Posted by BC on June 19th, 2008
Most of us know that at night we use averted vision to find our way around in the dark. Some of us have better night vision than others, and some of us have better vision in general than others of us. At night we naturally tend to use the rod-cells in our eyes, but doing so leaves us without good resolution. To focus directly on an object and gain details, we must use the cone-cells. If we focus on a star at night, that star may disappear from our vision if it is not bright enough to overcome the eye’s tendency to make it vanish from view. (More on cones and rods)
Try staring directly at stars of different magnitudes and see how bright a star must be to stay in your sight. Splitting stars this way is often more difficult that you may have imagined. Try your vision on Alcor and Mizar or the components of epsilon Lyrae. Eyesight was often tested at night by requesting that people split pairs or count members of clusters, such as the Pleiades Star Cluster. I have not tried splitting epsilon Lyrae yet, but I have counted members of the Pleiades. I counted 14 members with my naked eye.