved for mountaintops
and desert locations hundreds of kilometers from city lights. I never expected
to see it from my own backyard. But
observational barriers have a way of
falling when one decides to ignore convention and look anyway.
The gegenschein is a cousin of the
zodiacal light and may be the closest
thing to nothing that you'll ever lay
eyes on. An ovoid glow about 10° wide
by 5° high, the gegenschein is centered
on the ecliptic at the antisolar point
- the spot directly opposite the Sun's
location in the sky. Like the zodiacal
light, the gegenschein is composed of
minute grains of comet and asteroid
dust illuminated by sunlight. This
should come as no surprise, because
they're both manifestations of the same
phenomenon.
The zodiacal light shines brightest
nearest the sunset (or sunrise) point on
the horizon, and gradually fades as you
follow its conical form upwards. Seen
from the darkest locales, the light arcs
along the ecliptic across the entire sky
as the exceedingly faint zodiacal band.

Midway along its length, directly opposite the Sun, the band brightens as the
gegenschein, a German word meaning
"counterglow."
Much like with the full Moon or
planet at opposition, sunlight strikes
the dust particles in the gegenschein
head-on. And just as the full Moon
reaches its greatest altitude around local
midnight, so too does the gegenschein.
I ﬁrst noticed the phenomenon about
15 years ago from a dark-sky site. Once
I knew what to look for, I started noticing the gegenschein more often and
under less pristine skies. You can, too.
November is one of the best times
of year to look for the gegenschein,

Minima of Algol

29
30

because at midnight the ecliptic stands
high in the sky in Aries and Taurus.
Atmospheric extinction is at a minimum, no bright background stars get
in the way, and the Milky Way doesn't
muddle the view. The counterglow,
however, is high enough to seek by 11
p.m. local time, and remains wellplaced into the early morning hours.
While a dark sky is crucial, it
doesn't have to be perfect. As I mentioned earlier, I've seen the gegenschein
from home, where I deal with modest
light pollution in the southwest from
Duluth, Minnesota. Plan to look when
the Moon is absent from the midnight
sky, from November 9th to 21st. Early
in the month, the ghostly, oval-shaped
patch appears about 10° southwest of
the Pleiades, and by the 20th it hovers
southeast of the cluster. Use averted
vision and sweep around the area until
you discern a hazy presence that's a
little brighter toward its center.
Once you see the gegenschein, the
zodiacal band awaits as your next midnight challenge!

18

PERSEUS
38

Algol

21

TRIANGULUM

34

p Perseus climbs high in the northeastern
sky during evening hours in November.
Every 2.7 days, Algol (Beta Persei) dips from
its usual magnitude 2.1 to 3.4 and back.
Use this chart to estimate its brightness in
respect to comparison stars of magnitude
2.1 (Gamma Andromedae) and 3.4 (Alpha
Trianguli).

Oct.
1
4
6
9
12
15
18
21
24
27
29

UT
4:59
1:48
22:37
19:25
16:14
13:03
9:52
6:40
3:29
0:18
21:07

Nov.
1
4
7
10
13
16
18
21
24
27
30

UT
17:56
14:44
11:33
8:22
5:11
2:00
22:49
19:38
16:27
13:16
10:05

These geocentric predictions are from the
recent heliocentric elements Min. = JD
2445641.554 + 2.867324E, where E is any
integer. For a comparison-star chart and
more info, see skyandtelescope.org/algol.

sk yandtelescope.org * NOVEMBER 2 02 0

49
http://www.skyandtelescope.org/algol http://www.skyandtelescope.org
Sky & Telescope - November 2020

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