Transit
Transit, passage of one heavenly body over the disk of another, as of Mercury or Venus over the disk of the sun, or of a satellite over its primary. A transit of Mercury or Venus can take place only when either planet passes the sun at the time the sun is near one of the nodes of the planet; that is, when Venus or Mercury is in inferior conjunction, or is closer to earth than to the sun. The transit of Venus was first recorded in 1639. In 1679 the English astronomer Edmond Halley pointed out that these transits could be used to determine the distance of the sun. Usually two transits of Venus occur within 8 years of one another; then, after a lapse of 105 or 122 years, another two transits occur within 8 years. Transits of Mercury occur about 13 times in each century. The term transit also refers to an instrument for measuring the passage of an object past the local meridian (see Transit Instrument ). Likewise, the passage of an object past the local meridian is called a transit.