horology (hōrŏlˈəjē) [key], science of measuring time and technology of constructing instruments for its measurement or recording. Early measurements of the passage of time were based on observations of seasonal cycles and of the apparent motion of celestial bodies. Shorter intervals were measured by observing the shadow cast by an upright object; the shadow clock and the sundial were probably the first devices constructed. Later came the hourglass and the clepsydra and finally the clock and the watch. The most accurate type of timekeeping device in existence today is the atomic clock. Highly accurate time, which is necessary for such purposes as navigation and the tracking of artificial satellites, is provided throughout the world by time signals that are transmitted by certain radio stations.