If you insist on up/downness being with relation to something other than the earth, and want the sun to be the referent, then the sun would be the measure of downnness, since it pulls the earth toward it, in resistance to the centrifugal force of the earth's orbit.

Gravitational attraction as such is really in every direction where there is mass. Therefore, except with relation to the earth, "up" and "down" are meaningless.

Dave Oesterreich