Figure 24.21: Nicholas Copernicus. ( 13 )also responsible for this. As Earth turns, the Moon and stars change position in our sky.
Earth’s Day and Night
Another effect of Earth’s rotation is that we have a cycle of daylight and darkness approx-
imately every 24 hours. This is called a day. As Earth rotates, the side of Earth facing
the Sun experiences daylight, and the opposite side (facing away from the Sun) experiences
darkness or nighttime. Since the Earth completes one rotation in about 24 hours, this is
the time it takes to complete one day-night cycle. As the Earth rotates, different places on
Earth experience sunset and sunrise at a different time. As you move towards the poles,
summer and winter days have different amounts of daylight hours in a day. For example, in
the Northern hemisphere, we begin summer on June 21. At this point, the Earth’s North
Pole is pointed directly toward the Sun. Therefore, areas north of the equator experience
longer days and shorter nights because the northern half of the Earth is pointed toward the
Sun. Since the southern half of the Earth is pointed away from the Sun at that point, they
have the opposite effect—longer nights and shorter days.
For people in the Northern hemisphere, winter begins on December 21. At this point, it is
Earth’s South Pole that is tilted toward the Sun, and so there are shorter days and longer
nights for those who are north of the equator.