PLANET EARTH Nhe third planet from the Sun is Earth the home of all known life While it shares many characteristics with other planets its physical properties and his tory allow it to support life in its near surface environment Liquid water which is essential for all known forms of life is found in abun dance on Earth Deep salty oceans cover more than two thirds of the surface In addition Earth's oxygen rich atmo sphere is unique This characteristic is actually not surprising however since large amounts of oxygen exist in the atmosphere only because living things constantly supply it Earth s green plants take in carbon dioxide and give off oxygen which humans and other animals need to breathe This is one example of how life itself has altered the planet in ways that generally help maintain the conditions for life Scientists have come to view Earth as a dynamic world with many interacting systems Understanding these relationships will surely be important as human activi ties increasingly affect the planet s surface oceans and
PLANET EARTH SHAPE AND SIZE Many ancient cultures even sophisticated ones such as the Egyptians pictured Earth as being flat with the sky above being a separate abode of heavenly bodies the Sun Moon planets and stars By 2500 BC though some people correctly thought Earth to be roughly spherical in shape The Greek mathe matician Pythagoras is widely credited is with reaching this conclusion in the 6th century BC Two cen turies later Aristotle gave specific reasons for thinking Earth is round 1 matter is drawn to Earth's Astronauts aboard the Apollo 17 spacecray captured a stunning image of Earth as the center and would nat spacecraf beaded to the Moon in 1972 Vast urally compress it into oceans and seas surround the continent of Africa the island of Madagascar and at a spherical shape 2 top the Arabian Peninsula Also visible traveling south reveals are clouds in the atmosphere and the south new stars rising above polar ice cap NASA
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
