Gegenschein Peak Autumn Viewing Happens October and November

Summary: Gegenschein peak autumn viewing happens October and November for mid-northern latitude observers.

“False Dawn” image of the gegenschein (toward the left of the frame), the Milky Way (center) and the zodiacal light (to the right); image taken at European Southern Observatory’s La Silla Observatory on the outskirts of northern Chile’s Atacama Desert by Czech astrophotographer and ESO Photo Ambassador Petr Horálek: ESO/P. Horálek, CC BY 4.0 International, via ESO (European Southern Observatory)

Gegenschein peak autumn viewing happens October and November as the second of two annual peak viewing sessions for mid-northern latitude observers.

Gegenschein appears in astronomical terminology as the German word for “counter glow.” It is pronounced as gay-gen-shine. The gegenschein visualizes as a luminous, oval patch.

American amateur astronomer Bob King, known popularly as Astro Bob, describes the gegenschein as “cousin of the zodiacal light and one of the greatest night sky naked-eye challenges” in his Oct. 14, 2015, article in Sky & Telescope magazine. He explains that sunlight reflecting off dust particles from comets and rocky splinters from asteroids occasions the gegenschein and the zodiacal light. Back-scattered sunlight creates the gegenschein. Forward-scattered light produces its brighter cousin, the zodiacal light.

Both the gegenschein and the zodiacal light are centered on the ecliptic, Earth’s projected orbit on astronomy and navigation’s celestial sphere. King explains that the greater concentration of the small interplanetary particles account for the two optical phenomena’s favoring of the ecliptic.

The two optical phenomena differ in their placements with respect to the sun. The gegenschein appears at 180 degrees opposite the sun. The zodiacal light favors the same direction as spring’s sunsets and autumn’s sunrises.

The gegenschein illustrates astronomy’s opposition effect, which is also evinced by the full moon and by a planet at opposition. The opposition effect pertains to the brightness and shadow hiding that characterize the antisolar point, the abstract point on the celestial sphere lying directly opposite the sun, from the observer’s perspective. When the sun has set, the observer’s antisolar point is located above the horizon.

The gegenschein is positioned highest in the sky around midnight, local standard time. King notes: “You can see it an hour or two earlier or later, but it’s highest and easiest to spot during the midnight hour.” During daylight saving time, the highest gegenschein occurs at 1 a.m.

The gegenschein offers varying degrees of visibility throughout most of the year at mid-northern latitudes. The Milky Way camouflages the gegenschein in December and January, during the optical phenomenon’s crossing of the solar system’s home galaxy, according to King. American astronomer and gegenschein observer Edward Emerson “E.E.” Barnard (Dec. 16, 1857-Feb. 6, 1923) noted the gegenschein’s invisibility in “June and December, because in those months it is crossing the milky-way” in the June 1891 issue of the Astronomical Journal.

The gegenschein’s location high in the southern sky between September and November explains the autumn peak viewing session at mid-northern latitudes. King advises directing attention to the zodiac constellation appearing in the south around midnight.

Clear, moonless nights and sites devoid of, or with very little, light pollution facilitate successful perception of the gegenschein. Barnard also disfavored moonlight and artificial interference. In the 1918 issue of Popular Astronomy, he advised: “There must be no moonlight. The sky must be clear and not hazy. There must be no artificial light near to blind one.”

King echoes Barnard’s promotion of the averted gaze technique for spotting the gegenschein. Rather than looking directly at the targeted area, the gaze plays around the spot, with indirect glances. Barnard also found squinting as a valuable aid for detecting the gegenschein.

King announces that two annual peak viewing sessions of the gegenschein are available at mid-northern latitudes. The autumn peak of best visibility occurs in October and November. The late winter-early spring peak happens in February and March. Barnard only specified September through November as the best viewing months.

