Page 18 - Curriculum Visions Dynamic Book

18
 Multiple views of the Earth as it rotates about its axis. The South Pole and Antarctic are near the bottom of the picture. The top of the picture is at about latitude 35°N.
The Earth orbits the Sun on a more circular path than most of the other planets. When looked at from above the North Pole, the Earth revolves counterclockwise. That is the same direction as the Sun is revolving.
The Earth has a single satellite that we call the Moon. The Moon is large compared to the Earth, and much larger relative to its parent planet than the moons of other planets are. Some people therefore regard the Earth and Moon as a double planet system.
The Earth’s gravity
Every object in the Universe has a gravitational field. The size of the field varies with the mass of the object. The Earth’s gravity is much larger than that of the Moon, and it is this gravitational field that keeps the Moon orbiting the Earth in just the same way as all of the satellites do that we have recently put into space.
The gravitational field of the Earth causes tides to develop on the Moon. The line of greatest attraction by the Earth sweeps through the Moon as the Moon spins. That does, in fact, make the Moon change shape continually (bulge toward the Earth) by a small amount as it spins. The Moon, in turn, generates smaller but significant tides in the body of the Earth and in the water and air on the surface. The most important and noticeable result of this is to cause the Earth’s surface waters to flow toward the Moon and so create ocean tides (as shown on page 9).
The Earth’s size and shape
The Earth has a radiUs of 6,378 km (it is 12,756 km across) at the equator and a circUmference of 40,076 km. Because of the rapid spinning of the Earth centrifUgal forces make the Earth bulge at the equator, and so the Earth is not a true sphere. It is correspondingly flattened at the poles, where the radius is 6,371 km (it is 12,742 km across).