Chapter II: The Inner Group of Planets

Mercury.

216. _The Orbit of Mercury._--The orbit of Mercury is more eccentric than that of any of the larger planets, and it has also a greater inclination to the ecliptic. Its eccentricity (43) is a little over a fifth, and its inclination to the ecliptic somewhat over seven degrees. The mean distance of Mercury from the sun is about thirty-five million miles; but, owing to the great eccentricity of its orbit, its distance from the sun varies from about forty-three million miles at aphelion to about twenty-eight million at perihelion.

217. _Distance of Mercury from the Earth._--It is evident, from Fig. 251, that an inferior planet, like Mercury, is the whole diameter of its orbit nearer the earth at inferior conjunction than at superior conjunction: hence Mercury's distance from the earth varies considerably. Owing to the great eccentricity of its orbit, its distance from the earth at inferior conjunction also varies considerably. Mercury is nearest to the earth when its inferior conjunction occurs at its own aphelion and at the earth's perihelion.

218. _Apparent Size of Mercury._--Since Mercury's distance from the earth is variable, the apparent size of the planet is also variable. Fig. 252 shows its apparent size at its extreme and mean distances from the earth. Its apparent diameter varies from five seconds to twelve seconds.

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.

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.

Schröter, a celebrated German astronomer, at about the beginning of the present century, thought that he detected spots and shadings on the disk of the planet, which indicated both the presence of an atmosphere and of elevations. The shading along the terminator, which seemed to indicate the presence of a twilight, and therefore of an atmosphere, are shown in Fig. 256. It also shows the blunted aspect of one of the cusps, which Schröter noticed at times, and which he attributed to the shadow of a mountain, estimated to be ten or twelve miles high. Fig. 257 shows this mountain near the upper cusp, as Schröter believed he saw it in the year 1800. By watching certain marks upon the disk of Mercury, Schröter came to the conclusion that the planet rotates on its axis in about twenty-four hours. Modern observers, with more powerful telescopes, have failed to verify Schröter's observations as to the indications of an atmosphere and of elevations. Nothing is known with certainty about the rotation of the planet.

The border around Mercury, and the bright spot on its disk at the time of the transit of the planet across the sun, have been seen since Schröter's time, and the existence of these phenomena is now well established; but astronomers are far from being agreed as to their cause.

224. _Intra-Mercurial Planets._--It has for some time been thought probable that there is a group of small planets between Mercury and the sun; and at various times the discovery of such bodies has been announced. In 1859 a French observer believed that he had detected an intra-Mercurial planet, to which the name of _Vulcan_ was given, and for which careful search has since been made, but without success. During the total eclipse of 1878 Professor Watson observed two objects near the sun, which he thought to be planets; but this is still matter of controversy.

Venus.

225. _The Orbit of Venus._--The orbit of Venus has but slight eccentricity, differing less from a circle than that of any other large planet. It is inclined to the ecliptic somewhat more than three degrees. The mean distance of the planet from the sun is about sixty-seven million miles.

226. _Distance of Venus from the Earth._--The distance of Venus from the earth varies within much wider limits than that of Mercury. When Venus is at inferior conjunction, her distance from the earth is ninety-two million miles _minus_ sixty-seven million miles, or twenty-five million miles; and when at superior conjunction it is ninety-two million miles _plus_ sixty-seven million miles, or a hundred and fifty-nine million miles. Venus is considerably more than _six times_ as far off at superior conjunction as at inferior conjunction.

227. _Apparent Size of Venus._--Owing to the great variation in the distance of Venus from the earth, her apparent diameter varies from about ten seconds to about sixty-six seconds. Fig. 258 shows the apparent size of Venus at her extreme and mean distances from the earth.

228. _Volume and Density of Venus._--The real size of Venus is about the same as that of the earth, her diameter being only about three hundred miles less. The comparative sizes of the two planets are shown in Fig. 259. The density of Venus is a little less than that of the earth.

229. _The Greatest Elongation of Venus._--Venus, like Mercury, appears to oscillate to and fro across the sun. The angular value of the greatest elongation of Venus varies but slightly, its greatest value being about forty-seven degrees.

