  Affine linking numbers are the generalization of linking numbers to the case
of nonzero homologous linked submanifolds. They were introduced by Rudyak and
the first author who used them to study causality in globally hyperbolic
spacetimes. In this paper we use affine linking numbers to estimate the number
of times an observer sees light from a star, that is how many copies of the
star do they see on the sky due to gravitational lensing.
