As countless photos from space can prove, the Earth is round the "Blue Marble," as astronauts have affectionately dubbed it. Appearances, however, can be deceiving. Planet Earth is not, in fact, perfectly round. This is not to say the Earth is flat. Well before Columbus sailed the ocean, Aristotle and other ancient Greek scholars proposed that Earth was round. This was based on a number of observations, such as the fact that departing ships seemed to be smaller as they sailed away, as one might expect if sailing across a ball says scientist Bill Carstensen of Virginia Tech in Blacksburg. Isaac Newton first proposed that Earth was not perfectly round. Instead, he suggested it was an oblate spheroid(扁球体) a sphere that is squashed at its poles and swollen at the equator. He was correct and, because of this bulge, the distance from Earth’’s center to sea level is roughly 21 kilometers greater at the equator than at the poles. Our globe, however, is not even a perfect oblate spheroid, because mass is not distributed evenly within the planet. The greater a concentration of mass is, the stronger its gravitational(重力的) pull, "creating bumps around the globe," says scientist Joe Meert at the University of Florida. Also Earth’’s shape changes over time due to a number of other dynamic factors. Mass shifts around inside the planet. Mountains and valleys emerge and disappear due to plate construction. Occasionally falling stars make the suce cave in. And the gravitational pull of the moon and sun not only cause ocean tides but earth titles as well. In addition, the changing weight of the oceans and atmosphere can cause the crust to be out of shape. Moreover, to even out Earth’’s distribution of mass and stabilize its spin, "the entire suce of the Earth will rotate and try to redistribute mass along the equator, a process called true polar wander", Meert says. To keep track of Earth’’s shape, scientists now position thousands of Global Positioning System receivers on the ground that can detect changes in their rising of a few millimeters, Meert says. Another method, dubbed satellite laser ranging, fires visible-wavelength lasers from a few dozen ground stations at satellites. Any changes detected in their orbits correspond to gravitational pulls and thus mass distributions inside the planet. It may not take much technology to understand that Earth is not perfectly round, but it takes quite a bit of effort and technology related above to determine its true shape. Which of the following statements is true according to the first paragraph