The Approaching Transit of Venus.

Thames Star, Volume XIII, Issue 4344, 2 December 1882, Page 2

The Approaching Transit of Venus.

The astronomical importance of an accurate knowledge of the sun's distance from the earth can hardly be over-esti-mated. With the single exception of the moon, it is the unit in which the distances of the heavenly bodies is expressed. Knowing; the distance of the sun from the earth, then from Kepler's third law the distances of all the planets can be found; and from this will follow the determination of their dimensions, as well as of the orbits of their satellites.

For calculating the sun's distance from earth, several distinct methods are known, one of which is that afforded by the rare, occurrence of the transit of Venus.

Venus is much nearer to the sun than the earth is, and trarels over her whole orbit (360 degrees) in 22247 days. It is easy from this tojcalculate her rate of travelling from any interval of time, say an hour, a minute, &c, This is the basis of Delisle's method, which is the one that will be followed at the coming transit by the English and most of the European astronomers. Observations of the transit of Venus are mainly devoted to the accurate determination of the time of her first internal contact with the edge of the sun's disc, and of her last internal contact—that is when the planet has just entered completely upon the. sun's disc* 1 and when after passing across it, her circular edge just touohes the circular edge of the sun. From these observations, made at different parts of the earth, the parallax of the sun is determined—that is the angle subtended at the sun by the line of distance between the stations of observation. This being known, the sun's distance is easily determined. Without entering into detail, the following sketch will fairly indicate ho w the magnitudeofthatangleis to be determined. Suppose observers at two stations, A. and B, A. being at or about that part of the earth from which the planet can 'earliest be seen in contact with the sua, and B at the longest distance possible from A. from which the same contact can be seen latest. The line between A and B is known, and forms the base of the triangle whose apex is at that point in the sun at which the planet appeared to first touoh his disc. Suppose, then, the observer at A sees the planet just entered upon the sun's disc, and notes the time; some interval after* wards the observer at B sees the plant jast entered upon the suu's disc, and notes the time; and as it is well kown what.time Venua takes to describe Her orbit~of 360 degrees, the portion of it described in the time between the two observation* is calculable by a simple proportion. This is the magnitude of the angle subtended at the sun by the line of distance between the observers, and as the line of distance is known, the distance of the sun from the earth Is easily found. Of course a correction is made on account of the rotation of the earth on its axis during time of transit. rff. The sun will rise at Auckland 4% 7th with a black spot (Venus) on his disc at about one-sixth part of his diameter from the upper edge; at about 729 a.m. thft planet will be about making her final contact with his edge. The sun, Venus, and earth will not be in the same relative position again until the year 2004, that is 121? years hence, at which time it is more than probable that all that is mortal of the present human race will have returned to the. dust of Mother Earth.

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