Just as gravity causes objects near Earth to be pulled toward Earth’s
center, it also causes Earth and other objects near the Sun to be pulled
toward the Sun’s center. Fortunately, Earth does not move straight
into the Sun. Earth moves sideways, at nearly a right angle to the
Sun’s direction. Without the Sun’s gravitational pull, Earth would
keep moving in a straight line out into deep space. However, the
Sun’s pull changes Earth’s path from a straight line to a round orbit
about 300 million kilometers (200,000,000 mi) across.
Just as a day is the time it takes Earth to rotate once on its axis,
a year is the time it takes Earth to orbit the Sun once. In astronomy,
a is the motion of one object around another. The word
revolution can also mean the time it takes an object to go around once.
Earth’s rotation and orbit do not quite line up. If they did, Earth’s
equator would be in the same plane as Earth’s orbit, like a tiny hoop
and a huge hoop lying on the same tabletop. Instead, Earth rotates at
about a 23˚ angle, or tilt, from this lined-up position.