Mercury revolves round the sun in 87 days, 23 hours, 15 minutes, and 44 seconds in an eccentric orbit, so that his distance from that luminary varies from 43,350,000 to 28,570,000 miles. When in superior conjunction the apparent diameter of the planet is 4″.5; at inferior conjunction it is 12″.9, and at elongation 7″. His real diameter is 3,000 miles. Being situated so near to the sun, it is obvious that to an observer on the earth he must always remain in the same general region of the firmament as that body. His orbital motion enables him to successively assume positions to the east and west of the sun, and these are known as his elongations, which vary in distance from 18° to 28°. He becomes visible at these periods either in the morning or evening twilight, and under the best circumstances may remain above the horizon two hours in the absence of the sun. The best times to observe the planet are at his E. elongations during the first half of the year, or at his W. elongations in the last half; for his position at such times being N. of the sun’s place, he remains a long while in view. Occasionally he presents quite a conspicuous aspect on the horizon, as in February, 1868, when I thought his lustre vied with that of Jupiter, and in November, 1882, when he shone brighter than Sirius. The planet is generally most conspicuous _a few mornings after his W. elongations and a few evenings before his E. elongations_. In the course of his orbital round, Mercury exhibits all the phases of the moon. Near his elongations the disk is about half illuminated, and similar in form to that of our satellite when in the first or third quarter. But the phase is not to be distinctly made out unless circumstances are propitious. Galileo’s telescope failed to reveal it, and Hevelius, many years afterward, found it difficult. This is explained by the small diameter of the planet and the rarity with which his disk appears sharply defined. The phase is sometimes noted to be less than theory indicates; for the planet has been seen crescented when he should have presented the form of a semicircle. Several observers have also remarked that his surface displays a rosy tint, and that the terminator is more deeply shaded and indefinite than that of Venus.