Comet
A bright, rare visitor from the edge of the system — a once-in-a-long-time presence passing through.
A comet in a dream is the dream's image of a rare, bright presence passing through. Comets come from the edge of the system, take centuries to return, and are remembered. The dream brings the comet when something or someone in your life — an opportunity, a person, a moment of clarity — is passing through on a schedule that will not be back soon. A comet watched in time is the dream telling you the rare visit has been seen and taken in. A comet missed because you were not looking up is the dream noting that the rare thing came through your sky and you did not register it.