Apparent and Real Movement in Space
FOR many centuries, the stars were known as the fixed stars because they did not seem to change their relative positions in the heavens. They were contrasted by early astronomers with the planets. The latter could be seen to follow definite paths in the skies with reference to other heavenly bodies and therefore they were called "wanderers." (The literal meaning of the Greek word planetes, from which "planet" is derived, is "wandering.")
If the individual stars did not change their relative positions, they did apparently move around the Earth, which was thought of as a fixed body set in the center of the universe. The different constellations and other star groups rose and set just as the Sun did. They had to be looked for in different places in the heavens according to the time of day and also the season. These apparent motions of the stars have been known and recorded ever since the heavenly bodies were first studied.
The rising and setting of the stars and their positions in the skies have, from time immemorial, caused certain star groups to be associated with various events of human life. Thus the ancient Greeks began their navigation season when the Pleiades were first seen to climb above the horizon before snnrise in the month of May. To them, consequently, the Pleiades were the "sailing stars" Other peoples associated these stars with the planting season.
Chaucer and other medieval writers often used astronomical terms, based on apparent star motions, in referring to the time and season. This practice shows that in the Middle Ages the motions of the stars not only the concern of special students but were familiar to educated people in general. The men of the medieval period marveled at the wonderfully consistent patterns the stars formed as they wheeled majestically through the skies. No wonder that even learned men thought that these glittering celestial bodies were closely linked with human destinies.
The Copernican theory, which was advanced by the Polish astronomer Nicolaus Copernicus in the sixteenth century, dispelled the old belief that the Earth stood fixed in the center of the heavens. Copernicus pointed out that the Earth revolved around the Sun and that it held no more exalted place in the scheme of things than did Venus, or Mercury, or Mars or the other planets. Astronomers came to realize that the revolution of the stars around the Earth was only apparent. It was an optical illusion, based on the rotation of the Earth about its axis.
Yet modern science has shown that the stars in our galaxy actually move, so that they gradually change their positions with respect to one another. The British astronomer Edmund Halley, after whom perhaps the most famous of all comets was named, was the first to call attention to these individual motions of stars. He reported that the stars Arcturus and Sirius were both definitely south of the positions noted by the Alexandrian astronomer Ptolemy in the second century A.D. With the introduction, in the nineteenth century, of the photographic method for determining star positions, astronomers have been able to measure the individual motions of many thousands of stars.
The study of these motions is one of the most difficult branches of astronomy. There are several reasons for this. In the first place, the distances between stars are so immense that one can make out changes in their relative positions only with the most precise instruments and the most painstaking methods. Another problem is the lack of a reliable frame of reference in measuring stellar motion. Since we live on the planet Earth, we have to use it as a reference point. Obviously, it is not very satisfactory for this purpose. Not only does it make its annual journey around the Sun, but the entire solar system, as we shall see, is also moving constantly through the skies. Despite all obstacles, however, astronomers have made a great deal of progress in analyzing the different types of stellar motion.
