From the June 2013 issue

Test out your visual acuity

August 2013: The summer sky holds several naked-eye double stars that, once observed, can tell you something about the sharpness of your eyesight at night.
By | Published: June 24, 2013
Resolving naked-eye double stars presents an unusual visual challenge. To see fine detail, we usually look directly at the object of study so that light falls onto the eye’s fovea centralis — the pit in the retina that is responsible for sharp vision. But the fovea does not have rods, and the cones populating this tiny region are largely ineffective at night. Because some of the stars in this exercise may be too faint for you to detect in the dark with direct, foveal vision, you may have to explore the eye’s twilight zone. This area, which is about 12° away from the fovea’s center, includes not only cone cells near the fovea’s outer edge but also some night-sensitive rod cells that surround them.

Your eye’s hot spot
The best way to test your visual acuity with naked-eye doubles is to do so in stages, working from the easiest to the most difficult objects. Try starting with Alpha11) and Alpha2 Capricorni, now in the southeastern sky after sunset. These stars form the northwestern tip of the constellation’s Delta Wing asterism. Alpha2 shines at magnitude 3.6, and Alpha1 is its magnitude 4.3 companion 6.6′ to the west-northwest. While these stars are not difficult to resolve, spend time with them and see where in your field of vision they appear most distinct; this will be your eye’s hot spot.
Once you determine your hot spot, position in it our next target: magnitude 3.8 Omicron1 (ο1) Cygni and its line-of-sight companion, magnitude 4.8 30 Cygni, 5.6′ to the northwest. This attractive pair lies about 5° west and slightly north of Deneb (Alpha Cygni), which marks the tail of Cygnus the Swan. Not only are these stars slightly fainter than Alpha1 and Alpha2 Capricorni, but they’re also 1′ closer together. Again, take time to study these stars, and see if you need to adjust their placement in your field of vision.

Resolving a visual binary such as Alpha11) and Alpha 2 Capricorni (the bright stars near center) requires exploring the eye’s “twilight zone” about 12° away from the area of sharpest vision. // Sloan Digital Sky Survey
Probing deeper
If you are under a dark sky, shift your gaze high overhead to the Keystone of Hercules. Just 4.5° northwest of Zeta (ζ) Herculis (the southwesternmost star in the Keystone), you’ll find the neglected double Nu11) and Nu2 Coronae Borealis. Here, we have two 5th-magnitude stars separated by only 5′ and oriented roughly north-south.
These stars are, in fact, perfect test subjects for anyone wanting to try to resolve our next target: Epsilon11) and Epsilon2 Lyrae. You’ll find this famous naked-eye pair less than 2° east-northeast of brilliant Vega. Like slightly fainter Nu Coronae Borealis, 4th-magnitude Epsilon Lyrae consists of two equally bright stars in a north-south orientation; but these little gems lie only 3.5′ apart. They are, in fact, near the limit of resolution for many naked-eye observers. Seeing them may require watching in the deep twilight when the cone cells are still active.

If you have trouble with Epsilon Lyrae, turn your gaze to Zubenelgenubi (Alpha Librae), which lies about as high above the horizon as Alpha Capricorni but in the southwest. It is a slightly wider pair (3.9′) with a brighter primary (magnitude 2.8) and a magnitude 5.2 secondary to the northwest.

Near impossible?
Return to Alpha Capricorni and then drop your sights to Beta (β) Capricorni. While I often have seen this star referred to as a naked-eye double, I have yet to find an actual account of a positive sighting. I’ve tried several times and could only muster an elongation of the star. But I do think someone with younger or better eyes could achieve success. While Beta1 shines at 3rd magnitude, its partner star, Beta2, is a magnitude 6.1 sun only 3.4′ southwest of Beta1.

Of course, remember, if you don’t see any of these stars with your unaided eyes, they all make striking pairs when binoculars give them clarity. Let me know how you do. Send any reports to