If you've looked at the night sky for a prolonged period of time, you will notice all the stars, including the Big Dipper, seem to move around a fixed star directly overhead. Over the course of the year, the Big Dipper appears to rotate 360 degrees. Despite this, neither of these rotations take place; they are both illusions caused by the earth's movement.
Every day the earth makes one full rotation. The North star, Polaris, is directly aligned with the earth, so it appears stationary in the night sky. However, the rest of the stars, such as the Big Dipper, are not stationary, which causes the illusion of rotation around the fixed North star. To observe this phenomenon, locate the North star, which is always located near the Big Dipper.
While the Big Dipper appears to make a full rotation around the North star every night, it also appears to make a slower rotation every year. The earth takes one year to move around sun. As the earth completes this cycle, our view of the Big Dipper changes, giving the star formation the illusion of rotating in place. In the Northern Hemisphere, this means the Big Dipper appears upright in the summer and upside-down in winter.
Some cultures have mythological reasons why the Big Dipper rotates. Some Native American cultures believed the dipper was full of water in the summer, but would need to tip over in the fall. This is responsible for the autumnal change of colors. Historically, the rotation of the Big Dipper also had a practical use. Sailors used the Big Dipper's rotation as a way to gauge time. Because the constellation makes a full cycle every 24 hours, sailors could approximate the time by its position.