Earth & The Solar System

Quick Overview: What You Need to Know

Part 1: The Earth, Moon & Sun

1.1 — Day and Night

This is one of those things that seems obvious once you understand it, but is genuinely profound. Day and night are caused by Earth's rotation on its axis. Earth completes one full rotation every 24 hours. As it rotates:

• The half of Earth facing the Sun experiences daylight.
• The half facing away from the Sun experiences night.
• As Earth rotates, different parts of the planet move into daylight and darkness.

Think of it this way: imagine a ball slowly rotating while a torch shines on it. The lit side is in day, the dark side is in night. The torch (Sun) isn't moving around the ball—the ball is spinning.

1.2 — The Seasons

Here's a common misconception that examiners love to catch: many students think seasons are caused by Earth being closer to or farther from the Sun. This is wrong. Seasons are caused by two things working together:

1. Earth's tilted axis: Earth's axis is tilted at 23.5° from the vertical. This tilt never changes—it stays tilted the same way all year.
2. Earth's orbit around the Sun: As Earth orbits, different parts of the planet are tilted towards or away from the Sun at different times of year.

In summer (Northern Hemisphere):

• The Northern Hemisphere is tilted towards the Sun.
• The Sun's rays hit at a steeper angle.
• You receive more solar radiation per square metre.
• Days are longer; nights are shorter.
• Result: warm weather.

In winter (Northern Hemisphere):

• The Northern Hemisphere is tilted away from the Sun.