Life in the Universe

whose orbits they did not perturb, even during close encounters. He was the first to prove the planetary nature of Uranus (it was originally suspected to be a comet).

Lexell, Comet (D/1770 L1) First short-period comet on record. Discovered by Charles messier, it is one of the best-known cases of a cometary orbit having been altered considerably after being perturbed by the gravitational pull of the massive planet Jupiter. In 1770 this comet, in its approach towards perihelion, passed between the satellites of Jupiter. Later, on 1770 July 1, it came within 2.5 million km (1.6 million mi) of the Earth. As it passed by, the transient gravitational grasp of the Earth caused a decrease in the comet's period by almost three days, but the change in the period of the far more massive Earth was so small as to be immeasurable. The orbit of the comet was investigated by Anders lexell, who found it to have a period of 5.6 years. He showed that the comet had been highly perturbed by Jupiter in 1767 May, when its orbit had been changed from a much larger ellipse to its present shape, which explained why it had never been seen previously. The comet was never seen again after 1770. The reason for this was provided by Pierre laplace, who showed that a second close approach to Jupiter had occurred in 1779. On this occasion it had passed so close to Jupiter that its orbit was altered dramatically, with large changes in its orbital period and perihelion distance.

Libra See feature article libration Small oscillation of a celestial body about its mean position. The term is used most frequently to mean the Moon's libration. As a result of libration it is possible to see, at different times, 59% of the Moon's surface. However, the areas that pass into and out of view are close to the limb and therefore extremely foreshortened, so in practice libration has less of an effect on the features that can be clearly made out on the Moon's disk than this figure might suggest.

Physical libration results from slight irregularities in the Moon's motion produced by irregularities in its shape. Much more obvious is geometrical libration, which results from the Earth-based observer seeing the Moon from different directions at different times. There are three types of geometrical libration. Libration in longitude arises from a combination of the Moon's synchronous rotation and its elliptical orbit. As a result, at times a little more of the lunar surface is visible at the eastern or western limb than when the Moon is at its mean position. Libration in latitude arises because the Moon's equator is tilted slightly from its orbital plane, so that the two poles tilt alternately towards and away from the Earth. A smaller effect is diurnal libration, by which the Earth's rotation allows us to see more of the Moon's surface at its western limb when it is rising, and more at the eastern limb when it is setting.

Lick Observatory Major optical observatory, and the first in the United States to be built on a mountain-top site. Its telescopes are on Mount Hamilton, about 32 km (20 mi) east of San Jose, California, at an altitude of 1280 m (4200 ft). Its first director was Edward S. holden. The observatory was the result of a bequest to the University of California by an eccentric millionaire, James Lick (1796-1876), whose body lies beneath the pier of the 0.9-m (36-in.) Lick Refractor. Inaugurated in 1888, the telescope was for nine years the largest in the world, and it remains the world's second-largest refractor. With it, Edward E. barnard discovered Amalthea in 1892, the first Jovian satellite to be found since 1610. The front element of its objective lens was refigured in 1987, having suffered long-term damage from atmospheric corrosion.

Several other telescopes are on Mount Hamilton, including the Crossley 0.9-m (36-in.) reflector, once owned by Andrew common, and the 1-m (39-in.) Nickel Reflector, built in 1983. The largest telescope at Lick is

Libration in longitude

Libration in longitude

Libration in longitude

north pole of Moon

Moon's equator

plane of Moon's orbit

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