Understanding Earth’s Seasons: Causes, Motions, and Key Terms

Lesson Overview

• What Is Earth's Rotation and Revolution?
• Why Does Earth Move Around the Sun?
• What Causes Earth's Seasons?
• Why Are Seasons Opposite in Each Hemisphere?
• Key Seasonal Terms: Solstices and Equinoxes
• Common Misconceptions About Seasons

Have you ever wondered why we experience different seasons throughout the year? In this lesson, you'll discover the fascinating science behind Earth's seasonal changes. We'll explore how Earth's movements, rotation on its axis, and revolution around the Sun. You'll learn important terms like solstice and equinox, and clear up common misconceptions about why seasons happen. 

What Is Earth's Rotation and Revolution?

• Rotation is the Earth spinning on its axis.
  
  • It takes 24 hours (one day) for Earth to complete a rotation.
  • Rotation causes day and night. The Sun appears to rise in the east and set in the west due to Earth's rotation.
• Revolution is the movement of Earth around the Sun.
  
  • It takes about 365 days (one year) for Earth to complete one revolution around the Sun.
  • The path Earth takes around the Sun is called an orbit.

Why Does Earth Move Around the Sun?

• Earth stays in its orbit because of gravity, the force pulling objects toward each other.
  • Earth's orbit results from the gravitational pull between Earth and the Sun.
  • Gravity's strength depends on the mass and distance between objects.

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earths oceans Chapter 5 Section 1

What Causes Earth's Seasons?

• Earth's axis is tilted approximately 23.5 degrees from vertical.
  • Earth's tilt remains constant as Earth revolves around the Sun.
  • Seasons occur due to Earth's tilt, not because Earth is closer or farther from the Sun.

How the Tilt Causes Seasons:

• The hemisphere tilted toward the Sun experiences:
  
  • More direct sunlight (stronger and warmer).
  • Longer daylight hours (warmer days).
  • Spring and summer seasons occur when your hemisphere is tilted toward the Sun.
• The hemisphere tilted away from the Sun experiences:
  
  • Less direct sunlight (weaker and cooler).
  • Shorter daylight hours (cooler days).
  • Autumn and winter seasons occur when your hemisphere is tilted away from the Sun.
• Regions near the poles receive less direct sunlight compared to regions near the equator, which receive direct sunlight year-round.
  

Why Are Seasons Opposite in Each Hemisphere?

• The Northern and Southern Hemispheres experience opposite seasons at the same time.
  • When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere.
  • For example, December is winter in the United States but summer in Australia.

Key Seasonal Terms: Solstices and Equinoxes

Solstices:

• A solstice occurs when Earth's axis is most tilted toward or away from the Sun.
• Solstices happen twice a year:
  • June solstice (around June 21) marks the start of summer in the Northern Hemisphere and winter in the Southern Hemisphere.
  • December solstice (around December 21) marks the start of winter in the Northern Hemisphere and summer in the Southern Hemisphere.

Equinoxes:

• An equinox occurs when Earth's axis is not tilted toward or away from the Sun. This causes equal lengths of day and night worldwide.
• Equinoxes occur twice a year:
  • March equinox (around March 20) marks spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
  • September equinox (around September 22) marks autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

Common Misconceptions About Seasons

• Seasons are not caused by Earth's changing distance from the Sun.
• Earth's distance to the Sun changes slightly, but this does not significantly affect seasons.
• The main reason for seasonal changes is always Earth's axial tilt and its revolution around the Sun.

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Earth's Seasons - Ch 20 Lesson 1