Libration
Encyclopedia
In astronomy, libration is an oscillating motion of orbit
ing bodies relative to each other, notably including the motion of the Moon
relative to Earth
, or of Trojan asteroids relative to planets.
facing the Earth
, due to tidal locking
. Therefore, humans' first view of the far side of the Moon
resulted from lunar exploration
in the 1960s. 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.
There are three types of lunar libration:
'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 2011, 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, , in a stable orbit.
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
ing bodies relative to each other, notably including the motion of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
relative to Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, or of Trojan asteroids relative to planets.
Lunar libration
The Moon generally has one hemisphereHemisphere
Hemisphere may refer to:*Half of a sphereAs half of the Earth:*Any half of the Earth, see Hemispheres of the Earth, see:...
facing the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, due to tidal locking
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
. Therefore, humans' first view of the far side of the Moon
Far side of the Moon
The far side of the Moon is the lunar hemisphere that is permanently turned away, and is not visible from the surface of the Earth. The far hemisphere was first photographed by the Soviet Luna 3 probe in 1959, and was first directly observed by human eyes when the Apollo 8 mission orbited the Moon...
resulted from lunar exploration
Exploration of the Moon
The physical exploration of the Moon began when Luna 2, a space probe launched by the Soviet Union, made an impact on the surface of the Moon on September 14, 1959. Prior to that the only available means of exploration had been observation. The invention of the optical telescope brought about the...
in the 1960s. 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.
There are three types of lunar libration:
- Libration in longitudeLongitudeLongitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
results from the eccentricityOrbital eccentricityThe orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
of the Moon's orbit around Earth; the Moon's rotation sometimes leads and sometimes lags its orbital position. - Libration in latitudeLatitudeIn geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
results from a slight inclination between the Moon's axis of rotationRotation around a fixed axisRotation around a fixed axis is a special case of rotational motion. The fixed axis hypothesis exclude the possibility of a moving axis, and cannot describe such phenomena as wobbling or precession. According to Euler's rotation theorem, simultaneous rotation around more than one axis at the same...
and the normalSurface normalA surface normal, or simply normal, to a flat surface is a vector that is perpendicular to that surface. A normal to a non-flat surface at a point P on the surface is a vector perpendicular to the tangent plane to that surface at P. The word "normal" is also used as an adjective: a line normal to a...
to the plane of its orbitOrbitIn physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
around Earth. Its origin is analogous to how the seasonSeasonA season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...
s arise from Earth's revolutionRevolutionA revolution is a fundamental change in power or organizational structures that takes place in a relatively short period of time.Aristotle described two types of political revolution:...
about the Sun. - DiurnalDiurnal motionDiurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for...
libration is a small daily oscillation due to the 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—because the observer is on the surface of the Earth, not at its center.
Trojan libration
In 1772 LagrangeLagrange
La Grange literally means the barn in French. Lagrange may refer to:- People :* Charles Varlet de La Grange , French actor* Georges Lagrange , translator to and writer in Esperanto...
'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
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
. Trojan asteroids associated with Earth are difficult to observe in the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
, as their libration paths are such that they would be visible primarily in the daylight sky. In 2011, however, using infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
observation techniques, the asteroid 2010 TK7
2010 TK7
2010 TK7 is the first Earth trojan asteroid to be discovered; it precedes Earth in its orbit around the Sun. Trojan objects are most easily conceived as orbiting at a Lagrangian point, a dynamically stable location 60 degrees ahead of or behind a massive orbiting body, in a type of 1:1 orbital...
was found to be a Trojan companion of the Earth; it librates around the leading Lagrange point, , in a stable orbit.
External links
- 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 diameter
- National Maritime Museum: The Moon's Movement – discussion of the components of the Moon's libration with diagrams
- Observing the Lunar Libration Zones