The Ghostly Gegenschein

The gegenschein over Paranal Observatory in Chile. Image credit ESO/Y. Beletsky

3 March 2019

Owing to the encroachment of light pollution, sighting the gegenschein has almost become mythical and certainly makes me appreciate what an immense privilege it is to be observing under skies so dark and transparent that one can see the faint, diffuse, oval glow around midnight in the zodiacal constellation then highest in the sky.

I really enjoy the two ‘gegenschein seasons’ – once it has untangled itself from the Milky Way it’s visible from late January through April (when the anti-solar point moves from Cancer to Leo to Virgo) and from September through November (when it moves from eastern Capricorn through Aquarius and into Pisces, Aries and western Taurus). There is little to beat looking up at midnight, catching the faint illusive glow in whichsoever constellation and thinking of the Sun beneath my feet.

The gegenschein is caused by a retro-reflector phenomenon known as the Seeliger effect, which results in by sunlight glinting off dust particles scattered along the plane of the ecliptic. Although dust is scattered all along the solar ecliptic, it’s slightly brighter in the anti-sunward direction because, much like a Full Moon or planet at opposition, sunlight strikes the dust particles square on… like millions of tiny little ‘Full Moons’.

*Theodor Johann Christian Ambders Brorsen (1819-1895)*

The discovery of the gegenschein (‘counter-glow’ in German) has usually been attributed to Espirit Pezenas in 1730. He was a French Jesuit astronomer, hydrographer, and mathematician who worked at the observatory in Marseilles.

However, the inimitable ‘Celestial Sleuth’ Donald Olson (Texas State University astronomer, physics professor emeritus and Texas State University System Regents’ Professor) has determined that Danish astronomer Theodor Johann Christian Ambders Brorsen was actually the first person to give a clear and accurate description of the gegenschein, along with measurements of its position among the stars, in publications from the years 1854 and 1855 – more than a century after the supposed discovery by Pezenas.

He wrote: “… after 11 p.m. the Gegenschein appeared, which consisted of a brighter elongated round patch … the middle of it coincided almost exactly with the point opposite the Sun … a round glow, almost exactly opposite the Sun’s location.”

Olson also writes that based on his journal entries from March 1803 German explorer Alexander von Humboldt later recalled seeing a faint counter-glow in the east. In the original German of his account, counter-glow is “Gegenschein,” giving the phenomenon its modern name. However, astronomer Robert Roosen, an expert on night sky glows, rejected outright the possibility that Humboldt viewed the true gegenschein, as Humboldt described luminous pyramids near both the western horizon and the eastern horizon shortly after sunset, not near midnight when the oval of the true gegenschein is high overhead. But Humboldt gave us its name.

Olson published his findings in the October 2021 issue of Sky & Telescope magazine – a fantastic read!

Interestingly, Apollo 15 (July 26 – August 7, 1971) had planned, as one of its experiments, an attempt to photograph the gegenschein from lunar orbit. Command Module pilot Al Worden mentions this in his autobiography Falling to Earth: An Apollo 15 Astronaut’s Journey to the Moon, but alas, no images of the gegenschein from space were captured due to an error in pointing and tracking.

A diagram featuring Pioneer 10 observations of the gegenschein. The Gegenschein cannot be associated with Earth because seen from Pioneer and seen from Earth it is in different directions in space. Image credit: NASA/JPL

A very interesting observation of the gegenschein was made by the outbound Pioneer 10 mission as it headed to Jupiter. The gegenschein was observed on March 14, 1972, when the earth-sun-spacecraft angle was 3.4 degrees and the spacecraft was 9.2 million km from the earth and 1.011 AU from the Sun. That gegenschein was once thought to be solely an atmospheric phenomenon: possibly reflected sunlight shining off the very high atmosphere of Earth. The Pioneer 10 observation lent weight to the idea that dust is distributed pretty evenly across the solar plane, rather than ‘clumped’ exterior to the Earth’s orbit at the anti-solar point.

Astronomy has no shortage of the fleeting and ephemeral when it comes to observations, and seeing the ghostly gegenschein is such a strangely moving sight.