By telling you about the gegenschein, I’m afraid I’m sending you on something of a wild goose chase.
Worse than that, in fact, because I’m almost positive that you’ll see a wild goose before you catch a glimpse of the gegenschein.
But the gegenschein is fascinating and everyone needs long-term goals.
Have you ever sat around in your living room on a sunny day and been amazed (astonished, ashamed – pick an adjective) at all the tiny flakes of dust floating around in the room air? You can see the tiny airborne dust flakes that are just about everywhere when the sunlight streaming in the window hits them just right.
Try smacking the palm of your hand on the back of your couch the next time your living room is filled with sunlight. That can be pretty disgusting, even in the cleanest of homes.
This is an example of something scientists call ’scattering.’ Tiny dust particles scatter light – scattering is something like reflection, but not so organized.
When a particle scatters light, it absorbs momentarily part of a ray of light, then re-emits the same bit of light in a random direction. This is why you can’t see far on a foggy day even though there is plenty of light. The tiny droplets of water that make up a fog bank scatter and re-scatter light every which way so you can’t tell where or what the light came from.
Scattering is an important process in any astrophysical environment where there is both dust and light – just about everywhere. Close to home is the region of interplanetary space in our solar system between the orbits of Venus and Jupiter. Interplanetary space (indeed, space in general) is not nearly as empty as most people think. And under just the right conditions, if you look carefully, you can see dust flakes faintly flitting around the solar system just as they do around your apparently clean living room.
Scattering, although more random than reflection, is not completely random. It turns out that when very small dust flakes grab a bit of light from a light ray they scatter the light with a slight preference, either in the direction the original light ray was traveling or in exactly the opposite direction.
In more compact language, tiny dust flakes preferentially forward-scatter and back-scatter light. You can see this for yourself with some experimentation in your dusty, sun-lit living room.
To see this in the solar system, you need to set out to a very dark location on a very clear night and look into the dusty sky in the direction exactly opposite the sun. What you will see, if the conditions are just right, is the gegenschein – a faint ball of light in an otherwise dark sky exactly opposite the sun. The gegenschein is produced by tiny interplanetary dust grains that back-scatter sunlight.
Gegenschein is a German word that means “counter glow.” Late January through February is the best time of the year to spot the gegenschein because this is when the anti-solar point is high and far from other faint lights in the sky. Faint lights like the Milky Way are bright compared to the gegenschein.
From mid January through the end of February look for the gegenschein near the constellation Gemini, high in the sky around midnight. A nice picture and description of the gegenschein was recently posted on Astronomy Picture of the Day.
But remember, the gegenschein is not easy to spot; you’re more likely to see a wild goose.
