was discovered by Johannes Hevelius in 1648. It can reach 7°54′ in amplitude. Longitudal libration allows an observer on Earth to view at times further into the Moon's west and east respectively at different phases of the Moon's orbit. Libration in latitude results from a slight inclination (about 6.7°) between the Moon's axis of rotation and the normal to the plane of its orbit around Earth. Its origin is analogous to how the seasons arise from Earth's revolution about the Sun. Galileo Galilei is sometimes credited with the discovery of the lunar libration in latitude in 1632, although Thomas Harriot or William Gilbert might have done so before. Note Cassini's laws. It can reach 6°50′ in amplitude. The 6.7º depends on the orbit inclination of 5.15º and the negative equatorial tilt of 1.54º. Latitudal libration allows an observer on Earth to view beyond the Moon's north pole and south pole at different phases of the Moon's orbit. Parallax libration depends on both the longitude and latitude of the location on Earth from where the Moon is observed. Diurnal libration is the small daily libration, an oscillation due to Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's and the Moon's centers, allowing the observer to look first around one side of the Moon and then around the other—since the observer is on Earth's surface, not at its center. It reaches less than 1° in amplitude. Physical libration is the oscillation of orientation in space about uniform rotation and precession. There are physical librations about all 3 axes. The sizes are roughly 100 seconds of arc. As seen from the Earth, this amounts to less than 1 second of arc. Forced physical librations can be predicted given the orbit and shape of the Moon. The periods of free physical librations can also be predicted, but their amplitudes and phases cannot be predicted. Physical libration Also called real libration, as opposed to the optical libration of longitudal, latitudal and diurnal types, the orientation of the Moon exhibits small oscillations of the pole direction in space and rotation about the pole. This libration can be differentiated between forced and free libration, while forced is the libration due to the forces exerted in the course of the orbit of the Moon around the Earth and the Sun. Free liberation is an