Blue Collar Scientist

The ESO (European Organisation for Astronomical Research in the Southern Hemisphere, in case you aren’t familiar) has released some interesting images, including one of the best photographs of the gegenschein that I’ve ever seen.

I know I have a lot of non-astronomer readers, so bear with me for a second while I explain. That word is pronounced gay-gen-shine (hard “g”), or gay-gun-shine, depending on where you grew up and learned it. And what the gegenschein actually is, is pretty dang cool. The solar system was formed about 4.5 billion years ago from a great disk of dust and gas. The gas was blown out into interstellar space by the sun, but the dust is not so easy to drive out of the solar system. In fact, something called the Poynting-Robertson effect¹ causes this dust to fall inward toward the sun, so the dust has to be continually replenished, and we think that happens as a result of dust distributed by comets and the rare asteroid collision. In any case, there’s dust in our neighborhood of the solar system.

The gegenschein is what we see when sunlight illuminates this dust and reflects the light back to us. Actually, the zodiacal light is exactly the same thing. The zodiacal light appears in the west after sunset or the east before sunrise, and is awesomely bright from a dark sky at the right latitude. Part of this brightening is probably the result of dust between us and the sun backscattering light in our direction.

As you follow the zodiacal light farther from the sun in the sky, it gets quite dim, and perhaps becomes invisible. If you can see it at all, the faint wisp of light is called the zodiacal band, and you can track that all the way across the sky, until the end of the night when it re-connects with the morning zodiacal light. But along the way, something strange will happen - a bit football-shaped glow² will be found right in the middle of the zodiacal band. If you do your homework, you will find that this football is exactly 180° from the sun.

This football shaped glow is the gegenschein. The brightening of the zodiacal band here is caused by all the dust in this area being seen at nearly full phase. Neither the zodiacal band nor the gegenschein are easy to see - you need a dark site with clear skies at the proper latitude and season to reliably make it out.

And it is incredibly hard to photograph, which is why this photo blew my socks off:

That may not look like a big deal to a person who doesn’t spend much time under the night sky, but to someone who does, and who has tried to photograph the gegenschein many times, this is an impressive result. You can click the picture to get a high resolution version in all its glory.

There are more pictures associated with this ESO press release, including good shoots of the green flash and the blue flash. Enjoy!

1. Drag caused by radiation pressure tangential to a celestial body’s orbital motion. You do not want to see the math, or the relativistic model, of this phenomenon. Trust me. [↩]
2. American football - I’m referring to a shape that is approximately the cross-section of a prolate spheroid. [↩]

Tags: ESO, gegenschein, Yuri Beletsky

This entry was posted on Friday, May 2nd, 2008 at 9:24 am and is filed under astronomy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.