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The Reason for the Seasons
The Reason for the Seasons
Many people think we have seasons because Earth gets closer or farther from the Sun during the year. While it is true that the Earth changes its distance to the sun throughout the year, it doesn’t have the affect you might think. The Earth is closest to the Sun in early January (a point called perihelion), and farthest away from the Sun in early July (a point called aphelion). Perihelion and aphelion play little to no role in our seasons. The actual reason we have seasons is because of the Earth’s tilt and its position in its orbit around the Sun.
Earth’s axis—the imaginary line running from the North Pole to the South Pole—is tilted about 23.5°. As Earth travels around the Sun, it doesn’t wobble back and forth or tilt in different directions during the year. Instead, its axis stays pointed in the same direction in space at all times. This is called axial parallelism.
Imagine the Earth’s axis (the line from the North Pole to the South Pole) as an arrow. That arrow always points toward the same spot in the sky: Polaris, the North Star. Even as Earth moves along its 365-day orbit, the arrow never swivels.
If Earth’s orientation did change throughout the year, the seasons wouldn’t be predictable or consistent. But since the axis stays fixed, we always experience the same pattern of seasons in the same order every year.
Because of Earth’s tilt and position around the Sun:
In summer, your hemisphere is tilted toward the Sun. Sunlight hits the surface of the Earth more directly, like a flashlight aimed straight at the ground. The days are longer, the Sun is higher in the sky, and more energy reaches that part of Earth—so it gets warmer.
In winter, your hemisphere is tilted away from the Sun. Sunlight hits the Earth's surface less directly—arriving at a slant, spreading out, and delivering less energy. Days are shorter, the Sun is lower in the sky, and temperatures drop.
At the same time:
When the Northern Hemisphere has summer, the Southern Hemisphere has winter.
When the Southern Hemisphere has summer, the Northern Hemisphere has winter.
During spring and fall, neither hemisphere is tilted toward or away from the Sun, so both get nearly equal daylight.
Latitudes and the Seasons
Latitudes and the Seasons
Latitude measures how far a location on Earth is from the Equator, from 0° at the Equator to 90° at the poles. Latitude affects how the Sun’s energy hits the Earth, which changes the intensity of the seasons.
Important latitude lines on Earth include: Equator (0°), Tropic of Cancer (23.5° N), Tropic of Capricorn (23.5 S°), Arctic Circle (66.5° N), and Antarctic Circle (66.5° S).
Near the Equator (low latitudes): The Sun is almost always high in the sky. Sunlight is strong and fairly direct year-round, so temperatures stay warm and seasons are less noticeable. Many tropical regions don’t have the typical four seasons—they might just have rainy and dry periods.
Mid-latitudes (like the U.S., Europe, and China): The Sun’s angle changes more over the year. In summer, sunlight hits more directly, making days longer and warmer. In winter, sunlight is slanted, days are shorter, and temperatures drop. This produces distinct seasons: spring, summer, fall, and winter.
High latitudes (near the poles): The Sun stays very low in the sky, even in summer. During winter, the Sun can disappear completely for weeks or months (polar night). Summers have the midnight Sun, when the Sun doesn’t fully set. Because of the poor angle at which sunlight hits the poles, temperatures never get very warm even when the Sun never sets for months at a time. Seasonal changes are extreme, with very cold winters and cool or mild summers.
Special Days
Special Days
Summer Solstice - June 21st
Summer Solstice - June 21st
The summer solstice is the day of the year with the longest daylight and shortest night in a given hemisphere. It marks the official start of summer.
In the Northern Hemisphere, the summer solstice occurs around June 21st. On this day, the North Pole is tilted toward the Sun, so sunlight hits the Northern Hemisphere at its highest angle, creating the longest day of the year. The Sun’s rays hit the Earth most directly (at a 90o angle) at the Tropic of Cancer (23.5° north latitude), which is the farthest north the Sun’s direct rays reach during the year.
Daylight on June 21st at different locations:
Lockport, Illinois (42° N latitude): About 15 hours of sunlight, with the Sun’s rays striking at a maximum angle of roughly 72° above the horizon at solar noon.
