TheInvisibleMag n e t Raise your arm Keep it in that position for as long as you can What happens After some time your arm begins to hurt Something seems to be pulling it down Soon enough you have to lower your arm It’s a force called gravity that causes you to lower your arm Gravity acts something like a magnet tugging away at your arm as if it were a piece of metal We can’t see gravity or touch it We can only feel it The Earth has gravity that pulls down on everything on or near it It is this force that keeps us all on Earth The Moon and the Sun also have gravity All bodies in the universe have gravity In fact gravity helps hold all of them together Sir Isaac Newton first introduced the idea of gravity and Albert Einstein added to Newton’s ideas Gravity works in a two-way system This means that all bodies have a pull on each other For example Earth’s gravity forces the Moon to circle around it all the time In return the Moon’s gravity attracts the waters of Earth’s oceans to cause tides The force of gravity becomes weaker and weaker as we move away from its source That is partly why astronauts can float around in outer space They are too far away for the Earth to have much pull on them What do you think would happen if there were no gravity on Earth LEARN MORE READ THESE ARTICLES ALBERT EINSTEIN MOON SIR ISAAC NEWTON 10
GRAVITY DID YOU KNOW The 1961 Disney movie The Absent Minded Professorintroduced one of the most far-fetched and hilariously popular antigravity schemes flubber a superbouncy flying-rubber formula S EA R C H LIG H T Why do you think a ballpoint pen won’t work when you try to write with its point facing upward The upside-down pen’s point soon runs out of ink the wrong end of a pen that’s not facing toward the ground Answer Gravity causes the ink in the ballpoint pen to flow to 11
GALAXIES S tar Clusters When we look at the sky at night we may see thousands of stars shining brightly They look as if they are just scattered around the sky But actually most stars are clustered together in huge groups These groups are called galaxies S EA R C H LIG H T Find and correct the error in the following sentence There are many many universes in the galaxy Our Sun is part of a galaxy It is the Milky Way Galaxy On a very clear night if you look carefully at the sky you might see part of this whitish band of stars stretching from one side to the other The universe is so huge that the Milky Way Galaxy is only one of many Astronomers think that there are billions of galaxies in the universe Each of these galaxies may contain trillions of stars many much bigger than our own Sun The Milky Way itself contains several billion stars Some galaxies have no regular shape Others like the Milky Way are shaped somewhat like giant merry-go-rounds Each has a center around which stars move in circles It is hard to see the other galaxies in the sky with the naked eye Even though they are incredibly large they are also incredibly far away Scientists must use powerful telescopes to study other galaxies For this reason it takes a long time to learn even a little bit about another galaxy And there’s still a great deal we haven’t learned about our own galaxy LEARN MORE READ THESE ARTICLES SOLAR SYSTEM STARS UNIVERSE lations unl DID YOU KNOW ike galaxies are imagined Constel groups of stars that people in the as connecting to make pictures n i g h t s k y N a m e d m o s t l y f o r a n i m a l s lations and mythological figures constel l help astronomers and navigators l sti locate certain stars Our galaxy the Milky Way is shaped somewhat like a giant merry-go-round Its billions of stars move in circles around a center Myron Jay Dorf Corbis Answer There are many many galaxiesin the universe 13
