Notes

The origin of the earth has puzzled man since ancient times. This problem is astronomical as well as geological, for the origin of the earth cannot be divorced from that of the component members of our solar system. The earth is one of nine planets which revolve about the sun and rotate in the same direction as the sun. The position of the earth among the planets is not conspicuously prominent. It is the third planet out from the sun and in size is intermediate, for three planets, Mercury, Venus, and Mars are known to be smaller and Jupiter, Saturn, Uranus, and Neptune are larger. About most of the planets revolve smaller bodies called moons or satellites. The earth and Neptune each have one, Mars two, Uranus four, and Jupiter and Saturn each have nine. In comparison with the 88,600-mile diameter of Jupiter, the largest planet, and that of the sun, 864,000 miles, the earth's diameter of 7,900 miles appears small. But, even the largest body and the greatest distance in our solar system fade into insignificance when compared with the distances to the fixed stars of our universe. To record these distances the astronomer uses the light year, which is the distance traveled by light in one year with a velocity of 186,000 miles a second. Light travels the 93,000,000 miles from the sun to the earth in about 8 minutes and the 2,800,000,000 miles to Neptune in about 4 hours. The nearest star is 4 light years away and the bright stars seen by us are 10 to 500 light years away. Beyond the Milky Way that comprises our stellar universe are distributed other nebulae or universes which have been reported to extend 150,000,000 light years into space. The immensity of it all baffles visualization by human imagination.