What causes our four seasons—spring, summer, winter, fall? Why do our seasons change? You might guess that the seasons are caused by the distance between our planet and the Sun. That’s not it! We explain…
It’s all about astronomy. (Remember, astronomy is at the core of any almanac, which is a “calendar of the heavens.”)
What Causes the Seasons
Our seasons change because of the direction of the Earth’s tilt in relation to its orbit around 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.
Think of the Earth like a tilted, spinning top! In one year, Earth revolves completely around the Sun while rotating on an invisible axis.
Distance Does Not Cause Seasons
It seems logical that the distance between the Earth and Sun would be the reason for the seasons. Earth’s orbit is not a perfect circle. It is elliptical, or slightly oval-shaped. This means there is one point in the orbit where Earth is closest to the Sun, and another where Earth is farthest from the Sun.
So, you might assume that winter occurs when Earth is farthest from the Sun and summer occurs when Earth is closest to the Sun.
However, our planet’s distance from the Sun has little effect on the onset of seasons. In fact, Earth is closest to the Sun arond the winter solstice for our Northern Hemisphere! And, conversely, our planet is farthest away from the Sun around the summer solstice!
Earth’s Tilt Never Changes
Earth’s axis always tilts the same way. The direction of the Earth’s title never changes as Earth orbits the Sun.
What changes is the Earth’s position as it orbits around the Sun. For example, the Northern Hemisphere angles toward the Sun during summer and away from the Sun during winter. When our hemisphere tilts toward the Sun, the sun rays are more direct and we feel more warmth and light. (It’s summer!) There is less solar energy when we tilt away from the Sun. (It’s winter!)
If Earth did not tilt at all, we would not have the beautiful change in seasons!
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.
What if you live close to the equator? There are two tropic zones (see below). As you near the equator and tropic zones, there will less seasonal variation.
In some areas near the tropic zones, it appears there are just the two seasons. In a sense, each of these two seasons can be divided in half so there is a tempered dry season, followed by a more humid one.
More Seasonal Facts
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.)