Earth's Rotation and Day Types
Overview
This lecture explains the difference between Earth's rotation and its effects, focusing on how days are defined based on Earth's movement, and introduces key astronomical terms related to observing the Sun's motion in the sky.
Earth's Rotation and Day Definition
- Earth's rotation is responsible for the day and night cycle.
- One complete rotation of Earth defines a "day."
- A day based solely on Earth's rotation (not the Sun) is called a "sidereal day."
Sidereal Day vs. Apparent Solar Day
- A sidereal day is one full rotation of Earth relative to distant stars, not the Sun.
- It is difficult to perceive a sidereal day because we are on Earth's surface, moving with it.
- A more practical day for humans is based on the Sun’s position, called the "apparent solar day."
Defining the Apparent Solar Day
- The apparent solar day is measured by tracking the Sun's position in the sky.
- Using sunrise or sunset to define a day is unreliable because their times change throughout the year.
- Noon, when the Sun is highest in the sky, is a consistent reference point.
Zenith, Tropics, and Meridian
- The zenith is the point directly overhead.
- The Sun is only at zenith for people in the tropics, the region near the equator.
- The tropics are defined as regions where the Sun is at zenith at least once a year.
- The meridian is an imaginary line from the north to south pole, passing through your zenith.
- The Sun crosses your meridian at noon, even if it isn't exactly overhead unless you are in the tropics.
The Sun’s Daily Path
- The Sun rises in the east and sets in the west.
- Each day, the Sun's highest point (and crossing of the meridian) marks true noon.
- An apparent solar day is the time it takes for the Sun to cross your meridian twice (one noon to the next).
Key Terms & Definitions
- Rotation — The spinning of Earth on its axis.
- Sidereal Day — The time for one complete rotation of Earth relative to distant stars.
- Apparent Solar Day — The period between two successive noons, based on the Sun's position.
- Zenith — The point directly above an observer.
- Tropics — Earth regions where the Sun is directly overhead once a year.
- Meridian — The line in the sky from north to south through the zenith.
Action Items / Next Steps
- Review and understand the differences between sidereal day and apparent solar day.
- Prepare for the next class discussion on Earth's orbit.