EARTHANDITs MoON surface is 6 214 miles 10 002 kilometers In fact the meter was originally defined by the French Academy of Sciences in 1791 to be one ten millionth of the distance from the Equator to the North Pole This explains why this distance is almost exactly 10 million meters 10 000 kilometers MoTIONS For centuries Earth was simply the world the only one known Even most believers in a spherical Earth thought it to be a one of a kind object in the center of a spherical universe The Sun Moon planets and stars were generally thought to be of a very dif ferent nature from Earth In fact in the 4th century BC Aristotle proposed that they were made of a heavenly fifth element that he called quintessence This was in addition to his supposed earthly elements of earth water air and fire The Sun and Moon plus Mercury Venus Mars Jupiter and Saturn all easily visible to the naked eye were seen to gradually change position relative to the stars This earned them the name planets which meant wanderers Most thinkers including Aristotle believed that Earth was motionless in the 12
PLANET EARTH center of the universe This is called the geocentric Earth centered theory and it was developed in greater detail by Ptolemy of Alexandria in around AD 150 Almost all astronomers accepted the theory for the next 1 400 years In this view Earth was certainly not a planet because it was obviously not a wandering light in the sky RoTATION AND ORBIT In the 16th century AD Nicholas Copernicus of Poland proposed that Earth rotates on an axis through the North and South poles once a day actually once a sidereal day which is measured using the distant stars as a reference frame instead of the Sun Earth's sidereal day is 23 hours 56 minutes and 4 seconds which is a few minutes shorter than its solar day Copernicus also said that Earth orbits or revolves around the Sun once a sidereal year which is 365 256 days He believed that the Moon orbits Earth but that the other wan derers the planets not including the Sun revolve around the Sun like Earth does In this Earth is a planet because it too is a wanderer around the Sun Copernicus heliocentric Sun centered theory was slow to be accepted However 13
EARTHANDITs MoON Johannes Kepler of Germany assumed this basic view in developing his three laws of planetary motion in the early 17th century One of these laws states that a planet s orbit or path around the Sun is an ellipse with the Sun not at the exact center but at one of two points called foci Earth's orbit turns out to be more nearly a circle than the orbits of most of the other planets Earth's distance from the Sun varies by only a small percent age from about 91 4 million miles 147 1 million kilometers in early January to some 94 5 million miles 152 1 million kilometers in early July THE EFFECT OF GRAVITY In 1687 Isaac Newton of England published his law of universal gravitation in his major work the Principia This explained the plan ets motions as being caused by the Sun s gravitational pull on the planets By this point almost all scientists accepted the heliocentric theory Newton showed that any two objects attract each other with this gravitational force Its strength is proportional to the mass of each object and it becomes weaker with increasing distance between the objects 14
PLANET EARTH GRAVITATION The force that causes objects to drop and water to run downhill is the same force that holds the Earth the Sun and the stars together and keeps the Moon and artificial satellites in their orbits Gravitation the attraction of all matter for all other matter is both the most familiar of the natural forces and the least understood Gravity is the weakest of the four forces that are currently known to govern the way physi cal objects behave The other three forces are electromagnetism which governs such famil iar phenomena as electricity and magnetism the strong force which is responsible for the events in nuclear reactors and hydrogen bombs and the weak force which is involved with radioactivity Because of its weakness gravity is difficult to study in the laboratory Despite its weakness gravitation is impor tant because unlike the other three forces it is universally attractive and also acts over an infinite distance Over distances ranging from those mea surable on Earth to those in the farthest parts of the universe gravitational attraction is a signifi cant force and in many cases the dominant one Both Sir Isaac Newton in the 17th century and Albert Einstein in the 20th century initiated revolutions in the study and observation of the universe through new theories of gravity The subject is today at the forefront of theoretical physics and astronomy IS
