The advance of photography : its history and modern applications (1911)

314 THE ADVANCE OF PHOTOGRAPHY once the angular distance of Venus from this point or m a V. If this angle is substracted from the angle which the line a b makes with the direction of the sun's centre m a b, the remainder will give the angle V a b, which Venus makes with the line a b, which gives all the data necessary to calculate the distance of Venus, and, from that, the distance of the central body which forms the foundation and base of all astronomical measurements. The determination of this angle by photography is of special value, as this measurement can easily be made at any convenient time, whereas direct measurements can only be made while the phenomenon is visible, and hence many errors are introduced in the excitement of the moment. Measurements of this kind require apparatus of the most accurate description, and the adoption of many precautions. Parallax of Eros. — Besides the great disadvantage of the long interval "between transits of Venus across the sun's disc, this method has been considered far less accurate than that of measuring the distance of Mara or one of the brighter minor planets when in opposition. For this purpose too, only one observing station is necessary, and the positions with reference to certain stars can be measured on the photographs, and from this the angle subtended by the earth's radius at the planet (parallax), and so its distance can be determined. Since the discovery of Eros that heavenly body has been made use of in determining the distance of the sun in this way. Photographs were taken during its opposition 1900-1901, and these have been made the basis of calculations for rinding the solar parallax. Some account of the results obtained can be seen in the monthly notices of the Royal Astronomical Society for April 1907. Moon's Position relative to Stars. — Professor Turner l has suggested a method of photographing the moon 1 R.A.S. Notices, Nov. 1903.