There is a possibility that our sidereal universe, vast as it is known to be, may be but a unit in some still greater unit and that other similar systems lie beyond the reach of existing telescopes at unimaginable distances. The mind of man is overwhelmed by the thought of sidereal systems as vast as our own lying far beyond his ken. Whether or not such external systems do exist and are with our own sidereal system units in some still vaster creation we cannot know. So vast, indeed, is this one visible universe of ours that the mind of man, accustomed to earthly standards, cannot comprehend its magnitude or the infinitesimal size of our whole solar system compared to it. XXXI SOME ASTRONOMICAL FACTS WORTH REMEMBERING Kepler's Three Laws of Planetary Motion: I. The planets move in ellipses with the sun at one focus. II. The radius vector of a planet (line adjoining sun and planet) sweeps over equal areas in equal times. III. The square of the time of revolution (the year) of each planet is proportional to the cube of its mean distance from the sun. * * * * * Sir Isaac Newton discovered that the law of gravitation extends to the stars. That is, every mass in the universe attracts every other mass with an attraction directly proportional to the product of the masses and inversely proportional to the square of the distances between them. * * * * * Ocean tides are caused by the difference between the attraction of the sun and moon for the main body of the earth and their attraction for different particles of the earth's surface. The tide-raising force of the disturbing body is proportional to its mass and inversely proportional to the cube of its distance. The tides produced by the sun are, therefore, only two-fifths as great as the tides produced by the moon. * * * * * The celestial sphere is an imaginary sphere of infinite radius, with the earth at its center, upon which the celestial bodies are considered to be projected for convenience in determining their positions with respect to fixed points of reference in the heavens. The north and south poles of the heavens are the points on the celestial sphere directly above the north and south poles of the earth. The celestial equator is the great circle in which the plane of the earth's equator intersects the celestial sphere. It passes through the east and west points of the horizon and through the zenith--or point directly overhead--at the earth's equator. The ecliptic is the great circle in which the plane of the earth's orbit intersects the celestial sphere. The celestial equator and the ecliptic are inclined to each other at an angle of 23-1/2°, which is called the obliquity of the ecliptic. The two points in which the celestial equator and the ecliptic intersect are called respectively the vernal equinox and the autumnal equinox.