EARTHANDITs MoON the southern horizon and 3 Earth s shadow on the Moon at lunar eclipses is circular After Aristotle many people did not know about his teachings Belief in a flat Earth could persist and there was at least some debate on the issue However knowl edge of the spherical shape of Earth has been widespread among astronomers for most of the last 2 000 years at least Greek and Indian astronomers accepted the concept and by the 9th century AD Islamic astrono mers were using it Christian Europe had widely accepted the idea by the 13th century The field of measurement of Earth is called geodesy One of the earliest known attempts to calculate the circumference of Earth was that of Eratosthenes a librarian at the library of Alexandria in Egypt in about 250 BC He knew that at about noon on the day of the summer solstice the day the noon Sun was highest in the sky a vertical stick in Alexandria would cast a shadow that made an angle of about 7 degrees with the stick He also knew that in Syene now Aswan Egypt the Sun would pass directly overhead on that day shining to the bottom of a deep well By using an estimate of the north south distance between the two cities and assuming that the Sun was very far away he was able to calculate Io
PLANET EARTH Earth s circumference within a relatively small percentage of the value now known It is now known that the circumference at the Equator is 24 902 miles 40 075 kilo meters The diameter at the Equator is 7 926 miles 12 756 kilometers Actually Earth is not quite spherical but is an oblate spheroid squashed in the north south direction so that the pole to pole diameter is only 900 miles 12 714 kilometers The distance from the Equator to the North Pole along Earth s from Sun Alexandria Syene BEA Eratosthenes measurement of Earth The figure is not drawn to scale Encyclopedia Britannica Inc II
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
