Another method has been also used in these later centuries. Kepler’s law applies to all the planets. The planet next outside the earth is Mars, whose mean distance from the sun is about one‐third greater than that of the earth. It periodically happens that Mars is in opposition—that is, is precisely on the other side of the earth from the sun. In that case, he makes his nearest approach to our planet. Cannot his distance from the earth be then observed and determined, so that he will give us the means of calculating by Kepler’s formula the distance of the sun? It was tried, and with some success. The base‐line was found large enough; the observations were made at night, when the atmosphere is comparatively quiescent, and when fixed stars may be seen in the vicinity of the planet, to aid in taking the requisite angles. Yet, as in the case of Venus, there are, as we have stated, subsidiary calculations to be made on account of the eccentricity of his orbit and his varying velocity. In the case of Mars, these variations were too full of anomalies to allow confidence in the calculations. When afterwards these anomalies were understood to proceed from interplanetary attraction, they were so complicated that their numerical value almost escaped calculation. The whole subject has been gone over in our own day under the light of more perfect observations, and with the aid of the highest calculus. We doubt, however, if even now the results are sufficiently established to warrant a calculation as to the sun’s distance to which reasonable exception may not be taken. Anyhow, this method cannot compare, either in facility of calculation or in accuracy of result, with the method of determining the solar distance by observations for the transit of Venus. Of the theory and mode of such observations we will now say a few words. In 1677, while Halley, the great English astronomer, was at St. Helena, for the purpose of observing and cataloguing stars south of the equator, he observed a transit of Mercury across the face of the sun, and, from his efforts to measure its positions and movements, was led to believe that a transit of Venus could be so accurately observed and measured as to yield a precise and definite determination of the sun’s distance. From the knowledge he had of the movements of Venus, he knew that there had been a transit of Venus in 1631, as Kepler had predicted, although no eye in Europe had seen it; and another in 1639, which had been observed, but, of course, not for this purpose, which in 1639 was yet unthought of. The next transit would be in 1761. He could not hope to live to see it. But he did the next best thing. He studied out all the conditions of the question, published his plans, and made all the preliminary calculations required, so as to aid in securing, as far as possible, good observations and good results when the time came.