Day and night are caused by Earth's spinning motion. The "day" side of Earth faces the sun, and the "night" side faces away from the sun. As Earth spins, part of it moves from the night side to the day side. People see the sun rise in the east. At the same time, part of it moves from the day side to the night side. People see the sun set in the west.

Image to right: Earth makes one complete turn each day. When it is morning for you, your part of the world is turning out of the darkness into the light. When it is evening where you live, your part of the world is turning from the light and moving into darkness. Then it will be night. Credit: World Book illustration by James Conahan Each day begins at midnight. In most countries, the day is divided into two parts of 12 hours each. The hours from midnight to noon are the a.m. hours. The hours from noon to midnight are the p.m. hours. The letters a.m. stand for ante meridian, which means before noon. The letters p.m. stand for post meridian, which means after noon. Night is really a part of the whole day.

In addition to spinning around once a day, Earth travels around the sun once a year. This journey gives us the seasons. The lengths of days and nights are different in each season. This difference is caused by the tilt of Earth's axis. Earth's axis is an imaginary line that connects the North and South poles. When the North Pole tilts toward the sun, the northern part of Earth has summer, with long days and short nights. After six months, Earth has moved to the other side of the sun. The North Pole now tilts away from the sun, and the northern part of Earth has short days and long nights. How to cite this article: To cite this article, World Book recommends the following format: "Day and night." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.