sun, and as all the planets and planetoids are attendant upon the sun, they form a common system, the solar system. The facts regarding their size, their distance from the sun, and the inclination and position of the planes of their orbits with respect to the orbit of the earth are tabulated as follows: Notable Planetary Relations. That the orderly nature of this system implies some mode of evolution was seen by the framers of the nebular hypothesis. The more notable of these relations, following the summary by Young, are: I. The orbits are all nearly circular. 2. They are nearly in one plane (excepting the cases of some of the little planetoids). 3. The revolution of all is in the same direction. 4. There is a curiously regular progression of distances from the sun (expressed by Bode's law, which, however, breaks down at Neptune). See foregoing table. 5. There is a rough progression of density, increasing both ways from Saturn, the least dense of all the planets in the system. As regards the planets themselves, we have: 6. The plane of the planets' rotation, the plane of their equators, roughly coinciding with those of the orbits (probably excepting Uranus). 7. The direction of the rotation of the planets about their polar axes the same as that of their revolution in their orbits (excepting probably Uranus and Neptune). 8. The plane of orbital revolution of the satellites of each planet coinciding nearly with that of the planet's rotation, its equatorial plane. 9. The direction of the satellites' revolution in their orbits also coinciding with that of the planet's rotation about its axis, with exceptions in the case of the ninth satellite of Saturn and probably the seventh of Jupiter. 10. The largest planets rotating most swiftly. The sun, a member of the stellar system. The sun, as has been stated, is but a star and a member of the stellar system. What are the orders of magnitude in number, in size, in distance, in speed, in duration among these countless orbs, and how do these relations enter into the problem of the origin of the earth as one of that retinue of planets which attend upon the sun? The luminous stars of our system are estimated to be more than a hundred million in number. The number of the dark stars is unknown. Giving no ray of light to reveal their existence, they may for all we know be as numerous or more numerous than those in the radiant stages of their existence. The few stars whose sizes are known range in diameter from somewhat below a million to upward of ten million miles and more. Many of the stars are in reality double or multiple stars, consisting of companions so close that the two or more appear as one star to the naked eye, or even under the highest power of the telescope, the evidence of their composite nature being revealed only through the analysis of their light by the spectroscope. These double stars revolve swiftly about each other, but such internal motions must be sharply distinguished in thought from the streaming or drifting of the stars as parts of the great stellar system. Relatively to the sun they are found to move through space with speeds averaging between 10 and 30 miles per second, but ranging from less than 10 to more than 200 miles per second. They do not move, however, singly and in closed orbits, but rather in broadly scattered groups whose paths are almost straight lines. These courses of the stars must slowly curve under the aggregate attraction of the millions of stars, but can never return into themselves. The paths of groups of stars intersect other groups and are to some extent interwoven among themselves. These groups have been found to be integrated into two greater groups intermeshed among each other and forming two great star streams whose average motions are in opposite directions. With the passage of millions of years, the stars thus continually enter into new relations and build new configurations in the skies: a myriad host of stellar fireflies, the living and the dead, streaming through space hundreds of millions of miles per year. Although the stars are so great in number, their distances from each other average tens of millions of millions of miles, those in our part of the stellar system averaging between sixty and eighty trillions. The star nearest to the sun, a Centauri, happens, however, to be at a lesser distance of about twentysix trillions of miles. To bring down the dimensions of the universe to finite comprehension, we must divide the scale of nature by a thousand million. Then the earth would be represented by a pebble half an inch in diameter, circling once a year about a sun 4.5 feet in diameter, at a distance of 500 feet. The nearest star, a Centauri, would on this same scale be seen as two spheres revolving about each other at a distance apart equal to 2 miles, and each comparable in size to the sun. This double star would be situated at a distance of about 25,000 miles from our planetary system with its sun, but the other stars in this part of the stellar system would be separated from each other on the average by more than twice this distance. The Galaxy, or Milky Way, is the cloud-like zone of faint stars which extends as a belt around the sky. The stars in it appear faint and close together because of their remoteness. They seem to constitute the outer zone of our stellar system, and its dimensions are only vaguely known. On this diminutive scale the Milky Way might be found to be encompassed by a circle of a hundred million miles diameter, or it might be more or less. The nebula. All hypotheses of earth origin derive the planets and the sun from an antecedent nebulous or meteoritic state. The cloudy patches of light known as nebulæ, which are revealed especially by stellar photography, are, however, of several very different natures and it is a vital question as to which, if any, of these types, could have given birth to our planetary system. First are the irregular nebulæ, diffuse clouds of luminous matter, pervading whole groups of stars as in Orion and the Pleiades, shown in Plate I, A, denser about certain stars, but nevertheless enormously attenuated. This kind of nebulosity is associated with certain regions of the Milky Way. From the characteristics of their spectra, the stars in such nebulæ are regarded as young stars and the nebulous matter may represent the remains of an antecedent stage. The planetary nebulæ are a distinct type, comparatively few in number, and also found associated with the Milky Way. They show in the telescope faint, greenish, circular discs from which they derive their name rather than from any known |