Our Four Seasons: Winter, Spring, Summer, Autumn

Earth's Tilt, Not Distance from the Sun, Causes Our Seasons

Have you ever heard weather described as being seasonable or unseasonable?

The reason why is because we tend to feel particular weather patterns depending on what season it is. But what are seasons?   

What is a Season?

Different season
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A season is a period of time marked by changes in weather and hours of daylight. There are four seasons within a year: winter, spring, summer, and autumn. 

But while weather is related to the seasons, it doesn't cause them. Earth's seasons are a result of its changing position as it circles the Sun in the course of a year. 

The Sun: Essential to Weather and our Seasons

As the energy source for our planet, the sun plays an essential part in heating the earth. But don't think of Earth as a passive recipient of the sun's energy! On the contrary, it is Earth's motions that determine how this energy is received. Understanding these motions is the first step to learning why our seasons exist and why they bring changes in weather.

How Earth Moves Around the Sun (Earth's Orbit & Axial Tilt)

Earth travels around the Sun on an oval-shaped path known as an orbit. (One trip takes approximately 365 1/4 days to complete, sound familiar?) If it wasn't for Earth's orbit, the same side of the planet would directly face the sun and temperatures would remain either perpetually hot or cold year round.

While journeying around the sun, our planet doesn't "sit" perfectly upright -- rather, it leans 23.5° from its axis (the imaginary vertical line through Earth's center which points toward the North Star). This tilt controls the strength of sunlight reaching Earth's surface. When a region directly faces the sun, sunrays strike the surface head-on, at a 90° angle, delivering concentrated heat. On the contrary, if a region is located slantwise from the sun (for example, like Earth's poles are) the same amount of energy is received, but it intercepts Earth's surface at a shallower angle, resulting in less intense heating. (If Earth's axis wasn't tilted, the poles would also be at 90° angles to the sun's radiation and the entire planet would be heated equally.)

Because it greatly affects the intensity of heating, Earth's tilt -- not its distance from the sun -- is considered to be the primary cause of the 4 seasons.

The Astronomical Seasons

seasons-earth-orbit
Encyclopedia Britannica/UIG/Getty Images

Together, Earth's tilt and trip around the sun create the seasons. But if Earth's motions gradually change at each point along its route, why are there only 4 seasons? The four seasons correspond to four unique points where Earth's axis is tilted (1) at a maximum toward the sun, (2) at a maximum away from the sun, and equidistant from the sun (which happens twice).

  • Summer Solstice: Earth's max tilt gives us max heat

Observed on June 20 or 21 in the Northern Hemisphere, the summer solstice is the date on which Earth's axis points its innermost towards the sun. As a result, the sun's direct rays strike at the Tropic of Cancer (23.5° north latitude) and heat the Northern Hemisphere more efficiently than any other region on Earth. This means that warmer temperatures and more daylight are experienced there. (The opposite applies for the Southern Hemisphere, whose surface is curved farthest away from the Sun.)

More: Consider yourself a connoisseur of summer? Test your knowledge of the season

  • Winter Solstice: Earth leans toward the cold of space

On December 20 or 21, 6 months after the first day of summer, Earth's orientation has totally reversed. Despite Earth being its closest to the sun (yes, this happens in winter -- not summer), its axis now points its farthest away from the sun. This puts the Northern Hemisphere in a poor position for receiving direct sunlight, as it has now migrated its aim at the Tropic of Capricorn (23.5° south latitude). Decreased sunlight means cool temperatures and shorter daylight hours for locations north of the equator and more warmth for those located to its south.

  • Vernal Equinox & Autumnal Equinox

The mid-points between the two opposing solstices are known as the equinoxes. On both equinox dates, the sun’s direct rays strike along the equator (0° latitude) and Earth’s axis is neither tilted toward nor away from the sun. But if Earth's motions are identical for both equinox dates, why are fall and spring two different seasons? They're different because the side of the earth that faces the sun is different on each date. Earth travels eastward around the sun, so on the date of the autumnal equinox (September 22/23), the Northern Hemisphere is transitioning from direct to indirect sunlight (cooling temperatures), whereas on the vernal equinox (March 20/21) it is moving from a position of indirect to direct sunlight (warming temperatures). (Once again, the opposite applies for the Southern Hemisphere.)

No matter what the latitude, the length of daylight experienced on these two days is evenly balanced with the length of night (thus the term "equinox" meaning “equal night.”)

Meet the Meteorological Seasons

We've just explored how astronomy gives us our 4 seasons. But while astronomy explains earth's seasons, the calendar dates it assigns them aren't always the most accurate way of organizing the calendar year into four equal periods of similar temperatures and weather. For this, we look to the "meteorological seasons." When are the meteorological seasons and how do they differ from "regular" winter, spring, summer, and fall? Click the underlined text to learn more.