North Pole: 24 hours of sunlight (continuous daylight, known as the midnight Sun)
South Pole: 0 hours of sunlight (24 hours of darkness)
Equator: About 12 hours of sunlight, with the Sun high in the sky at an angle of about 66.5° at solar noon.
After the summer solstice, the days gradually start getting shorter as Earth continues its orbit, leading toward fall.
Autumnal or Fall Equinox - September 22nd
Autumnal or Fall Equinox - September 22nd
The fall equinox is one of the two days of the year when day and night are nearly equal in length all over the world. It marks the official start of autumn in the Northern Hemisphere.
The fall equinox occurs around September 22nd each year. On this day, Earth’s axis is not tilted toward or away from the Sun, so sunlight shines directly (at a 90o angle) on the Equator (0° latitude). This means the Sun’s rays hit most directly at the Equator, and day and night are about equal everywhere.
Daylight on September 22nd at different locations:
Lockport, Illinois (42° N latitude): About 12 hours of sunlight, with the Sun’s rays striking at a maximum angle of roughly 48° above the horizon at solar noon.
North Pole: About 12 hours of sunlight
South Pole: About 12 hours of sunlight
Equator: About 12 hours of sunlight, with the Sun directly overhead at 90° at solar noon.
After the fall equinox, the Northern Hemisphere starts receiving less daylight than the Southern Hemisphere, and temperatures gradually cool as autumn progresses toward winter.
Winter Solstice - December 21st
Winter Solstice - December 21st
The winter solstice is the day of the year with the shortest daylight and longest night in a given hemisphere. It marks the official start of winter.
In the Northern Hemisphere, the winter solstice occurs around December 21st. On this day, the North Pole is tilted farthest away from the Sun, so sunlight hits the Northern Hemisphere at its lowest angle, creating the shortest day of the year. The Sun’s rays hit most directly (at a 90o angle) at the Tropic of Capricorn (23.5° south latitude).
Daylight on the winter solstice at different locations:
Lockport, Illinois (42° N latitude): About 9 hours of sunlight, with the Sun’s rays striking at a maximum angle of roughly 24° above the horizon at solar noon.
North Pole: 0 hours of sunlight (24 hours of darkness)
South Pole: 24 hours of sunlight (continuous daylight)
Equator: About 12 hours of sunlight, with the Sun low in the sky at an angle of about 66.5° from the horizon at solar noon.
After the winter solstice, the days gradually get longer as Earth continues its orbit, leading toward spring.
Fun Fact:
Fun Fact:
Christmas, celebrated on December 25, marks the birth of Jesus Christ in the Christian tradition. Interestingly, the exact date of Jesus’ birth is not recorded in the Bible, so early Christians chose a date later. December 25 is close to the winter solstice in the Northern Hemisphere—the shortest day of the year. Ancient peoples celebrated the return of longer days and the “rebirth” of the Sun. Early Christians may have chosen December 25 to align Jesus’ birth with these celebrations of the “return” of the sun.
Vernal or Spring Equinox - March 20th
Vernal or Spring Equinox - March 20th
The spring equinox is one of the two days of the year when day and night are nearly equal in length all over the world. It marks the official start of spring in the Northern Hemisphere.
The spring equinox occurs around March 20th each year. On this day, Earth’s axis is not tilted toward or away from the Sun, so sunlight shines directly (at a 90o angle) on the Equator (0° latitude). This means the Sun’s rays hit most directly at the Equator, and day and night are about equal everywhere.
Daylight on March 20th at different locations:
Lockport, Illinois (42° N latitude): About 12 hours of sunlight, with the Sun’s rays striking at a maximum angle of roughly 48° above the horizon at solar noon.
North Pole: About 12 hours of sunlight
South Pole: About 12 hours of sunlight
Equator: About 12 hours of sunlight, with the Sun directly overhead at 90° at solar noon.
After the spring equinox, the Northern Hemisphere starts receiving more daylight than the Southern Hemisphere, and temperatures gradually warm as spring progresses toward summer.
Remember:
Remember:
Because of the tilt of the Earth, the Southern hemisphere experiences the opposite seasons of the Northern hemisphere. When it is winter in the north, it is summer in the south. When it is spring in the north, it is fall in the south. This is why Santa always wears a Hawaiian shirt during Christmas in Australia!
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