Kimi Kawamoto, age 12; of Omaha, Nebraska, for her question:
How can the earth be part of the_Milky Way?
Back in 1610, Galileo turned his small homemade telescope on the Milky Way and discovered that its blurry light comes from a teeming host of faraway stars. When we behold its pale arch over the sky, this seems believable. But we are told that this crowded starry realm is 20,000 light years or more from our solar system. With this up to date information in mind, it is hard to believe that our small, remote planet can be a part of the Milky Way. Nevertheless, it is.
The stately city of Omaha is part of Nebraska. Someone told you this and maybe you verified its location by visiting parts of the state beyond the visible horizon. As an earthling, you can comprehend these dimensions and test them. But the dimensions of the glimmering Milky Way occupy an enormous pie shaped region of the infinite oceans of space that surround our planet. We must strain our brains to comprehend its scope and, at least at present, we cannot explore and test it in person.
The diameter of our earth is less than 8,000 miles, but our solar system measures about 10 billion miles from side to side. Our dazzling sun is merely one of an estimated 100 billion stars that belong to the Milky Way. The diameter of the enormous star system is estimated to be 80,000 light years. Traveling non stop at almost 700 million miles per hour, a spaceship could cross it in 80,000 earth years. We live in the suburbs in one of the thinly populated spokes that spiral out from the teeming center. Our nearest starry neighbors are about 25 million million earth miles beyond the orbit of Pluto. The crowded center of the Big Wheel is 26,000 light years away, somewhere behind the constellation Sagittarius.
From the earth, we have an edgewise view of our enormous home in the heavens. We see its teeming stars as the pale glimmering arch of the Milky Way. Our view is limited to a pie slice section of the wheeling system and the single bright stars we see are those that share our suburban neighborhood. As the big wheel rotates, its separate stars move at different speeds and those near the hub move faster. The earth moves in step with the solar system, completing one galactic orbit in 200 million years.
Our little planet is indeed part of the rotating 1tilky Way and can never escape from its big beauteous home in the heavens. The next nearest galactic star systems are enormous distances beyond the outer rim of our Milky Way out in the infinite ocean of extragalactic space.
The crowded nucleus of our galaxy is about 25,000 light years wide and 8,000 light years thick. From it, two thinly populated spokes spiral out to the rim. The Big Wheel rotates around an axis through the nucleus. It has a north and south galactic pole, a galactic equator around its wide waist, plus galactic lines of latitude and longitude. Facing spaceward from the earth's North Pole, the wheeling Milky Slay rotates in a clockwise direction.