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Day and night happen because Earth rotates, or spins, on its axis. At any moment, the half of Earth facing the Sun is in daylight, while the half turned away is in darkness. This daily pattern affects temperature, weather, biological rhythms, and human activity. Understanding day and night helps explain many other Earth science ideas, including time zones and the apparent motion of the Sun across the sky.
Earth completes one full rotation in about 24 hours, so most places experience a regular cycle of sunrise, daytime, sunset, and nighttime. The boundary between the lit half and the dark half is called the terminator. As Earth spins from west to east, the Sun appears to rise in the east and set in the west. This rotation, not the Sun moving around Earth each day, is the main reason we experience day and night.
Key Facts
- Earth rotates once in about 24 h.
- Day occurs on the side of Earth facing the Sun.
- Night occurs on the side of Earth facing away from the Sun.
- Earth rotates from west to east.
- Angular speed of Earth = 360 degrees / 24 h = 15 degrees per hour.
- Time difference = longitude difference / 15 degrees per hour.
Vocabulary
- Rotation
- Rotation is the spinning of Earth around its axis.
- Axis
- The axis is the imaginary line through Earth from the North Pole to the South Pole that Earth spins around.
- Terminator
- The terminator is the moving boundary between the daylight side and the nighttime side of Earth.
- Longitude
- Longitude is the east-west position of a place on Earth measured in degrees.
- Time zone
- A time zone is a region where people use the same standard time based mainly on longitude.
Common Mistakes to Avoid
- Thinking day and night are caused by Earth orbiting the Sun, which is wrong because orbit explains the year while rotation explains the daily cycle.
- Believing the Sun moves around Earth each day, which is wrong because the Sun only appears to move across the sky as Earth spins.
- Assuming all places on Earth have exactly 12 hours of day and 12 hours of night every day, which is wrong because day length changes with latitude and season.
- Confusing rotation with revolution, which is wrong because rotation is Earth spinning on its axis and revolution is Earth traveling around the Sun.
Practice Questions
- 1 Earth rotates 360 degrees in 24 hours. How many degrees does Earth rotate in 3 hours?
- 2 Two cities differ by 45 degrees of longitude. What is the time difference between them if Earth rotates 15 degrees each hour?
- 3 A student says nighttime happens because clouds block sunlight. Explain why this idea is incorrect using Earth's rotation and the positions of Earth and the Sun.