saw the same thing at work (i work outdoors) yesterday. Its a halo (or corona) and i've seen it before a fair few times. Happens when the light interacts with the moisture in the air (clouds), same as a rainbow in prinicple, 'cept it's in the sky around the object (sun or moon) rather than touching the ground.

see www.atoptics.co.uk for some lovely images.

tis a beautiful thing :-)