Page:Transactions NZ Institute Volume 16.djvu/32

xxvi at the point where the phase is maximum, the eclipse of the 6th May last, which was to have a duration three times as great, was looked forward to the more eagerly, and scientific parties were sent out from England, France, and America to examine the phenomenon from Flint and Caroline Islands, situated to westward of the Marquesas, and the nearest points of land to the central line. We have learnt, by telegram, that the observations made by them were successful, especially as regards the photographs taken; but it is impossible to discuss the details of the results until further accounts reach us.

Next, as regards the transit of Venus. I need scarcely mention that the object of observing the transit of Venus across the sun's disc is to determine the distance of the sun from the earth;—in the words of Sir George Airey, "the noblest problem in astronomy."

Although, through the sublime discoveries of Copernicus and Kepler, we have a just conception of the order of the solar system and its relative dimensions as expressed in Kepler's Law: that the squares of the periodic times of the planets in their orbits are to each other as the cubes of their distances from the sun; yet we know not with anything like absolute certainty any of these distances. But, in virtue of the law just quoted, given the true distance of any of the planets from the sun or from each other, and we have all the rest. The distance we seek, therefore, is not alone that of the earth from the sun, but in reality the base-line of the universe.

The sun's distance was to the ancient astronomers an insoluble problem, owing to the want of adequate instrumental means. Aristarchus gave the distance as nineteen times that of the moon, which, according to our value of the moon's distance, would give that of the sun under 5,000,000 miles. Even so late as the time of Kepler, not 300 years ago, the estimate of the sun's distance was 13,000,000 miles, or less than one-seventh of what is now accepted.

Indeed, in Kepler's time, the idea of utilizing the transit of Venus, as astronomers now do, was not thought of. It was reserved for the celebrated Scottish philosoper, James Gregory, in 1663, to point out the probability of determining the sun's parallax by means of the transit of Venus.

As is well known, these transits occur in pairs, the first and second of a pair being divided by an interval of only eight years, whilst between one pair and another there are successively intervals of 105½ and 121½ years. Thus, there were transits of Venus in 1631 and 1639; next in 1761 and 1769; the present generation has been specially favoured by having seen the transits of 1874 and 1882; and the next will not take place till 2004 and 2012.