When any celestial object is concealed by the interposition of another, it is said to be occulted, and the phenomenon is called an occultation. Strictly speaking, an eclipse of the Sun is an occultation of that luminary by the Moon, but usage has given to it the exceptional name of “ eclipse.” The most important phenomena of this kind are the occultations of the planets and larger stars by the Moon, but the occultation of one planet by another, on account of the rarity of such an occurrence is exceedingly interesting. Inasmuch as the Moon's apparent diameter is about ½°, it follows that all stars and planets situated in a zone extending ¼° on each side of her path, will necessarily be occulted during her monthly course through the ecliptic. The great brilliancy of the Moon entirely overpowers the smaller stars, but the disappearance of the more conspicuous ones are visible in a telescope, a table of which is inserted every year in the Nautical Almanac.

It must be remembered, that the disappearance always takes place at that limb of the Moon which is presented in the direction of its motion.