OPOD - Gegenschein, Counterglow

Gegenschein, Counter Glow

Radek Grochowski captured this rare sight from the Low Beskid mountains of SE Poland on 8th October.

Mouse over the image for an enhanced view. The gegenschein is the faint glow at left, far dimmer than the massed star clouds of the Milky Way. There is a hint of the even rarer zodiacal band trailing upwards from it and along the ecliptic towards the Pleiades at top left. Radek's unenhanced image captures the atmosphere of the unaided eye's impression of the night sky.

"The mountains offered perfect conditions for stargazing. After two days of rain, when the sky clarified at last the air was crystal-clear and the sky dark. The gegenschein was easily visible with averted vision as an oval brightening in the constellation of Pisces, it was approximately a dozen times fainter than the brightness of the Milky Way. Photograph details: 45 seconds at ISO 2000, Nikon D300 and Sigma 10/2.8 fisheye lens."

©Radek Grochowski, shown with permission.

The antisolar point opposite the sun is home to the opposition effect, the heiligenschein, the centre of the rainbow, the glory and the convergence of shadows and anticrepuscular rays. All are from scattering by the ground or dust or water droplets in the atmosphere. The gegenschein, another antisolar effect, is not of the Earth � it is light scattered by interplanetary dust grains extending out in a huge disk almost to the orbit of Jupiter.

Each grain is 1- 300 micron (0.001 - 0.3 mm) across miles from a neighbour. By the standards of visible light it is large and mostly scatters sunlight forwards, only slightly deflected, but some is scattered backwards though less strongly.

Sunlight forward scattered by particles between Earth and Mercury's orbit give us the pearly cone seen before sunrise and after sunset, the zodiacal light. The gegenschein is backwards scattered light by particles outwards from Earth to the asteroid belt.

The brighter zodiacal light is elusive enough. It used to be described as visible only in or near the tropics where the ecliptic makes a steep angle with the horizon. However, at the right time of year and with skies unpolluted by artificial lights it can be seen in Northern Europe, Canada and USA. The much fainter gegenschein is much harder and Radek�s image from Poland is possibly a first from so far north.