Angular
units are given as degrees, arc minutes, arc seconds.
An
arcsec is about the angular size of a quarter seen 5 km away.
The
resolution of your eye (which has a diameter of 2.5mm) at the wavelength of
visible light (which is 500 nm) is therefore:
2x10-3 x 5x102/2.5x10-3=400arcsec=6.5arcmin
(the Moon has an angular diameter of 30 arcmin).
To reach the
practical limit of what can be seen (~1 arcsec) through our atmosphere, you need
a telescope with 400 times that diameter, or 1 meter. Telescopes larger
than that only gain you light gathering power, unless you do something to
reduce the blurring by the atmosphere. Radio telescopes, which operate at wavelengths
10,000 times longer, will have far worse resolution.
