Sky Watch: Seeing in the dark
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 in this column. Experienced stargazers have little trouble with these because they use some clever tricks-of-the-trade to maximize the power of their eyes, and sometimes forget that most people aren't accustomed to seeing in the dark.
So here are a couple of "secrets" that should help improve your nighttime vision -- both in the sky and on Earth.
One is to use a process that most have experienced; astronomers call it dark adaptation. Enter a dark movie theater on a bright sunny afternoon and you know how tough it can be to find your way to a seat. After watching the movie for a while, however, finding your way out to the refreshment stand or restroom is no challenge.
What has changed? Has the theater slowly become brighter without you noticing? Not at all; in darkness, our pupils dilate to allow in more light. This process takes time -- often more than 20 minutes -- but it does eventually happen.
Step back onto a sunny street again, however, and the sudden shift from dark to light can be stunning. Yet within only seconds you're seeing normally again.
Before beginning their nighttime observing sessions, astronomers always allow plenty of time for this process to take place. 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. You can find red LED headlamps or flashlights in most large stores, but wrapping a piece of dark red cellophane around a white flashlight will work, too.
Another "secret" trick of astronomers is called averted vision. The sensors at the center of the retina are known as cones; they see colors quite 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 more clearly, astronomers use their rods by glancing off to the side of dim objects.
Try out these techniques under a dark sky. Allow yourself to 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 for viewers in the Earth's Northern Hemisphere 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, by someone who is sufficiently dark adapted. Even then, you may find you must avert your vision to spot some of its stars.
Unless you're an astronomer, you probably weren't aware of the tricks used by us "night people," but you are now. So no more excuses; get out there and enjoy those feeble photons raining down upon us from afar!