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)))
  

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