That we can't resolve something does NOT mean that it is invisible.
It just means that you can't distinguish it from a point. Grab your
binoculars and you can resolve Mars into planetary disk.
You can't resolve the lights of an airplane high in the sky, but you
sure can still see them.
As far as Mars goes, you can directly measure the distance to Mars
using parallax - not the yearly one around the Earth's orbit - which I
know you all dispute - but an instantaneous one from opposite (or nearly
so) sides of Earth. Add the angular diameter observed with a telescope
at that same time, and you can also find it's absolute diameter. Violá!
Regards,
Regner
