Rest and exercise, however, temporarily restored his health, and he gave proof of the undiminished vigour of his powers by carrying off, in 1764, the prize offered by the Paris Academy of Sciences for the best essay on the **libration** of the moon.

His first important astronomical work was a careful investigation of the **libration** of the moon (Kosmographische Nachrichten, Nuremberg, 1750), and his chart of the full moon (published in 1775) was unsurpassed for half a century.

The most important of these are his memoirs "Sur les inegalites seculaires des moyens mouvements des planetes," "Sur la variation des constantes arbitraires dans les questions de mecanique," both published in the Journal of the Ecole Polytechnique (1809); "Sur la **libration** de la lune," in Connaiss.

By De la Rue's advice, Pritchard began his career there with a determination of the physical **libration** of the moon, or the nutation of its axis.

This apparent oscillation is called **libration**, and its amount on each side of the mean is commonly between 6° and 7°.

There is also a **libration** in latitude, arising from the fact that the axis of rotation of the moon is not precisely perpendicular to the plane of her orbit.

This **libration** is more regular than that in longitude, its amount being about 60° 44' on each side of the mean.

The other side of the moon is therefore invisible from the earth, but in consequence of the **libration** about six-tenths of the lunar surface may be seen at one time or another, while the remaining four-tenths are for ever hidden from our view.