Term for a momentary geometric pattern formed by astronomical objects Few days back, the Pleiades, the Sun and the Moon were forming an almost perfect equilateral triangle.

Is there a term that describes such momentary geometrical patterns? Asterism and conjunction are close, but not exactly applicable here.
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.