230. _Sidereal and Synodical Periods of Venus._--The _sidereal_ period of Venus, or that of a complete revolution around the sun, is about two hundred and twenty-five days; her orbital motion being at the rate of nearly twenty-two miles a second. Her _synodical_ period, or the time it takes her to pass around from her greatest eastern elongation to the same elongation again, is about five hundred and eighty-four days, or eighteen months. Venus is a hundred and forty-six days, or nearly five months, in passing from her greatest elongation east through inferior conjunction to her greatest elongation west.

231. _Venus as a Morning and an Evening Star._--For a period of about nine months, while Venus is passing from superior conjunction to her greatest eastern elongation, she will be east of the sun, and will therefore set after the sun. During this period she is the _evening star_, the _Hesperus_ of the ancients. While passing from inferior conjunction to superior conjunction, Venus is west of the sun, and therefore rises before the sun. During this period of nine months she is the _morning star_, the _Phosphorus_, or _Lucifer_, of the ancients.

232. _Brilliancy of Venus._--Next to the sun and moon, Venus is at times the most brilliant object in the heavens, being bright enough to be seen in daylight, and to cast a distinct shadow at night. Her brightness, however, varies considerably, owing to her phases and to her varying distance from the earth. She does not appear brightest when at full, for she is then farthest from the earth, at superior conjunction; nor does she appear brightest when nearest the earth, at inferior conjunction, for her phase is then a thin crescent (see Fig. 258). She is most conspicuous while passing from her greatest eastern elongation to her greatest western elongation. After she has passed her eastern elongation, she becomes brighter and brighter till she is within about forty degrees of the sun. Her phase at this point in her orbit is shown in Fig. 260. Her brilliancy then begins to wane, until she comes too near the sun to be visible. When she re-appears on the west of the sun, she again becomes more brilliant; and her brilliancy increases till she is about forty degrees from the sun, when she is again at her brightest. Venus passes from her greatest brilliancy as an evening star to her greatest brilliancy as a morning star in about seventy-three days. She has the same phase, and is at the same distance from the earth, in both cases of maximum brilliancy. Of course, the brilliancy of Venus when at the maximum varies somewhat from time to time, owing to the eccentricities of the orbits of the earth and of Venus, which cause her distance from the earth, at her phase of greatest brilliancy, to vary. She is most brilliant when the phase of her greatest brilliancy occurs when she is at her aphelion and the earth at its perihelion.

233. _The Atmosphere and Surface of Venus._--Schröter believed that he saw shadings and markings on Venus similar to those on Mercury, indicating the presence of an atmosphere and of elevations on the surface of the planet. Fig. 261 represents the surface of Venus as it appeared to this astronomer. By watching certain markings on the disk of Venus, Schröter came to the conclusion that Venus rotates on her axis in about twenty-four hours.

It is now generally conceded that Venus has a dense atmosphere; but Schröter's observations of the spots on her disk have not been verified by modern astronomers, and we really know nothing certainly of her rotation.

234. _Transits of Venus._--When Venus happens to be near one of the nodes of her orbit when she is in inferior conjunction, she makes a transit across the sun's disk. These transits occur in pairs, separated by an interval of over a hundred years. The two transits of each pair are separated by an interval of eight years, the dates of the most recent being 1874 and 1882. Venus, like Mercury, appears surrounded with a border on passing across the sun's disk, as shown in Fig. 262.

Mars.

235. _The Orbit of Mars._--The orbit of Mars is more eccentric than that of any of the larger planets, except Mercury; its eccentricity being about one-eleventh. The inclination of the orbit to the ecliptic is somewhat under two degrees. The mean distance of Mars from the sun is about a hundred and forty million miles; but, owing to the eccentricity of his orbit, the distance varies from a hundred and fifty-three million miles to a hundred and twenty-seven million miles.

236. _Distance of Mars from the Earth._--It will be seen, from Fig. 263, that a _superior_ planet (or one outside the orbit of the earth), like Mars, is nearer the earth, by the whole diameter of the earth's orbit, when in opposition than when in conjunction. The mean distance of Mars from the earth, at the time of opposition, is a hundred and forty million miles _minus_ ninety-two million miles, or forty-eight million miles. Owing to the eccentricity of the orbit of the earth and of Mars, the distance of this planet when in opposition varies considerably. When the earth is in aphelion, and Mars in perihelion, at the time of opposition, the distance of the planet from the earth is only about thirty-three million miles. On the other hand, when the earth is in perihelion, and Mars in aphelion, at the time of opposition, the distance of the planet is over sixty-two million miles.

