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
AsTRONOMY UNDERSTANDING THE UNIVERSE The Moon's Path Around the Sun Earth's orbit Around Sun Moon's Path Cycle of Moon s Orbit Begins new moon 3 days Sun the Moon and the stars appear to be rising and setting Earth is accompanied by the Moon which moves around the planet at a dis tance of about 30 Earth diameters At the same time Earth moves around the Sun Every year Earth completes one revolution around the Sun This motion along with the tilt of Earth s rotation axis relative to the axis of its revolution around the Sun accounts for the changes in the seasons When the northern half of Earth is tipped toward the Sun the Northern Hemisphere experiences summer and the Southern Hemisphere which is tipped away from 7 days 114 days 15 days 6 18 days 22 days third quarter As the moon revolves in an almost circular path around Earth Earth moves in a sim ilar path around the sun Both motions combine to give the moon a wavy orbit Encyclopaedia Britannica Inc 26 days 30 days new moon 14 New Cycle of Moon's Orbit Begins
THE VISIBLE SKY the Sun experiences winter When Earth has moved to the other side of the Sun six months later the seasons are reversed because the Southern Hemisphere is then tipped toward the Sun and the Northern Hemisphere is tipped away from the Sun The Moon does not always look the same from Earth Sometimes it looks round sometimes like a thin curved sliver These apparent changes are called the phases of the Moon They occur because the Moon shines only when the Sun's light bounces off its sur face This means that only the side of the Moon that faces the Sun is bright When the Moon is between Earth and the Sun the light side of the Moon faces away from Earth This is called the new moon and it is not visible from Earth When the Moon is on the other side of Earth from the Sun its entire light side faces Earth This is called the full moon Halfway between the new and full moons in locations on either side of Earth are the first quarter and the last quarter which look like half disks as viewed from Earth ECLIPSES In ancient times people often were terri fied when the Sun or the Moon seemed 15
