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Humans use day and night as signals for our daily activities.
When the sun comes up in the morning,
we get out of bed and go to school, work, or play.
When night arrives, we end our activities and go to sleep.
We use the cycle of the day
as a way to identify when we should be active
and when we should rest.
But what causes the change from day to night?
The Earth is rotating as it orbits around the sun.
Every 24 hours, it rotates one time.
As it rotates,
one half of the Earth faces the sun
and one half faces away from the sun.
The half facing the sun receives sunlight and has daytime.
The half of the Earth facing away from the sun experiences night.
Since the Earth is constantly rotating,
the part of the Earth that is illuminated by the sun
is always changing.
Take a look.
Here is a model of the Earth and sun.
(In reality,
the Earth is much smaller than the sun and farther away from it,
but this model, even though it's not to scale,
will help us review the cause of day and night.)
The United States and Japan
are both marked in red on our model of the Earth.
Now, watch what happens as the Earth rotates.
When it is day in the United States, it is night in Japan.
That is because the United States is on the side of the Earth
directly facing the sun and Japan is not.
But as the Earth rotates, the United States moves away from the sun,
and other parts of the globe are lighted by it.
In roughly half a day,
the Earth has rotated far enough that Japan experiences day
and the United States has night.
As the Earth continues to spin,
the part of the globe where Japan is located
will turn away from the sun and return to nighttime,
and other parts of Earth will turn toward the sun.
This constant rotating of the Earth
is what causes us to have day and night.