In this paper it is proposed that the ratio between the apparent radius
of any two celestial bodies separated by a very big distance, as seemed each
other at a distance, is numerically equal to the ratio of the respective real
values of their radius. That relationship is what is called in the paper the
Reciprocity Principle. In other words, the apparent size of heavenly bodies, as
seemed at a distance, plays a special role in the gravitational interactions.
This is so because of some kind of effect over the size due to the very big
distances in space. When a couple of celestial bodies interact each other, both
of them can be considered as sources of the gravity attraction. In that situation,
each body acts upon the other in such a way that it can be considered that
there are some degree of reciprocal interaction. In this paper, a formal
definition of that reciprocal effect is given.