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By comparison, the human eye can resolve structures separated by at best about 40 seconds of arc, and even the Hubble Space Telescope can achieve a resolution of only about 0.04 second of arc.

But measures of parallax made by Herman S. Davis and Wilsing seem to show a difference of parallax between the two components of about 0·08 of a second of arc.

So accurate were his measures that he was probably certain of the star's position to the nearest second of arc.

Antares and Betelgeuse appear to be the most inviting for investigation, because their estimated angular diameters are about one-twentieth of a second of arc.

By applying the interferometer method to Capella, arc distances of much less than one-twentieth of a second of arc were measured.