Gravitation

Abstract

The long-standing close relationship between physics and astronomy has been fruitful for both sciences. Astronomy is probably the oldest of the sciences, and throughout most of recorded history its central problem has been the explanation of the nature of the members of the solar system and of their curious motions through the sky. The members of the solar system visible to the naked eye are the sun, moon, and five bright planets — Venus, Jupiter, Mars, Saturn, and Mercury. The other objects in the sky are stars, similar to the sun but at distances too great to be visualized by the human mind. The nearest, Alpha Centauri, is 1.4 × 10¹³ miles away, so far away that light traveling at 186 000 miles sec⁻¹ takes 4.4 years to travel from that star to us. (The distances in astronomy are so vast that the distance light travels in a year, the light-year, forms a convenient unit. Thus Alpha Centauri is 4.4 light-years away, the distance of Rigel is 540 light-years, and so on.) There are about 9000 stars visible (to the naked eye) from the earth, about 2500 of these being visible at any location on a clear night, and maps of the relative locations of these stars have been made for thousands of years. The maps made today differ so slightly from those made by the Greeks three thousand years ago that the stars are called fixed. (If the prehistoric artists, who left pictures of animals in French caves, had been interested enough in stars to leave some sketches of the constellations as they saw them, it would have been very helpful to contemporary astronomers, for over that much longer time scale the stars would not appear fixed.)