Page:Popular Science Monthly Volume 58.djvu/657

Rh possibility of determining the solar parallax by observations of the transits of Venus, well knew when he developed the methods that he himself could not live to see the experiment tried, since he was then sixty-three years of age, and the next transit of Venus did not come for forty-two years. Perhaps few of the observers who are so enthusiastically at work on Eros at this opposition will be alive to make observations at a really close approach of that interesting body.

At the Paris meeting of the International Astrophotographic Congress, in August, 1900, a committee was appointed to suggest the most favorable course to be pursued. The committee later advised that work be done by the micrometer, the heliometer and by photographs. The observations in each case give the distance of Eros in seconds of arc from adjacent stars. The simplest case is where simultaneous observations are made by observers at widely separated stations. Let A and B (Fig. 3) be two stations on the earth. The observer at A will see Eros projected on the celestial sphere at E1, and the observer at B, at

E2. It is only necessary for each observer to measure the distance in seconds of arc between Eros and some adjacent stars, as 1, 2, 3 and 4. The positions of the stars must be known with the greatest precision, so that the observations give the value of the arc E1E2, which equals the angle AEB. We have then the necessary material for computing the distance of Eros from the earth in miles. Given this and the orbit of Eros, the distances of the earth and all the other planets from the sun in miles follow from the known laws of gravitation. The distance AB may lie in a north and south direction, or in an east and west direction, or more probably in a combination of the two. In the first case there must be two observers, widely separated, as, for example, at Arequipa, Peru, latitude south 16°, and Helsingfors, Finland, north 50°. In the second case there may be two stations, as, one in Europe and the other in the United States, or the whole work may be done at one station by allowing the earth's diurnal motion to carry the observer to a new position. Suppose, for example, that one observation is made when the planet is rising in the