Here's why:

* One full rotation: It takes approximately 24 hours for the Earth to complete one full rotation on its axis.

* Day and Night: This rotation causes the cycle of day and night as different parts of the Earth face the Sun.

* Solar Day: We define a day as the time it takes for the Sun to appear in the same position in the sky, which is slightly longer than 24 hours due to the Earth's orbit around the Sun.

Therefore, the Earth's rotation is the fundamental motion that defines the length of a day.