THE VISIBLE SKY ince the beginnings of humankind peo ple have gazed at the heavens Before the dawn of history someone noticed that certain celestial bodies moved in orderly and predictable paths and astronomy an ancient science was born Yet some of sci ence s newest discoveries have been made in this same field which includes the study of all matter outside Earth s atmosphere From simple observations of the motions of the Sun and the stars as they pass across the sky to advanced theories of the exotic states of matter in collapsed stars astronomy has spanned the ages For centuries astronomers concentrated on learning about the motions of heavenly bodies They saw the Sun rise in the east and set in the west In the night sky they saw tiny points of light Most of these lights the stars seemed to stay in the same place in relation to one another as if they were all fastened to a huge black globe surround ing Earth Other lights however seemed to travel going from group to group of sta tionary stars They named these moving Io
THE VISIBLE SKY New stars are forming from the hot gas and dust of the Orion nebula a major stellar nursery only some 1 500 light years from Earth Our sun probably formed in a similar environment More than 500 separate images were combined to create this mosaic NASA ESA M Robberto Space Telescope Science Institute ESA and the Hubble Space Telescope Orion Treasury Project Team II
AsTRONOMY UNDERSTANDING THE UNIVERSE points planets which means wanderers in Greek Ancient astronomers thought that the positions of celestial bodies revealed what was going to happen on Earthwars births deaths and good fortune or bad This system of belief is called astrology Most scientists no longer believe in astrology but they have found that some ancient astrologers were good at observing the motions and positions of stars and planets When people today look at the sky with out a telescope or other modern instrument they see basically the same things the ancient astronomers saw During a clear day one can see the Sun and sometimes a faint Moon On a clear night one can see stars and usually the Moon Sometimes a star may seem to be in different positions from night to night it is really a planet one of the wanderers of the ancients The planets all circle the Sun just as Earth does They are visible from Earth because sunlight bounces off them The stars are much farther away Most stars are like the Sun large hot and bright They shine from their own energy A broad strip of dim light is also visible across the night sky It is a clustering of faint stars known as the Milky Way The Milky Way 12
THE VISIBLE SKY is part of the Milky Way galaxy an enormous cluster of stars of which the Sun is only one member out of more than 100 billion stars Other galaxies exist far beyond the Milky Way EARTH IN SPACE The apparent westward motion of the Sun the Moon and the stars is not real They seem to move around Earth but this apparent motion is actually caused by Earth s move ment Earth rotates eastward complet ing one rotation each day This may be hard to believe at first because when one thinks of motion one usually also thinks of the vibrations of moving cars or trains But Earth moves freely in space with out rubbing against anything so it does not vibrate It is this Planet Earth rises above the moon's bori gentle rotation unin son in an unprecedented view captured in December 1968 by Apollo 8 astronauts as hibited by significant their orbit carried them clear of the far friction that makes the side of the moon NASA 13
