TRANSIT, in astronomy, the passage of a heavenly body over the meridian. Also the passage of one of the inferior planets, Mercury or Venus, over the sun's disk, Mercury being so near the sun, and so difficult to observe with accuracy, its transits are not nearly so important to astronomers as those of Venus. In 1716 Dr. Halley published a paper in the "Philosophical Transactions," advising that the transits of Venus over the sun's disk which would occur in A. D. 1761 and 1769 should be taken advantage of for the purpose of ascertaining the sun's distance from the earth. Though he was dead long before these dates arrived, the government of the day acted on his sug gestion. In 1769 the celebrated Captain Cook was sent to Otaheite for the pur pose of noting the transit, another ob server being dispatched to Lapland. The observations of the latter being erroneous the distance of the sun was exaggerated by about three millions of miles. In 1874, when the next transit occurred, all civil ized nations sent out scientific men to observe it. It was known that it would be invisible at Greenwich, but expeditions were sent out by the British Government to the Sandwich Islands, to New Zealand, Egypt, Rodriguez, and Kerguelen Island. Other nations occupied other stations, and the weather proved suitable at most places for accurate observation.
Transits of Venus come, after long in tervals, in pairs, eight years apart; and another transit took place on the after noon of Dec. 6, 1882. In the British Isles the weather was generally unfavorable, clouds with occasional snowflakes obscur ing the sky at Greenwich, and through nearly all Great Britain, except on the W. coast. At Dublin, partial observa
tions were obtained; and of various Brit ish expeditions sent abroad complete suc cess was obtained in Madagascar and at the Cape of Good Hope. Observers from the United States and other countries were also successful. The observation of the distance the planet moves to the right and left of the sun, in describing its orbit, enables an astronomer to ascertain the relative distance of the two luminaries. The relative breadth of the sun's diame ter as compared with his distance from the earth is also easily ascertained. If, then, two observers on the surface of our sphere take their stations at ju diciously selected points, as widely apart as possible, and note a transit of Venus, the planet will have a lesser line to traverse at the one place than the other, and will do it in a shorter time. From accurate notation of the difference in time taken in connection with the dif ference in length it is possible to calcu late, first the breadth of the sun and secondly his distance from the earth. When the materials obtained in connec tion with the two transits were worked out, it was found, as Hansen had sus pected, that the sun's distance had been over-estimated, and it was reduced from 95,300,000 to 92,700,000 miles.
In engineering, a portable instrument resembling a theodolite, designed for measuring both horizontal and vertical angles. It is provided with horizontal and vertical graduated circles, one or two levels, and a compass, and is mounted upon a tripod-stand.