Astroblog: A Dark Adapted Eye

Friday, June 19, 2009

A Dark Adapted Eye

If you were reading a particularly interesting article, say on how to find the constellation of the Emu [1], or about the Geminid meteor shower, and you get enthused so you walk outside ... and stand blinking in the dark. You can see a few bright points of light and if you are like me, a glowing square where you were staring at the computer screen, but the sky is a dark wasteland.

You have come up against the phenomena of dark adaption. Most people have encountered the phenomenon in the opposite direction, walking in from a brightly lit garden to indoors, for a moment you stand blinking in the dark.

Our eyes are amazing things. They can operate over a range of intensity of nine orders of magnitude. But they take time to adapt. In bright light the photopigments (rhodopopsins) are partially bleached, and they must be chemically regenerated [2]. It takes 30 minutes for eyes to become fully dark adapted, which is why amateur astronomers get so annoyed when someone turns on a torch, or heaven forfend, sweeps car headlights across them while they are observing. It will take at least half an hour before their eyes are as sensitive again.

If you are thinking, "Half an hour? What's the point of looking?", don't despair. For the casual observer, you only need six minutes to be able to see stars down to magnitude 5 or 6. This is more than enough to the see sky in its magnificence. If you want to see the faint, pretty clusters, or get the best out of meteor showers, then it's well worth waiting. Even from my suburban location, by waiting patiently I can see the Milky Way aching faintly overhead. And while you are waiting, you can spend your time gazing at the wonderful constellations of bright stars. Within a few minutes you can see enough stars to pick out most constellations.

I think this is part of the reason we are disconnected from the sky. Apart from the persuasive light pollution reducing the number of stars we can see, in the suburbs and the city we usually emerge briefly from brightly lit homes to take out the rubbish, feed the cat etc. Not enough time for us to adapt to the darkness even if we do look up.

If you have been staring at a computer screen for some time, there is little you can do to speed up the process of dark adaption. Oh, you could turn off all the lights inside and put a sheet of red cellophane over the screen [3], but that makes watching YouTube difficult and will raise concern from your housemates or partner (or both).

The best thing to do is to simply wait. I walk out into the back yard, make sure the backyard lights are off, and I close my eyes for a minute or so. Then I open my eyes and watch the sky unfold around me.

[1] Uh, what? There is no constellation of the Emu, you say. I'll tell you about it next week.
[2] there are also neural processes involved, but these are quite rapid compared to reversing the bleaching, for a full technical explanation see here.
[3] red light doesn't ruin your dark adaption, that's why amateur astronomers put red cellphane over their torches. So they can see stuff without ruining their dark adaption.

Labels: Astronomy, unaided eye, unaided eye observation, where to look