The sun, I notice on this day when Daylight Saving has reverted to Standard Time, has imperceptibly set.
early 15c., from French imperceptible (early 15c.), from Medieval Latin imperceptibilis, from assimilated form of in- "not, opposite of" (see in- (1)) + perceptibilis (see perceptible). Related: Imperceptibly. OED marks imperceivable as "Now rare."