Overview of Seasonality
Seasonality refers to the periodic variations in climate, weather, and daylight duration throughout the year. On Earth, this phenomenon is primarily driven by the interaction between the tilt of the Earth’s axis and its revolution (orbit) around the Sun.
Key Mechanisms
- Axial Tilt: The Earth’s axis is tilted at an angle of approximately $23\frac{1}{2}^{\circ}$. This tilt is constant, but its orientation relative to the Sun changes as the Earth orbits.
- Sun’s Vertical Rays: Because of this tilt, the point where the Sun’s rays strike the Earth directly at a $90^{\circ}$ angle (the “subsolar point”) shifts throughout the year between the Tropic of Cancer ($23\frac{1}{2}^{\circ}$ N) and the Tropic of Capricorn ($23\frac{1}{2}^{\circ}$ S).
- Day Length and Intensity: The tilt causes different latitudes to receive varying intensities of sunlight and lengths of daylight. If the axis were not tilted, every location on Earth would receive 12 hours of daylight every day of the year.
Seasonal Milestones
The workbook defines four critical points in Earth’s orbit:
| Event | Approximate Date | Solar Position |
| March Equinox | March 21–22 | Sun is vertical at the Equator |
| June Solstice | June 21–22 | Sun is vertical at the Tropic of Cancer ($23\frac{1}{2}^{\circ}$ N) |
| September Equinox | Sept 22–23 | Sun is vertical at the Equator |
| December Solstice | Dec 21–22 | Sun is vertical at the Tropic of Capricorn ($23\frac{1}{2}^{\circ}$ S) |
Important Considerations
- Winter vs. Distance: Even though the Earth is physically closer to the Sun in January (perihelion), the Northern Hemisphere experiences winter during this time because it is tilted away from the Sun.
- Temperature Lag: The coldest and hottest days often occur weeks after the winter and summer solstices, respectively. This is due to the thermal inertia of the atmosphere and the Earth’s surface, which requires time to absorb or release energy before reaching peak temperatures.
- Local Factors: While the tilt determines general seasonal patterns, local terrain (such as whether a slope is south-facing or north-facing in the Northern Hemisphere) can significantly alter solar exposure and local temperatures.