PHASES

, in Astronomy, the various appearances, or quantities of illumination of the moon, Venus, Mercury, and the other planets, by the sun. These Phases are very observable in the moon with the naked eye; by which she sometimes increases, sometimes wanes, is now bent into horns, and again appears a half circlé; at other times she is gibbous, and again a full circular face. And by help of the telescope, the like yariety of Phases is observed in Venus, Mars, &c.

Copernicus, a little before the use of telescopes, foretold, that after ages would find that Venus underwent all the changes of the moon; which prophecy was first fulsilled by Galileo, who, directing his telescope to Venus, observed her Phases to emulate those of the moon; being sometimes full, sometimes horned, and sometimes gibbous.

Phases of an Eclipse. To determine these for any time: Find the moon's place in her visible way for that moment; and from that point as a centre, with the interval of the moon's semidiameter, describe a circle: In like manner find the sun's place in the ecliptic, from which, with the semidiameter of the sun, describe another circle: The intersection of the two circles shews the Phases of the eclipse, the quantity of obscuration, and the position of the cusps or horns.