Overview
This lecture explains the true causes of Earth's seasons, clarifying common misconceptions and highlighting the roles of daylight hours and sunlight angle.
Common Misconceptions About Seasons
- Many people mistakenly believe Earth's distance from the Sun causes seasons.
- Earth's orbit is nearly circular, so its distance from the Sun changes very little throughout the year.
- Earth is actually closest to the Sun in early January (northern hemisphere winter), not during summer.
- If distance caused seasons, both hemispheres would have the same season at the same time, which is not the case.
True Causes of Earth's Seasons
- Seasons result primarily from two factors: the tilt of Earth's axis and varying daylight hours.
- Earth's axis is tilted about 23.5° relative to the plane of its orbit around the Sun.
- This tilt causes one hemisphere to tilt toward the Sun (receiving more direct sunlight) while the other tilts away.
- Number of daylight hours varies with both latitude and time of year, affecting temperature.
Variation of Daylight and Sunlight Angle
- In the northern hemisphere mid-latitudes, daylight peaks at the summer solstice (late June) and is lowest at the winter solstice (late December).
- On equinoxes (spring and fall), both hemispheres receive about 12 hours of daylight.
- At the equator, daylight hours and sunlight angle remain nearly constant throughout the year, leading to minimal seasonal temperature change.
- More direct sunlight (high angle) during summer leads to higher temperatures; less direct sunlight (low angle) during winter leads to lower temperatures.
Temperature and Seasonality Differences by Latitude
- Mid-latitudes experience significant temperature differences and distinct seasons due to changing daylight and sunlight angles.
- At the equator, minimal variation in sunlight angle and daylight hours means little seasonal temperature change.
- Southern hemisphere seasons are opposite those in the northern hemisphere.
Key Terms & Definitions
- Axis Tilt — The 23.5° angle between Earth's rotation axis and its orbital plane around the Sun.
- Solstice — When the Sun is at its most northern or southern point, resulting in maximum or minimum daylight hours.
- Equinox — When day and night are approximately equal length everywhere on Earth, occurring in spring and fall.
- Direct Sunlight — Sunlight hitting Earth's surface at a high angle, resulting in concentrated energy and higher temperatures.
- Elliptical Orbit — An oval-shaped orbit; Earth's is nearly circular, not affecting seasons.
Action Items / Next Steps
- Complete the seasons lecture tutorial.
- Review related simulations or diagrams showing Earth’s tilt and orbit.