OJ ong ago Earth was the subject of great speculation Ancient astronomers such las Ptolemy thought that Earth was the center of the universe Likewise there have been all sorts of assumptions about Earth s satellite the Moon One of the most inter esting beliefs was that moonlight was capable of driving a person crazy Using a telescope Galileo Galilei went on to show that the Sun was actually the center of the universe And while some folks still believe that a full Moon brings out the weird in people they have plenty of solid evidence that proves the Moon has real natural effects on Earth For instance the gravitational pull of the Moon dur ing its many phases influences ocean tides Many additional facts about Earth and the Moon have been discovered over the years The results of years of research into the nature of Earth and the Moon are the subject of this book More is known about Earth than any other planet in the solar system mainly because scientists have had the advantage of living here Even before the advent of helpful tech nology such as the telescope and computers scientists managed to find ways to figure out certain characteristics of the planet These included its size shape and movement For instance ancient Egyptians measured
INTRODUCTION shadow length and factored in the distance between cities to obtain a rough estimate of Earth's circumference To find out about Earth s physical struc ture scientists test and observe the planet s surface layer known as the crust Using mate rial from this and the top of the next deepest layer the mantle they can work out formu las that tell them what s happening all the way down to Earth's center the core To study the atmosphere the envelope of gases surrounding Earth scientists have an array of other tech niques and tools including radar and tethered balloons that carry sophisticated instruments Manned space missions have provided scientists with plenty of information about the Moon s physical characteristics Testing of Moon rocks revealed the age and composi tion of the satellite s surface Long before the rocket age ancient astronomers used math observation and simple deduction to guess timate measurements of the Moon's size and its distance from Earth Exploration observation and experimen tation have answered many questions and dispelled many myths concerning Earth and its satellite Yet there are still many myster ies to be solved regarding this ever evolving planet and its
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