The takeaway for the gegenschein peak autumn viewing that happens in October and November for observers at mid-northern latitudes is that the optical phenomenon also offers peak viewing in February and March and only cloaks visibility during Milky Way crossings.

mid-month approximate shapes and sizes for the ecliptic-centered gegenschein: Martin McKenna @martinastro2005, via Twitter Oct. 18, 2018

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

“False Dawn” image of the gegenschein (toward the left of the frame), the Milky Way (center) and the zodiacal light (to the right); image taken at European Southern Observatory’s La Silla Observatory on the outskirts of northern Chile’s Atacama Desert by Czech astrophotographer and ESO Photo Ambassador Petr Horálek: ESO/P. Horálek, CC BY 4.0 International, via ESO European Southern Observatory @ https://www.eso.org/public/images/potw1707a/

mid-month approximate shapes and sizes for the ecliptic-centered gegenschein: Martin McKenna @martinastro2005, via Twitter Oct. 18, 2018, @ https://twitter.com/martinastro2005/status/1052926421973250048

For further information:

Barnard, E.E. (Edward Emerson). “The Gegenschein and its Possible Origin.” Popular Astronomy, vol. XXVII, no. 2, whole no. 262 (February 1919): 109-112.
Available via Harvard ADSABS (NASA Astrophysics Data System) @ http://adsabs.harvard.edu/full/1919PA.....27..109B

Barnard, E.E. (Edward Emerson). “The Gegenschein in the First Half of October.” Popular Astronomy, vol. XXVI, no. 8, whole no. 258 (October 1918): 561.
Available via Harvard ADSABS (NASA Astrophysics Data System) @ http://articles.adsabs.harvard.edu/full/1918PA.....26..561B
Available via HathiTrust @ https://hdl.handle.net/2027/njp.32101075380277?urlappend=%3Bseq=627

Barnard, E.E. (Edward Emerson). “The Gegenschein or Zodiacal Counterglow.” Popular Astronomy, vol. VII, no. 4, whole no. 64 (April 1899): 169-179.
Available via Harvard ADSABS (NASA Astrophysics Data System) @ http://adsabs.harvard.edu/full/1899PA......7..169B

Barnard, E.E. (Edward Emerson). “Observations of the Zodiacal Counterglow, or Gegenschein, Made at Mt. Hamilton During the Years 1888, 1889, 1890, 1891.” Astronomical Journal, vol. 11, issue 243 (June 1891): 1920.
Available via Harvard ADSABS (NASA Astrophysics Data System) @ http://articles.adsabs.harvard.edu/full/1891AJ.....11...19B

Barnard, E.E. (Edward Emerson). “Periodical Changes in the Form of the Gegenschein.” Astronomical Journal, vol. 20, issue 472 (November 1899): 131-132.
Available via Harvard ADSABS (NASA Astrophysics Data System) @ http://articles.adsabs.harvard.edu/full/1899AJ.....20..131B

King, Bob. Night Sky With the Naked Eye. Salem MA: Page Street Publishing Co., 2016.

King, Bob. “Take the Gegenschein Challenge.” Sky & Telescope > Observing. Oct. 14, 2015.
Available @ https://www.skyandtelescope.com/observing/take-the-gegenschein-challenge101420151410/

Ley, Willy. “The Puzzle Called Gegenschein.” Galaxy Magazine, vol. 19, no. 4 (April 1961): 74-79.
Available via Internet Archive @ https://archive.org/stream/Galaxy_v19n04_1961-04#page/n37/mode/1up

Marriner, Derdriu. “Thomas William Backhouse Clearly Saw Gegenschein Sept. 28, 1875.” Earth and Space News. Wednesday, Sept. 25, 2019.
Available @ https://earth-and-space-news.blogspot.com/2019/09/thomas-william-backhouse-clearly-saw.html

Martin McKenna @martinastro2005. “The Gegenschein was spectacular last night as a huge oval elongated on both ends as it merged into the zodiacal band 25 degrees long. Easy to see despite being in proximity to Maghera town. Check it out after moonset between Taurus, Aries Pisces & Cetus.” Twitter. Oct. 18, 2018.
Available @ https://twitter.com/martinastro2005/status/1052926421973250048

Nemiroff, Robert (MTU); and Jerry Bonnell (UMCP). “The Gegenschein Over Chile.” NASA APOD (Astronomy Picture of the Day). Jan. 14, 2014.
Available @ https://apod.nasa.gov/apod/ap140114.html