Two very tiny soaring birds were just discernible against the dramatic sky this morning. They were probably gulls or buzzards with a wingspan of about 1 metre. I normally use the little finger of my outstretched arm which subtends 1 degree to estimate how far away they are. But the fraction of a degree was so tiny, I couldn't estimate it. I went back to
resolving power. I measured the diameter of my pupil and it was 3mm. That gives Rayleigh criterion resolving power for visible light wavelength of say 500nm as 2 x 10^-4 radians, roughly 0.01 degree.
The furthest distance resolvable between the wingtips, shown as stars above, can be approximated by arc length/radius for small angles and conveniently given in radians. For a 1 metre wingspan, 2 x 10^-4 = 1/radial distance. The furthest they could be turns out to be about 5km away.
