Handy Dinosaur Answer Book 在线电子书 pdf 下载 txt下载 epub 下载 mobi 下载 2024

THE<br >BEGINNING<br >BIRTH OF THE SOLAR SYSTEM<br >What do we mean by the solar system?<br >Our solar system--and presumably others--consists of a star， plan<br >satellites， comets， asteroids， meteoroids， and interplanetary dust<br >gas. Our solar system includes the following:<br >Sun--The Sun is at the center of our solar system， and is an average<br >when compared with other stars in the universe. It is classed as a typ<br >G2-class star， represents about 99.86 percent of the mass of the s(<br >system， and is about 864，000 miles (1，390，180 kilometers) in diame<br >or about 109 Earth diameters across. It is thought that the Sun arri<br >at its present brightness about 800 million years ago， and will proba<br >burn for another seven billion years.<br >Nine known planets--From the Sun outward， the planets are M<br >cury， Venus， Earth， Mars (the inner， or terrestrial， planets); Jupi<br >Saturn， Uranus， Neptune (the outer， or gaseous， planets， or<br >giants); and Pluto (which is more similar in characteristics to the t<br >restrial planets).<br >Moons--More than 64 moons (satellites) circle the various planets<br >addition， a moon has also been found around the small minor pla<br >(asteroid) Ida， and some scientists believe such tiny moons arou<br >asteroids may be common. Saturn has the most known moons (m<br ><br >