219. _Volume and Density of Mercury._--The real diameter of Mercury is about three thousand miles. Its size, compared with that of the earth, is shown in Fig. 253. The earth is about sixteen times as large as Mercury; but Mercury is about one-fifth more dense than the earth. 220. _Greatest Elongation of Mercury._--Mercury, being an _inferior_ planet (or one within the orbit of the earth), appears to oscillate to and fro across the sun. Its greatest apparent distance from the sun, or its _greatest elongation_, varies considerably. The farther Mercury is from the sun, and the nearer the earth is to Mercury, the greater is its angular distance from the sun at the time of its greatest elongation. Under the most favorable circumstances, the greatest elongation amounts to about twenty-eight degrees, and under the least favorable to only sixteen or seventeen degrees. 221. _Sidereal and Synodical Periods of Mercury._--Mercury accomplishes a complete revolution around the sun in about eighty-eight days; but it takes it a hundred and sixteen days to pass from its greatest elongation east to the same elongation again. The orbital motion of this planet is at the rate of nearly thirty miles a second. In Fig. 251, _P'''_ represents elongation east of the sun, and _P'_ elongation west. It will be seen that it is much farther from _P'_ around to _P'''_ than from _P'''_ on to _P'_. Mercury is only about forty-eight days in passing from greatest elongation east to greatest elongation west, while it is about sixty-eight days in passing back again. 222. _Visibility of Mercury._--Mercury is too close to the sun for favorable observation. It is never seen long after sunset, or long before sunrise, and never far from the horizon. When visible at all, it must be sought for low down in the west shortly after sunset, or low in the east shortly before sunrise, according as the planet is at its east or west elongation. It is often visible to the naked eye in our latitude; but the illumination of the twilight sky, and the excess of vapor in our atmosphere near the horizon, combine to make the telescopic study of the planet difficult and unsatisfactory. [Illustration: Fig. 254.] 223. _The Atmosphere and Surface of Mercury._--Mercury seems to be surrounded by a dense atmosphere. One proof of the existence of such an atmosphere is furnished at the time of the planet's _transit_ across the disk of the sun, which occasionally happens. The planet is then seen surrounded by a border, as shown in Fig. 254. A bright spot has also been observed on the dark disk of the planet during a transit, as shown in Fig. 255. The border around the planet seems to be due to the action of the planet's atmosphere; but it is difficult to account for the bright spot. [Illustration: Fig. 255.] [Illustration: Fig. 256.]