(1) The partial or total shadowing (obscuration), relative to an observer, of one celestial body by another. The term occultation is used for those eclipses in which some star other than the Sun provides the light, and the light is obscured by the Moon or by some other body in the Solar system. Occultations and eclipses have been used in the determination of longitude and latitude. The principle is the same for either phenomenon. The time at which the Moon's shadow passes a point of known longitude and latitude is measured, as is the time the same part of the shadow passes a second point whose longitude and latitude are to be determined. The difference in times is a function of (a) the differences in longitude and latitude, and (b) the velocity of the Moon's shadow across the Earth's surface. The last of these quantities can be calculated from the data in astronomical ephemerides. Observations of several eclipses give enough data for calculating the longitudinal and latitudinal differences. In the past, the necessary calculation have used the Besselian elements listed in ephemerides and almanacs. (2) The period of time during which one celestial body obscures the light received from another.