Sky watch: Dark adaptation and the elusive Little Dipper

Anyone who has ever spent time under a dark night sky knows how challenging it can be to find some of the sights I suggest. Astronomers have little trouble with these because they use some clever tricks of the trade to maximize the power of their eyes.

One of these is dark adaptation. Enter a dark movie theater on a bright sunny afternoon and it can be tough to find your way to a seat. After watching the movie a while, however, finding your way out to the refreshment stand or restroom is no challenge.

That's because, in darkness, our pupils dilate to allow in more light. This process takes time - often more than 20 minutes - but it does eventually happen.

Astronomers always allow plenty of time for this process to take place before they begin their observing. Once dark adaptation is completed, they protect their night vision by using only red light to find their way around or to read star maps or log books.

Another of astronomers' "secret" tricks is something called averted vision. The sensors at the center of the retina are known as "cones"; they see colors well, but only under bright conditions. Only the "rods" - the gray sensors surrounding the cones - can see faint light, but they do so at the expense of color.

So, in order to see faint objects much more clearly, astronomers use their rods by always glancing off to the side of dim objects.

Try out these techniques under a dark sky. Become fully dark adapted, and avoid looking at any white light. Take with you a flashlight that is covered with red cellophane, or get yourself a red LED flashlight. Then find some faint objects in the sky and begin averting your vision to see them.

One of the best tests of these techniques lies in the northern sky: the Little Dipper. The stars of this famous grouping are quite dim and can be seen only from a dark location far from the blinding lights of a city, and by someone who is well dark-adapted. Even then, you might find you must avert your vision to spot some of its stars.