To take an observation

(Naut.) to ascertain the altitude of a heavenly body, with a view to fixing a vessel's position at sea.

See also: Observation

References in classic literature ?
But as I had no instruments to take an observation to know what latitude we were in, and not exactly knowing, or at least remembering, what latitude they were in, I knew not where to look for them, or when to stand off to sea towards them; otherwise I might now easily have found some of these islands.