Seasons
Earth's seasons are caused by its 23.5° axial tilt as it orbits the Sun — not by its changing distance from the Sun. When the Northern Hemisphere tilts toward the Sun it receives more direct sunlight (summer); when it tilts away it receives oblique, weaker sunlight (winter).
Why Do Seasons Occur?
As Earth travels along its elliptical orbit, its axis always points toward Polaris. This consistent tilt means each hemisphere alternately leans toward and away from the Sun, changing:
- Angle of incidence of sunlight → more direct = more energy per m²
- Length of daylight → longer days mean more total solar energy received
- Path of the Sun across the sky → higher arc = less atmosphere to penetrate
Key Events in Earth's Orbit
| Event | Approx. Date | Orbital Angle | Solar Declination | NH Season | SH Season |
|---|---|---|---|---|---|
| March Equinox | ~Mar 20 | 0° | 0° | Spring | Autumn |
| June Solstice | ~Jun 21 | 90° | +23.5° | Summer | Winter |
| September Equinox | ~Sep 23 | 180° | 0° | Autumn | Spring |
| December Solstice | ~Dec 21 | 270° | −23.5° | Winter | Summer |
| Perihelion | ~Jan 3 | ~283° | −23.1° | Earth closest to Sun (147.1 M km) | |
| Aphelion | ~Jul 4 | ~103° | +22.8° | Earth farthest from Sun (152.1 M km) | |
Key Formulas
Solved Examples
Example 1 — Solar Altitude at Summer Solstice in London
Find the noon solar altitude at London (φ = 51.5°N) on the June Solstice (δ = +23.5°).
Solution:
α = 90° − |φ − δ| = 90° − |51.5° − 23.5°| = 90° − 28° = 62°
Compare to winter: α = 90° − |51.5° − (−23.5°)| = 90° − 75° = 15°
The Sun is 47° higher in summer — much stronger heating per m².
Example 2 — Daylight Hours at Arctic Circle on Summer Solstice
At φ = 66.5°N, δ = +23.5°:
Solution:
H = (2/15) × arccos(−tan 66.5° × tan 23.5°)
tan 66.5° ≈ 2.30, tan 23.5° ≈ 0.435 → product = −1.00
arccos(−(−1)) = arccos(1) = 0° → H = 0/15 × 2 → wait: product = −tan φ × tan δ = −2.30 × 0.435 = −1.00
arccos(−(−1.00)) = arccos(1.00) = 0° → H = 0 → Actually H = arccos(−1) = 180° → H = (2/15)×180 = 24 hours (Midnight Sun!)
Example 3 — Insolation at Equator vs 60°N in December
December Solstice δ = −23.5°. Compare noon insolation (I₀ = 1361 W/m²).
Solution:
Equator (φ = 0°): α = 90° − |0 − (−23.5°)| = 66.5°; I = 1361 × sin 66.5° ≈ 1249 W/m²
60°N: α = 90° − |60 − (−23.5°)| = 90° − 83.5° = 6.5°; I = 1361 × sin 6.5° ≈ 154 W/m²
The equator receives ~8× more intense winter sunshine than 60°N.
Practice Questions
- Why is Earth's distance from the Sun NOT the main cause of seasons? Use evidence from perihelion/aphelion timing.
- A location at 30°S experiences winter when the Southern Hemisphere tilts away from the Sun. Which orbital position (in degrees) causes this?
- Calculate the noon solar altitude at New York City (40.7°N) during the September equinox.
- Explain why the Arctic experiences 24-hour daylight in summer but 0-hour daylight in winter.
- If Earth's axial tilt were 0°, what would the seasons be like? What if it were 45°?
- Mars has an axial tilt of 25.2°. Compare its seasons to Earth's, and explain why Martian seasons are more extreme in one hemisphere.
Frequently Asked Questions
Why does the Northern Hemisphere have summer while the Southern has winter?
The 23.5° axial tilt means when the Northern Hemisphere leans toward the Sun (around June), it receives direct sunlight and has summer. Simultaneously the Southern Hemisphere leans away, receiving oblique sunlight — winter. Six months later the positions reverse.
Doesn't Earth being closer to the Sun cause warmer temperatures?
No. Earth is actually closest to the Sun (perihelion, ~147.1 M km) in early January — winter in the Northern Hemisphere. The ~3.3% variation in distance changes insolation by about 6.8%, which is much smaller than the effect of tilt. Seasons are primarily caused by tilt, not distance.
What is solar declination?
Solar declination is the latitude at which the Sun is directly overhead at solar noon. It ranges from +23.5° (June Solstice, Tropic of Cancer) to −23.5° (December Solstice, Tropic of Capricorn) and is 0° at both equinoxes.
Why are the solstices called the "longest" and "shortest" days?
At the summer solstice, the Sun traces its longest arc across the sky, spending the most time above the horizon — giving the most hours of daylight. At the winter solstice the arc is shortest. The word "solstice" means "Sun stands still" in Latin, referring to the Sun appearing to pause at its maximum/minimum declination.
What is the difference between an equinox and a solstice?
An equinox occurs when the Sun is directly above the equator (declination = 0°), giving approximately equal day and night worldwide — happens ~March 20 and ~September 23. A solstice is when the Sun reaches its maximum or minimum declination (±23.5°), giving the longest or shortest day — happens ~June 21 and ~December 21.
Why do equatorial regions not have strong seasons?
Near the equator, the Sun is always nearly overhead regardless of Earth's orbital position. The solar altitude only varies from ~66.5° to 90° throughout the year — a small change. Daylight also stays close to 12 hours year-round. So temperature variation is small; equatorial regions experience wet and dry seasons rather than hot and cold ones.
What causes polar day and polar night?
Beyond the Arctic/Antarctic Circles (66.5°N/S), during summer the Sun never sets (polar day / Midnight Sun) because Earth's tilt keeps the pole permanently facing the Sun. During winter the pole faces away and the Sun never rises (polar night). The duration increases toward the poles, reaching 6 months each at the geographic poles.
How does axial tilt affect the intensity of seasons?
Greater tilt means greater variation in solar altitude between summer and winter, and greater variation in day length. If tilt were 0° there would be no seasons. If tilt were 90° (like Uranus), one pole would face the Sun for 6 months then the other — extreme seasons. Earth's 23.5° tilt provides moderate, life-supporting seasons.