  An isogeny class of elliptic curves over a finite field is determined by a
quadratic Weil polynomial. Gekeler has given a product formula, in terms of
congruence considerations involving that polynomial, for the size of such an
isogeny class. In this paper, we give a new, transparent proof of this formula;
it turns out that this product actually computes an adelic orbital integral
which visibly counts the desired cardinality. This answers a question posed by
N. Katz.
