Seasons

Big Idea

 Seasons happen because Earth is tilted on its axis AND Earth revolves (orbits) around the Sun.

 The tilt changes the angle and amount of sunlight a place receives during the year.

Key Terms

 Axis: an imaginary line through Earth from the North Pole to the South Pole.

 Rotation: Earth spinning on its axis (causes day and night).

 Revolution / Orbit: Earth moving around the Sun (about one year).

 Hemisphere: half of Earth (Northern Hemisphere or Southern Hemisphere).

 Tilt: Earth is not upright; its axis leans (about 23.5°).

How Earth’s Tilt Creates Seasons

 For part of the year, one hemisphere is tilted toward the Sun.

 The hemisphere tilted toward the Sun gets more direct sunlight (light hits more straight-on).

 More direct sunlight = warmer temperatures + longer days (summer).

 The hemisphere tilted away gets more indirect sunlight (light hits at an angle).

 More indirect sunlight = cooler temperatures + shorter days (winter).

 Seasons are opposite in the Northern and Southern Hemispheres.

Northern Hemisphere Example (from the video)

 June: Northern Hemisphere tilted toward the Sun → more direct sunlight → summer (warm, long

days).

 December: Northern Hemisphere tilted away from the Sun → more indirect sunlight → winter (cool,

short days).

Spring and Autumn

 In spring and autumn, a hemisphere is neither tilted strongly toward nor away from the Sun.

 Sunlight is more balanced, so temperatures are more moderate (not too hot, not too cold).

Why the Angle of Sunlight Matters (Flashlight Demo)

 Shine a flashlight straight down on a table: the light is smaller and brighter (more concentrated) →

like direct sunlight in summer.

 Shine it at an angle: the light spreads out and looks dimmer (less concentrated) → like indirect

sunlight in winter.

 When the same energy is spread over a bigger area, it warms the surface less.

Using a Graph to Show Seasons

 A temperature graph over 12 months often shows low temperatures in winter months and higher

temperatures in summer months.

 This matches when a hemisphere receives more indirect vs more direct sunlight.

Common Misconception (Important!)

 Seasons are NOT caused by Earth being closer to or farther from the Sun.

 They are caused mainly by Earth’s tilt changing the sunlight angle and day length.

Quick Revision Questions

1. 1) What two Earth motions/conditions cause seasons?

2. 2) What does ‘direct sunlight’ mean?

3. 3) Why are days longer in summer (in your hemisphere)?

4. 4) What season is it in the Southern Hemisphere when it is winter in the Northern Hemisphere?

5. 5) Why does sunlight at an angle warm the ground less?

Answers

 1) Earth’s tilt on its axis + Earth’s revolution (orbit) around the Sun.

 2) Sunlight hitting more straight-on, so the energy is concentrated on a smaller area.

 3) Your hemisphere is tilted toward the Sun, so the Sun stays above the horizon longer.

 4) Summer.

 5) The same energy is spread over a larger area, so it is less intense.

Additional Notes: Orbit Shape, Daylight Hours & Solstices

1) Earth’s Orbit Shape (Ellipse) — But Almost a Circle

 Earth’s orbit is slightly elliptical (oval-shaped), but it is very close to a circle.

 Some diagrams exaggerate how “stretched” the orbit is.

2) Distance From the Sun Is NOT the Main Cause of Seasons

 Earth is actually closer to the Sun in early January than in July (by about 5 million km).

 So winter in the Northern Hemisphere is not because we are farther from the Sun.

3) Daylight Hours Change With Season (and With Latitude)

 Earth’s axis stays pointed in nearly the same direction in space as Earth orbits the Sun.

 Because of Earth’s tilt + orbit, the number of daylight hours changes through the year: more

daylight in summer, less in winter.

 Higher latitudes (closer to the poles) have bigger changes in daylight hours than places closer to

the equator.

 Near the North Pole in northern summer, the Sun can stay above the horizon all day (the “midnight

Sun”).

Example from the video (to show the idea)

 Around 40° north latitude, summer can have about 15 hours of daylight and 9 hours of darkness.

 Farther north, places can get even more daylight in summer (for example, Alaska can get around

19 hours on some summer days).

4) Why 24 Hours of Sun Still Doesn’t Make the Poles Hot

 Temperature depends on both (a) daylight hours AND (b) how high the Sun is in the sky (the

sunlight angle).

 When the Sun is low in the sky, sunlight is spread over a larger area and is less intense, so it warms

the ground less.

 That’s why polar regions can stay cold even with long summer daylight.

5) Solstices (Key Dates Mentioned)

 The day when the Sun reaches its highest noon position for the Northern Hemisphere is around the

June solstice (often June 20–21).

 The opposite happens around the December solstice (often December 21–22): summer in the

Southern Hemisphere and winter in the Northern Hemisphere.

Extra Idea: Where Do We Get Four Distinct Seasons?

 Four distinct seasons are most noticeable in many temperate (middle-latitude) regions.

 Many places near the equator have smaller temperature changes and may have wet/dry seasons

instead.