Epsilon Lyrae is known as the "double-double" star. It can be used as a test of good eyesight.

Exceptionally good vision on a clear, dark night will reveal Epsilon as undoubtedly two tiny stars (designated Epsilon 1 and Epsilon 2) that are very close together. The separation is 3.5 arc minutes, which is approximately one-ninth the apparent diameter of the full Moon.

This feat is probably right at the limit of perfect vision. Binoculars will make the two stars clearly visible. But if you train a small telescope on them, then each of these two stars are themselves shown to be double stars.

So here, in what initially might appear as a single speck of light in the sky, we have a system of four stars, revolving intricately about each other. The two stars that make up Epsilon 1 take at least several hundred years or more to orbit each other. An even longer interval of nearly a thousand years has been assigned to the two stars that make up Epsilon 2. Meanwhile, both pairs of stars appear to be revolving about each other and have a common center of gravity with a period that probably is on the order of a million years or more!

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