“Eustasy” is a relatively recent term, coming to English from German as a back-formation of “eustatic,” which means “relating to or denoting changes in sea level throughout the world.” Although “eustasy” is the standard spelling in scientific circles (and in the Oxford dictionary), “eustacy” is the only spelling of this word in other dictionaries. The “-cy” suffix evolved from the Latin “-atia” suffix. Since the early 1990s, eustasy has been measured using satellite altimeters. These use the return speed and intensity of a radar pulse directed at the ocean to determine the height of the sea surface. Before satellites, tide gauge stations measured the daily low and high tides using mechanical floats and recorders. These stations still exist, although today they use more advanced acoustics and electronics.