Why do we Have Day and Night?

If a comedian or a smart-aleck were to answer this question, he or she might say something like, “We have day and night because the Sun shines during the day and doesn’t shine during the night.”

This is true, but only partly true. The Sun actually shines all the time. The difference between day and night is this: Some people see the Sun at a particular time, while at this same moment others don’t see the Sun. Night is simply the absence of sunlight in a particular place on the planet. How does this happen? It has to do with the position of the Earth in relation to the Sun and a specific type of movement called “rotation.”

The Earth is turning or spinning constantly, similar to a top or a gyroscope. The difference of course is that the Earth’s rotation is constant. Fortunately, it doesn’t slow down, at least not enough for us to notice! If the pace of the Earth’s rotation slowed down considerably or if the Earth stopped rotating all living things would be at risk. Death would be almost certain.

If we choose a point on the Earth’s surface, such as where we are sitting or standing, and think about the rotation of the Earth we should be able to understand that the spot we are on points toward the Sun for several hours and gradually turns until our spot is no longer facing the Sun. When we can see the sunlight, it is day. When the spot we are standing or sitting on rotates enough so that our view of the Sun is blocked it is night. We are on the dark side of the Earth for several hours.

All planets in our solar system have day and night but the amount of light and heat from the Sun varies, depending on how close the planet is to the huge star we call the Sun. It just so happens that the Earth is the correct distance from the Sun to provide us with heat and light that is sufficient to keep us alive. The length of our days and nights are generally determined by how quickly the Earth turns. We have set our complete cycle of day and night at 24 hours. This is the amount of time it takes for our planet to make one complete rotate.

Other planets rotate more quickly or more slowly. Days and nights on the other planets are not the same length as days and nights on Earth. For example, Saturn rotates completely in 11 hours while Pluto makes one rotation in a time equal to 6.4 of our days.

The length of our days and nights is made a bit more complex by the fact that the Earth doesn’t “stand” perfectly upright on its imaginary axis. It tilts just a bit from being perfectly upright. When the “top” of the Earth, the North Pole, is pointing toward the Sun we have summer. Winter arrives when the North Pole is pointing away from the Sun. The length of the sunlit day varies as the Earth moves around the Sun, in addition to rotating on its axis.