6.1.1 The Earth
Basic Facts about Earth
The Earth is a planet that rotates on its axis.
- The axis of the Earth is tilted at an angle of about 66° to its orbital plane.
- The Earth completes one full rotation in approximately 24 hours.
- This rotation causes the cycle of day and night.
Explanation of Day and Night
- The Earth rotates from west to east on its axis once every 24 hours.
- The side of the Earth facing the Sun experiences day, while the side facing away experiences night.
- As Earth continues to rotate, the same regions alternately move into and out of sunlight.
The imaginary boundary separating the lighted part of the Earth from the dark side is called the circle of illumination.
Revolution of the Earth and Seasons
The Earth orbits the Sun once every 365 days. This revolution, combined with the tilt of its axis, causes the periodic cycle of the seasons.
- As the Earth orbits the Sun, each hemisphere alternately tilts toward or away from the Sun.
- The hemisphere tilted toward the Sun experiences summer (direct solar energy).
- The hemisphere tilted away experiences winter (indirect solar energy).
- Countries near the equator remain hotter throughout the year.
The average orbital speed of Earth is given by:
v = 2πr / T
- where
r= average orbital radius, andT= orbital period (1 year).
Variation in Earth's Tilt and Its Effects
- The Earth's axis tilt slowly shifts between 22° and 25° over about 41,000 years.
- Greater tilt → more extreme seasons (hotter summers, colder winters).
- Smaller tilt → milder seasons (warmer winters, cooler summers).
The Moon and Its Motion
The Moon orbits the Earth in about one month. This causes the periodic nature of the phases of the Moon.
| Phase | Description |
|---|---|
| New Moon | The Moon is between the Earth and Sun; the unlit side faces Earth. Not visible. |
| Waxing Crescent | A thin crescent of the Moon becomes visible and increases in size (“waxes”). |
| First Quarter | Half of the Moon’s lit portion is visible; occurs about a week after New Moon. |
| Waxing Gibbous | More than half of the Moon’s visible surface is lit and continues increasing. |
| Full Moon | The Moon is on the opposite side of Earth from the Sun; the full lit face is visible. |
| Waning Gibbous | More than half the Moon is lit, but the illuminated area decreases (“wanes”). |
| Third Quarter | Half of the lit portion is visible again after the waning gibbous phase. |
| Waning Crescent | Less than half the Moon’s face is illuminated before returning to New Moon. |
Key Astronomical Facts
- The circle of illumination separates day and night on Earth.
- One complete rotation gives day and night.
- One complete revolution gives the cycle of seasons.
- The axial tilt affects the amount of solar radiation received by each hemisphere.
Exam Focus
- Explain why Earth experiences day and night — due to its rotation on a tilted axis.
- Explain how seasons are caused by the tilt and revolution of Earth around the Sun.
- List the main phases of the Moon in correct order.
- Be able to define rotation, revolution, and circle of illumination.
Explore more at Smart Exam Resources.