The next step is to determine the size of the solar system in terms of that of the earth. When the sun is eclipsed by the moon, the time at which the moon first begins to cover the sun’s disc is different for different stations on the earth’s surface, and the observed differences of time enable us to measure the moon’s distance in terms of known distances on the surface of the earth. In this way the mean distance of the moon is found to be 384,403 kilometres or 238,857 miles. In the same way the transit of the planet Venus across the disc of the sun provides an opportunity for determining the scale of the solar system in terms of the dimensions of the earth. The asteroid Eros provides still better opportunities. The Paris Conference (1911) adopted 149,450,000 kilometres, or 92,870,000 miles, as the most likely value for the mean distance of the earth from the sun. The next and final step, which the year 1838 saw accomplished, is that of using the diameter of the earth’s orbit as base-line, and determining the distances of the stars. The first step, from the standard yard or metre to the measured base-line on the earth’s surface, involves an increase of several thousand-fold in length. The increase involved in the next step, from the base-line to the earth’s diameter, is again one of thousands. And again the next step, from the diameter of the earth to that of the earth’s orbit, involves an increase of thousands. But the last step of all, from the earth’s orbit to stellar distances, involves a million-fold increase. Recent measurements shew that the nearest stars are at almost exactly a million times the distances of the nearest planets. At its nearest approach to the earth, Venus is 26 million miles distant, while the nearest star, Proxima Centauri, is 25,000,000 million miles away; this latter star is a faint companion of the well-known bright star α Centauri in the southern hemisphere. The distances of the planets when at their nearest, and of the nearest stars, are shewn in the following table: