Tilted!

Tilted!

Quick quiz: Why does Earth have seasons?

If you said "because Earth is closer to the Sun in summer," you're not alone—but you're also not correct! This is one of the most common misconceptions in science.

Here's a surprising fact: Earth is actually CLOSEST to the Sun in early January (winter in North Carolina) and FARTHEST in early July (summer)!

So if distance doesn't cause seasons, what does? In this lesson, you'll discover the true cause of seasons.

Essential Question: What causes Earth to have seasons?

The Real Cause: Earth's Axial Tilt

Earth is Tilted!

Earth doesn't spin perfectly upright. Instead, our planet is tilted at 23.5 degrees from vertical. This tilt, called axial tilt (or obliquity), is the PRIMARY cause of seasons.

Key concept: The tilt itself never changes direction—Earth's axis always points toward the same spot in space (near the North Star, Polaris).

As Earth orbits the Sun over the course of a year:
• Sometimes the Northern Hemisphere tilts TOWARD the Sun → Northern summer
• Sometimes the Northern Hemisphere tilts AWAY from the Sun → Northern winter

Why Tilt Matters

The tilt affects TWO critical things: 1. The angle at which sunlight hits the surface 2. How many hours of daylight each day has

Both of these determine how much solar energy reaches a location—and that's what creates warm summers and cold winters.

How Sunlight Angle Affects Temperature

The Flashlight Experiment

Imagine shining a flashlight straight down onto a table versus shining it at an angle.
• Straight down: Light is concentrated in a small circle = BRIGHT and INTENSE
• At an angle: Same light spreads over a larger oval = DIMMER and LESS INTENSE

This is exactly what happens with sunlight:
• Direct sunlight (summer): Rays hit at a steep angle, concentrating energy = WARMER
• Angled sunlight (winter): Rays hit at a shallow angle, spreading energy = COOLER

The Math of Sunlight (North Carolina example)

Summer Solstice (around June 21):
• Sun is high in the sky at noon
• Rays hit at about 75° angle
• Energy is concentrated
• Long days (~14.5 hours of daylight)

Winter Solstice (around December 21):
• Sun stays low in the sky
• Rays hit at about 28° angle
• Energy spreads over 2.5x more area!
• Short days (~9.5 hours of daylight)

Solstices and Equinoxes

The Four Key Positions

As Earth orbits the Sun over a year, it passes through four important positions:

Summer Solstice (~June 21)

Winter Solstice (~December 21)

Equinoxes (~March 20 & September 22)

Opposite Seasons in Hemispheres

Critical concept: When it's summer in the Northern Hemisphere, it's winter in the Southern Hemisphere—and vice versa!

When the North Pole tilts toward the Sun:
• Northern Hemisphere gets more direct sunlight → Summer
• Southern Hemisphere gets less direct sunlight → Winter

This is why:
• Christmas in Australia happens during hot summer weather
• July is the coldest month in Argentina
• Ski season in Chile runs from June to September

Daylight Hours in North Carolina

Summary

Key Concepts Review

The Cause of Seasons: Earth's 23.5° axial tilt—NOT distance from the Sun

Two Effects of Tilt: 1. Changes the ANGLE of sunlight (direct vs. spread out) 2. Changes the NUMBER of daylight hours

Four Key Orbital Positions:
• Summer Solstice (~June 21): Longest day, summer begins, North tilts toward Sun
• Winter Solstice (~Dec 21): Shortest day, winter begins, North tilts away from Sun
• Spring Equinox (~March 20): Equal day/night, spring begins
• Fall Equinox (~Sept 22): Equal day/night, fall begins

Hemisphere Effects:
• Seasons are OPPOSITE in Northern and Southern Hemispheres
• Equator experiences little seasonal change
• Poles experience extreme seasonal daylight differences