Earth's seasons

1. Structure of Earth

Earth has three main internal layers.

Crust

• Outermost layer where life exists.

• Thickness: 2–40 miles (3–64 km).

• Thinnest: under oceans

• Thickest: under mountains

• Made mainly of granite and basalt.

• Major elements:

  • Oxygen

  • Silicon

  • Aluminum

  • Iron

  • Calcium

  • Sodium

  • Potassium

• Divided into tectonic plates that float on the mantle.

• Plate movement causes earthquakes and volcanic activity.

• Oceans cover ~70% of the crust.

Mantle

• Located below the crust.

• Depth: about 1,800 miles (2,900 km).

• Made of silicate rocks rich in magnesium and iron.

• Divided into:

  • Upper mantle

  • Lower mantle

Important process:

Convection currents

• Hot material rises

• Cooler material sinks

• Drives tectonic plate movement

• Causes hotspot volcanism

Core

Earth’s innermost layer.

Two parts:

Inner Core

• Solid

• Radius: about 760 miles

Outer Core

• Liquid

• Thickness: about 1,350 miles

• Made mainly of iron and nickel

Movement of liquid metal in the outer core produces Earth’s magnetic field.

Magnetic poles do not align perfectly with geographic poles.

2. Atmosphere Composition

Main gases:

Trace gases:

• Carbon dioxide

• Methane

• Neon

• Helium

• Hydrogen

• Ozone

• Nitrous oxide

• Krypton

• Xenon

• Carbon monoxide

• Ammonia

3. Earth’s Rotation and Revolution

Rotation

• Earth spins on its axis.

• Time: 24 hours

• Causes day and night.

Revolution

• Earth orbits the Sun.

• Time: ~365 days

• Orbit shape: ellipse.

Average distance from Sun:

• 93 million miles

• 150 million km

Closest distance: 91.4 million miles
Farthest distance: 94.5 million miles

Distance difference: 3.1 million miles.

4. Earth’s Axial Tilt

Earth’s axis is tilted 23.5°.

This tilt is the main cause of seasons, not the distance from the Sun.

When a hemisphere tilts toward the Sun:

• receives more direct sunlight

• experiences summer

When tilted away from the Sun:

• receives less sunlight

• experiences winter

5. The Four Seasons

Summer

• Longest days

• Highest temperatures

• Strong sunlight

• More evaporation

Autumn (Fall)

• Temperatures decrease

• Leaves change color and fall

• Harvest season

Winter

• Short days, long nights

• Cold temperatures

• Snow in many regions

• Some animals hibernate or migrate

Spring

• Temperatures rise

• Plants grow again

• Animals return from migration

Northern Hemisphere seasonal start:

Southern Hemisphere seasons are reversed.

6. Solstices

Occur when Earth's tilt is maximum toward or away from the Sun.

Summer Solstice

• Around June 20–21

• Longest day in Northern Hemisphere

Winter Solstice

• Around December 21–22

• Shortest day in Northern Hemisphere

7. Equinoxes

Occurs when the Sun is directly above the equator.

Day and night are approximately equal.

Vernal (Spring) Equinox

• March 20–21

Autumnal (Fall) Equinox

• September 22–23

8. Latitude and Solar Energy

Latitude lines

• Run parallel to the equator.

• Measure north–south position.

Key values:

• Equator: 0°

• Poles: 90°

Sunlight distribution:

Reasons:

• Sunlight spreads over larger area near poles.

• Rays travel through more atmosphere.

9. Solar Radiation and Temperature

When sunlight is direct:

• concentrated energy

• higher temperature

When sunlight is indirect:

• spread over larger surface

• lower temperature

This variation produces seasonal temperature changes.

10. Key Concept Summary

• Earth has crust, mantle, and core.

• The atmosphere contains mostly nitrogen and oxygen.

• Earth’s 23.5° axial tilt causes seasons.

• Solstices and equinoxes mark seasonal transitions.

• Latitude affects solar energy received.

• Direct sunlight near the equator causes warmer climates.