Moon, in astronomy, one of the heavenly bodies, usually, though erroneously, classed among the planets ; for it is with more propriety considered as a satellite, or secondary planet.

The mean distance of the moon from the earth;is computed by astronomers, to be about 240,000 miles: her diameter is as 100 to365, that is, about 2180 miles.

The moon appears to us under | various forms; being sometimes horned, at others semi-circular then full and round. Sometimes this celestial body increases ; at others, it is on the wane ; illumining the globe, we inhabit, at certain periods throughout the whole night, at others only for a few hours.-The cause of these appearances is believed to arise from the moon being an opaque dark body, which shines only with the light she receives from the sun ; hence only the part turned towards that luminary can be illuminated, and consequently transmit light; as the other side is involved in its native darkness. These phenomena, however, it is not within our province to explain ; and. as the inquisitive reader wi!l doubtless consult the profound writings of MaskelynE, HerschEl, and other astronomers, who have devoted their sleepless nights to the observation of this satellite, we shall not enter into any farther details.- See also AstroNOMY.

Formerly, the moon was believed to possess considerable influence on the human body; but such opinion, though sanctioned by the name of Mead, an 1 other eminent physi-cians, is now generally exploded. This luminary, however, greatly influences the vegetable creation, and likewise appears to affect quadrupeds, especially horses. It is well known, that these useful animals are subject to a weakness of the eyes (particularly when they are somewhat advanced in years) ; and that such debility increases, or decreases, according to the course of the moon; whence they are said to have moon-eyes. Thus, in the wane, they are dull and muddy, or troubled but, on the approach of the new moon, they re-assume their former lustre- No remedy has hitherto been discovered for this affection, which generally terminates in total blindness.