The Reason for the Seasons
Why do some places have winter while others have summer?
We all learn about the four seasons, but have you ever stopped to consider why they are opposite in the Northern and Southern Hemispheres? The answer lies not in the Earth's distance from the sun, but in its **axial tilt**.
The Earth's Tilt: A Simple Explanation
Our planet's axis of rotation is tilted at approximately $23.5^\circ$ relative to its orbital plane around the sun. This tilt is the primary cause of the seasons. As the Earth orbits the sun throughout the year, different parts of the planet receive more direct sunlight at different times.
During a part of the year, the Northern Hemisphere is tilted towards the sun, receiving more direct sunlight and experiencing longer days. This is our summer. At the same time, the Southern Hemisphere is tilted away from the sun, receiving less direct light and experiencing shorter days—their winter. Six months later, the roles are reversed.
Equinoxes and Solstices
- Summer Solstice: The day with the most daylight hours in your hemisphere. It occurs around June 21st in the Northern Hemisphere and December 21st in the Southern Hemisphere.
- Winter Solstice: The day with the fewest daylight hours. It occurs around December 21st in the Northern Hemisphere and June 21st in the Southern Hemisphere.
- Vernal (Spring) and Autumnal Equinoxes: The two days of the year when the sun is directly above the equator, resulting in nearly equal day and night hours across the globe. They occur around March 20th and September 22nd.
While this tool provides a simple approximation based on the three-month calendar, the actual start and end of seasons are tied to these astronomical events. Understanding this simple physics helps us appreciate the beautiful and predictable cycle of life on our planet.