About 200 axioms are used to describe the domain of astronomy.
lightlike?(e1,e2) holds if a photon could leave the event
(position and time) e1 and arrive at event e2.
ephemeris-object-and-time-to-event yields an event corresponding
to the position of a given astronomical object (planet or spacecraft)
at a given time.
a-sent(o,d,ta) computes the time a photon must leave object o
to arrive at destination d at time ta.
Axiom lightlike?-of-a-sent:
(all (o d ta)
(lightlike?
(ephemeris-object-and-time-to-event
o (a-sent o d ta))
(ephemeris-object-and-time-to-event d ta)))