the pendulum itself could not change its motion, so he concluded that the Earth, underneath
the pendulum was moving.Figure24.9shows how this might look.
It takes 23 hours, 59 minutes and 4 seconds for the Earth to make one complete rotation
on its axis, if we watch Earth spin from out in space. Because Earth is moving around the
Sun at the same time that it is rotating, Earth has to turn just a little bit more to reach
the same place relative to the Sun, so we experience each day on Earth as 24 hours. At the
equator, the Earth rotates at a speed of about 1,700 kilometers per hour. Thankfully, we do
not notice this movement, because it would certainly make us dizzy.
Earth’s Revolution
Earth’s revolution around the Sun takes much longer than its rotation on its axis. One
complete revolution takes 365.24 days, or one year. The Earth revolves around the Sun
because gravity keeps it in a roughly circular orbit around the Sun. The Earth’s orbital path
is not a perfect circle, but rather an ellipse, which means that it is like a slight oval in shape
(Figure24.10). This creates areas where the Earth is sometimes farther away from the Sun
than at other times. We are closer to the Sun at perihelion (147 million kilometers) on about
January 3rd and a little further from the Sun (152 million kilometers) at aphelion on July
4th. Students sometimes think our elliptical orbit causes Earth’s seasons, but this is not the
case. If it were, then the Northern Hemisphere would experience summer in January!
Figure 24.10: Earth and the other planets in the solar system make regular orbits around
the Sun; the orbital path is an ellipse and is controlled by gravity. ( 8 )
During one revolution around the Sun, the Earth travels at an average distance of about 150
million kilometers. Mercury and Venus take shorter times to orbit the Sun than the Earth,
while all the other planets take progressively longer times depending on their distance from
the Sun. Mercury only takes about 88 Earth days to make one trip around the Sun. While
Saturn, for example, takes more than 29 Earth years to make one revolution around the Sun.
Earth revolves around the Sun at an average speed of about 27 kilometers (17 miles) per
second. Our planet moves slower when it is farther away from the Sun and faster when it is
closer to the Sun. The reason the Earth (or any planet) has seasons is that Earth is tilted