THE PLANETS
WE have just obtained a close view of one of the stars of the Galaxy,—the nearest star, our own sun,—and have gazed in admiration at the pearly setting of its corona, the turbulent waves of its crimson atmosphere, its black spots and moving belts, and our admiration increases as we visualize in our mind's eye, the eight fine planets which swing unceasingly around in its beautiful light.
In comparison with the sun, these planets are very small; either the earth, Mars, Venus or Mercury could be dropped into an average-sized sun-spot, or all four into an extra large sun-spot, while a thousand globes huge as Jupiter would scarcely balance the globe of the sun. Indeed, it has been computed that if the total mass of the whole solar system, with the exception of the sun, were called I, then the mass of the sun on the same scale would be 744.
All of the planets of the solar system roll as if on a great shield, one orbit set within the other at widely separated intervals. These orbits are not round but elliptical, with the sun shining at one focus of each elliptical orbit.
An unseen force called gravitation holds the planets in their pathways, always just so near and just so far from the sun. This is the same force that holds the stars in a Galaxy and keeps the objects on the earth from sliding off into space. Since the attractive power of the sun is tremendous, the planets must travel around their pathways at a tremendous rate in order to develop enough centrifugal force to offset the gravitational pull, and the closer the planet the faster it must fly.
The orbits of the four outer planets, Neptune, Uranus, Saturn and Jupiter, and the orbits of Mars and the Earth, drawn to scale. (Within the orbit of the Earth would appear the orbits of Venus and Mercury, too small to be shown in this drawing.)
Mercury, for instance, travels at a rate of 35 miles a second while Neptune, seventy-five times as far off, has an orbital velocity of only 3.4 miles a second. The following table shows the distances of each planet from the sun and the time it takes it to complete its journey around its orbit, beginning with Mercury, the nearest planet to the sun, and continuing outward.
| Distances from the Sun | Period of Revolution | |
|---|---|---|
| Mercury | 35,750,000 miles | 88 days |
| Venus | 66,750,000 " | 225 " |
| Earth | 92,500,000 " | 365 " |
| Mars | 141,000,000 " | 687 " |
| Jupiter | 457,000,000 " | 12 years |
| Saturn | 881,000,000 " | 29½ " |
| Uranus | 1,771,000,000 " | 84 " |
| Neptune | 2,775,000,000 " | 165 " |