In astronomy, libration is a perceived oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of trojan asteroids relative to planets. Lunar libration is distinct from the slight changes in the Moon's apparent size viewed from Earth. Although this appearance can also be described as an oscillating motion, it is caused by actual changes in the physical distance of the Moon because of its elliptic orbit around Earth. Lunar libration is caused by three phenomena detailed below.
The Moon keeps one hemisphere of itself facing the Earth, due to tidal locking. Therefore, the first view of the far side of the Moon was not possible until the Soviet probe Luna 3 reached the Moon on October 7th, 1959 and further lunar exploration by the U.S. and the Soviet Union. However, this simple picture is only approximately true: over time, slightly more than half (about 59%) of the Moon's surface is seen from Earth due to libration.
Libration is manifested as a slow rocking back and forth of the Moon as viewed from Earth, permitting an observer to see slightly different halves of the surface at different times.
The following are the three types of lunar libration:
- Libration in longitude results from the eccentricity of the Moon's orbit around Earth; the Moon's rotation sometimes leads and sometimes lags its orbital position. The lunar libration in longitude was discovered by Johannes Hevelius in 1648.
- 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 could have done so before.
- Diurnal libration is a small daily 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.
In 1772, Lagrange's analyses determined that small bodies can stably share the same orbit as a planet if they remain near Lagrange points, which are 60° ahead of or behind the planet in its orbit. Such ‘trojan asteroids’ have been found co-orbiting with Earth, Jupiter, Mars, and Neptune. Trojan asteroids associated with Earth are difficult to observe in the visible spectrum, as their libration paths are such that they would be visible primarily in the daylight sky. In 2010, however, using infrared observation techniques, the asteroid 2010 TK7 was found to be a trojan companion of the Earth; it librates around the leading Lagrange point, L4, in a stable orbit.
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- Libration of the Moon from educational website From Stargazers to Starships
- Astronomy Picture of the Day: 2005 November 13 – time-lapse animation of moon through one complete cycle, hosted by NASA
- Libration: 2 years in 2 seconds – 24 full moon pictures taken over two years, compiled in an animation (linked on page) showing the Moon's libration and variations in apparent diameter
- Observing the Lunar Libration Zones