Page:Popular Science Monthly Volume 87.djvu/217

Rh satellites, and all the asteroids with a very few exceptions, would perform their motions entirely within the box. The exceptional asteroids and the majority of the comets would dip out of the box because the planes in which their orbits lie make considerable angles with the central plane of the solar system.

It is an equally remarkable fact that the eight planets and the 800 asteroids are all revolving around the Sun in the same direction, which we call west to east. Likewise, the Sun rotates on its axis from west to east, and so also do Mercury, Venus, the Earth and its Moon, Mars, Jupiter and Saturn. Our moon, Mars's two moons, the seven inner moons of Jupiter, Saturn's rings and eight of its moons, revolve around their plants from west to east. From Jupiter out to Neptune we come upon exceptions to the rule. The eighth and ninth moons of Jupiter go around the planet from east to west. The ninth moon of Saturn is similarly reversed in direction. The four moons of Uranus move in a plane making an angle of 98° with the principal plane of the solar system; that is, nearly at right angles to the principal plane. The one moon of Neptune moves in a plane inclined 145° to the plane of the system; in effect, from the east toward the west. The equatorial planes of Uranus and Neptune are, without doubt, essentially coincident with their satellite planes.

Our solar system is very completely isolated from other systems. Light travels from our Sun out to Neptune in less than 4 hours, yet it requires 4 years to travel from our Sun to the nearest star, α Centauri. Stating the case differently, the nearest star is more than 9,000 times as far from our system as our farthest planet, Neptune, is from the Sun. We should have to go 7 light-years from our Sun in another direction to reach the second-nearest star. It is 9 light-years in a still different direction to Sirius. The average distances between neighboring stars, at least in our part of the universe, is 6 or 7 or 8 light-years. We can see that the stars themselves occupy very little space, and that they have an abundance of room to move about. Recalling, further, that the average speed of the stars is about 26 kilometers per second, which means that about 80,000 years would be required for the average star to travel over the average distance to its neighbor, we can see that collisions of two stars must be exceedingly rare; and that close approaches of two stars, approaches so close as to disturb each other violently, must also be rare. However, when we consider the number of stars in the stellar system, we should perhaps expect a few close approaches to occur in a human life time; possibly also a grazing collision, but probably no full collision.

The universe of stars—our stellar system—is believed by students of the subject, all but unanimously, to occupy a limited volume of space