Is this sentence correct ?

Sunspots appear on the surface of the sun as dark spots , although never sighted on the Sun's poles or equator .

It says in my book that " Although introduces a subordinate clause , which has a subject and a verb " .
First of all , what is a subordinate clause ? Does it mean that , in the above example , although must be followed by " they are" ?
as in "Sunspots appear on the surface of the sun as dark spots , although THEY ARE never sighted on the Sun's poles or equator ."


1 more doubt. Is it correct to say "at the poles " or " on the poles " ? We use "on" only when it refers to the surface of the sun , don't we ?