terminology - Term for a momentary geometric pattern formed by astronomical objects

Momentary events in general (comets, supernovae, solar flares, etc.) can be lumped under the general heading of "transient phenomena". 'Transient asterisms' may be a candidate term for the concept you are describing.

Then again, we now know that on a large enough time scale all asterisms are eventually transient in nature. So one might justifiably take the point of view that the adjective "transient" is redundant. So perhaps calling the patterns you describe simply as "asterisms" is not as bad of a candidate term as you might suspect.