A large observatory transit telescope, with an astronomer recording the transit of Venus in the Hartwell Observatory Transit Room. By measuring the precise time (note the clock) that a star or planet passes the meridian (south), the object's celestial "longitude," called right ascension, can be measured. Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. Only eight such events have occurred since the invention of the telescope. A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth (or another planet), becoming visible against the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun. Venus transits are historically of great scientific importance as they were used to gain the first realistic estimates of the size of the Solar System.