The mean distance of Mars from the earth when in conjunction is a hundred and forty million miles _plus_ ninety-two million miles, or two hundred and thirty-two million miles. It will therefore be seen that Mars is nearly five times as far off at conjunction as at opposition.

237. _The Apparent Size of Mars._--Owing to the varying distance of Mars from the earth, the apparent size of the planet varies almost as much as that of Venus. Fig. 264 shows the apparent size of Mars at its extreme and mean distances from the earth. The apparent diameter varies from about four seconds to about thirty seconds.

238. _The Volume and Density of Mars._--Among the larger planets Mars is next in size to Mercury. Its real diameter is somewhat more than four thousand miles, and its bulk is about one-seventh of that of the earth. Its size, compared with that of the earth, is shown in Fig. 265.

The density of Mars is only about three-fourths of that of the earth.

239. _Sidereal and Synodical Periods of Mars._--The _sidereal_ period of Mars, or the time in which he makes a complete revolution around the sun, is about six hundred and eighty-seven days, or nearly twenty-three months; but he is about seven hundred and eighty days in passing from opposition to opposition again, or in performing a _synodical_ revolution. Mars moves in his orbit at the rate of about fifteen miles a second.

240. _Brilliancy of Mars._--When near his opposition, Mars is easily recognized with the naked eye by his fiery-red light. He is much more brilliant at some oppositions than at others, for reasons already explained (236), but always shines brighter than an ordinary star of the first magnitude.

241. _Telescopic Appearance of Mars._--When viewed with a good telescope (see Plate IV.), Mars is seen to be covered with dusky, dull-red patches, which are supposed to be continents, like those of our own globe. Other portions, of a greenish hue, are believed to be tracts of water. The ruddy color, which overpowers the green, and makes the whole planet seem red to the naked eye, was believed by Sir J. Herschel to be due to an ochrey tinge in the general soil, like that of the red sandstone districts on the earth. In a telescope, Mars appears less red, and the higher the power the less the intensity of the color. The disk, when well seen, is mapped out in a way which gives at once the impression of land and water. The bright part is red inclining to orange, sometimes dotted with brown and greenish points. The darker spaces, which vary greatly in depth of tone, are of a dull gray-green, having the aspect of a fluid which absorbs the solar rays. The proportion of land to water on the earth appears to be reversed on Mars. On the earth every continent is an island; on Mars all seas are lakes. Long, narrow straits are more common than on the earth; and wide expanses of water, like our Atlantic Ocean, are rare. (See Fig. 266.)

Fig. 267 represents a chart of the surface of Mars, which has been constructed from careful telescopic observation. The outlines, as seen in the telescope, are, however, much less distinct than they are represented here; and it is by no means certain that the light and dark portions are bodies of land and water.

In the vicinity of the poles brilliant white spots may be noticed, which are considered by many astronomers to be masses of snow. This conjecture is favored by the fact that they appear to diminish under the sun's influence at the beginning of the Martial summer, and to increase again on the approach of winter.

242. _Rotation of Mars._--On watching Mars with a telescope, the spots on the disk are found to move (as shown in Fig. 268) in a manner which indicates that the planet rotates in about twenty-four hours on an axis inclined about twenty-eight degrees from a perpendicular to the plane of its orbit. The inclination of the axis is shown in Fig. 269. It is evident from the figure that the variation in the length of day and night, and the change of seasons, are about the same on Mars as on the earth. The changes will, of course, be somewhat greater, and the seasons will be about twice as long.

243. _The Satellites of Mars._--In 1877 Professor Hall of the Washington Observatory discovered that Mars is accompanied by two small moons, whose orbits are shown in Fig. 270. The inner satellite has been named _Phobos_, and the outer one _Deimos_. It is estimated that the diameter of the outer moon is from five to ten miles, and that of the inner one from ten to forty miles.

Phobos is remarkable for its nearness to the planet and the rapidity of its revolution, which is performed in seven hours thirty-eight minutes. Its distance from the centre of the planet is about six thousand miles, and from the surface less than four thousand. Astronomers on Mars, with telescopes and eyes like ours, could readily find out whether this satellite is inhabited, the distance being less than one-sixtieth of that of our moon.

It will be seen that Phobos makes about three revolutions to one rotation of the planet. It will, of course, rise in the west; though the sun, the stars, and the other satellite rise in the east. Deimos makes a complete revolution in about thirty hours.