What causes the seasons? Here are the key astronomical reasons from The Old Farmer’s Almanac. (Remember, astronomy is at the core of any almanac, which is a “calendar of the heavens.”)
First, seasons are determined by the direction of Earth’s tilt in relation to the Sun and the angle of the Sun’s light as it strikes Earth.
- At one end of the axis is the North Pole; at the other, the South Pole.
- The axis is tilted at a 23.5° angle away from the Sun during winter in the Northern Hemisphere; it’s the opposite in the summer.
In one year, Earth revolves completely around the Sun while rotating on an invisible axis, like a tilted, spinning top.
The Equator’s Position
The equator is an imaginary line dividing Earth into the Northern and Southern Hemispheres. On 2 days each year, on or around March 21 and September 23, the Sun is directly above the equator. Here is how the equator relates to the seasons:
- In the Northern Hemisphere, spring starts on the March date, which is called the vernal equinox.
- Fall begins on the September date, which is called the autumnal equinox.
- Summer in the Northern Hemisphere begins on or around June 21, the summer solstice, when the Sun is directly above an imaginary line 23.5° north of the equator called the Tropic of Cancer.
- Winter begins on or around December 21, the winter solstice, when the Sun is above the Tropic of Capricorn, 23.5° south of the equator.
- The seasons are the opposite in the Southern Hemisphere.
Here are some seasonal facts—the long and the short of it, as it were.
- The summer solstice is one of the longest days of the year in the Northern Hemisphere.
- The winter solstice is one of the shortest days of the year in the Northern Hemisphere.
- Daytime and nighttime on the equinoxes are not equal; this is a myth. However, within a few days of each equinox, there is a day with nearly equal daytime and nighttime. (This depends on the latitude.)