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 ASTRONOMY point of water, in order to explain the evaporation of a thin deposit of frost during the Martian summer. Our general conclusions as to the physical condition of the surface of Mars may be summed up as follows: (1) The dark regions on the planet are not oceans, as was formerly supposed, the more probable hypothesis being that the dark colour is simply that of the solid surface of the planet. The amount of heat received from the sun on each unit of surface is less than half that received by the earth, and it is commonly supposed that the surface of our globe is, in the general average, warmer than it would be if our atmosphere were less dense or less rich in aqueous vapour. It is therefore very unlikely that the surface of Mars can be warm enough to allow of the presence of large bodies of water in the liquid state. (2) The so-called canals or channels are probably not the definite features that appear on the drawings, but rather the result of slight suggestions made to the eye by more or less irregular differences in the minute shadings and colour tints on the surface of the planet. Probably no astronomical discovery since that of Neptune has so surprised the astronomical world as the announcement, made in August 1877, that ProSateiutes fessor Asaph Hall had discovered two satellites °t ars. Mars. The most remarkable feature of these

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approach of the two orbits, though this could not always have been the case in the past, nor will it be the case in the future. One result of the secular variations must be that the two orbits will, at some period, actually intersect, and near this time Mars and Eros will approach so close to one another that the orbit of the latter may be completely changed. Not unlikely this was the case at some unknown epoch in the past, previous to which the orbit of Eros may have been very different from what it is now. Approximate elements of this body referred to the equinox of 1900 are these :— Epoch, 1898, August 31, 5 G.M. T. Mean long, at epoch .... 342° 48 221° 37' ,, anom. ,, 121° 11' Long, of perihelion 303° 31' node 10° 50' Inclination. 12° 52' Angle of eccentricity 2015-2326" Daily mean motion 0-163798" Log. mean distance 0-22275" Eccentricity From the mean distance and eccentricity it follows that the least distance1 of the planet from the sun is IT333 ; the greatest, 1'7829. At the descending node on the ecliptic its distance from the earth’s orbit is only 0-149, a distance much less than that of any other known planet. The earth passes this point about 24th January of each year; but it is only on rare occasions that the planet will be very near the corresponding part of its orbit at this date. A tantalizing fact for the astronomer is that such a passage occurred about 24th January 1894, before the planet was recognized, and that so near an approach will not occur again until 1975. Oppositions not much less favourable than that of 1894 will occur in 1931 and 1938. The epochs of these nearest approaches can be utilized for more exact measures of the solar parallax than have yet been made. Seven years before and after each most favourable opposition another occurs at which, though less favourable, the planet comes so near as to offer an excellent opportunity for the measures in question. A study of the distribution of the mean motions of these bodies shows a singular feature, apparently connected with their origin, on which it may throw some light. About 1870 it was Qistribunoticed by Kirkwood that when these mean motions plon Qf are arranged in the order of their magnitude, gaps are meaa found in the series corresponding to the points of com- mofjoaSm mensurability with the mean motion of Jupiter ; that is to say, the period of Jupiter being in round numbers twelve Distribution of 400 Minor Planets into Groups, according to Mean Motions and Mean Distances. No. of Commens. Limits of Mean Limits of Mean Breadth with of Zone. Planets. Jupiter. Distance. Motion.

bodies is their minuteness, their estimated diameter being only 8 or 10 miles, and they are the smallest known bodies in our system, except possibly a few of the more recently discovered minor planets. Among satellites the inner one is unique, in that its time of revolution round the planet is less than the Martian day, being in fact less than nine hours. The result is that to an observer on the planet it would rise in the west and set in the east. They are, of course, difficult objects to observe, especially at those oppositions when the planet is farthest from the sun; but at the favourable oppositions, when Mars is near its perihelion, they may easily be seen with a telescope of 12 inches’ aperture or upwards, if the observer takes the precaution to cut off the light of Mars from his field of vision. Particulars relating to the elements and motions of these bodies are given on pages 744-5. The number of known member’s of the group of minor planets now approaches 500, and is increasing so rapidly that no trustworthy estimate of its total can yet be Minor planets. made. The mean opposition magnitude of those most recently discovered generally lies between the 11th and 13th, and sometimes is of the 14th or 15th. New discoveries are now made entirely by photography. A sensitized plate is exposed in the focus of an equatorial photographic telescope, moved by clockwork so as to follow the stars in their diurnal motion, and on this when developed the stars appear as minute dark dots. But if the image of a planet is imprinted on the plate, it will 31 Below 2 "33 Exceed 1000 64 •15 2-33 to 2-48 910 to 1000 appear as a short line, owing to the combined effect of its 3:1 6 •07 2-48 2-55 870 ,, 910 orbital motion and that of the earth.. The ease with 79 •17 2-55 2-72 790 ,, 870 which such a plate can be examined under a microscope, 65 •07 2-72 2-79 760 ,, 790 renders this method much simpler and more expeditious 9 •03 2-79 2-82 750 „ 760 5:2 •02 1 2-82 2-84 than the old one of searching for the planet visually. 740 „ 750 29 •08 2-84 2-92 710 ,, 740 One of the bodies recently added to the list is, in the 7:3 5 •06 2-92 2690 ,, 710 character of its orbit, quite unique. The orbits of all 96 •26 23610 ,, 690 the minor planets known up to 1898 were situated 2 :1 0 •10 3-34 3580 „ 610 10 T8 3-34 3-52 between those of Mars and Jupiter, but in August of that 540 ,, 580 5:3 0 •36 3-52 460 ,, 540 year, Witt, of Berlin, found a planet which at perihelion 5 •40 3-88 28 400 ,, 460 comes considerably within the orbit of Mars. The name of Eros has been assigned to it. A remarkable feature is that the two orbits, those of Eros and of Mars, are inter- years, there are few or none of these bodies which have periods of one-third, or two-fifths of twelve years. If there were any linked in such a way that if both were material they one-half, of the group having any of these periods, it is known that the would pass through each other like two links in a chain; 1 Distances among the planets are expressed in radii of the earth s but the inclination of the orbits and the large eccentricity of Eros are such that, at present, there is no very near orbit.