#OTD in 1789.
William Herschel discovers a new moon of Saturn: Enceladus, but little was known about it until the two Voyager spacecrafts, Voyager 1 and Voyager 2, flew by Saturn in 1980 and 1981. Like many satellites of Saturn discovered prior to the Space Age, Enceladus was first observed during a Saturnian equinox, when Earth is within the ring plane.