3.1.4: Seasons

How Earth’s Tilt Creates the Seasons

Figure \(\PageIndex{1}\) shows the tilt of the Earth and its impact on sun angle are the reasons the Northern and Southern Hemispheres experience opposite seasons. Summer occurs when a hemisphere is tipped toward the Sun, while winter occurs when it is tipped away. This orientation also determines day length. Together, sun angle and day length dictate the total amount of solar radiation striking the surface. Following the Earth through its orbit reveals how these conditions shift for both hemispheres.

Figure \(\PageIndex{1}\): Earth's Tilt and it's impact on sun angles . (CC BY 4.0; Image created by OpenAI ChatGPT. (2026))

Figure \(\PageIndex{1}\) is an educational infographic titled “Earth’s Tilt and Its Impact on Sun Angle, Seasons, and Day Length.” The diagram explains how Earth’s 23.5° axial tilt and annual revolution around the Sun create the seasons, change the angle of sunlight, and alter day length.

At the center is a bright yellow Sun. Around it, four Earths are positioned along a dashed orbital path, each tilted at the same 23.5° angle.

• March Equinox (March 21) at the top: The Sun is directly overhead at the Equator (0°). Day and night are each 12 hours long everywhere on Earth.
• June Solstice (June 21) on the left: The Sun is directly overhead at the Tropic of Cancer (23.5° N). This is the longest day of the year in the Northern Hemisphere and the shortest in the Southern Hemisphere. Areas north of the Arctic Circle experience 24 hours of daylight, while areas south of the Antarctic Circle experience 24 hours of darkness.
• September Equinox (September 23) at the bottom: The Sun is again directly overhead at the Equator. Day and night are equal worldwide.
• December Solstice (December 22) on the right: The Sun is directly overhead at the Tropic of Capricorn (23.5° S). This is the longest day of the year in the Southern Hemisphere and the shortest in the Northern Hemisphere. Areas south of the Antarctic Circle experience 24 hours of daylight, while areas north of the Arctic Circle experience 24 hours of darkness.

A legend identifies Earth’s axis, orbital path, and the North and South Poles. An inset labeled “Earth’s Axial Tilt” illustrates the 23.5° tilt relative to a perpendicular line.

Across the bottom, three panels explain the effects of Earth's tilt:

• Sun Angle: More direct sunlight delivers more energy to a smaller area, while lower-angle sunlight spreads energy over a larger area.
• Day Length: Longer days provide more daylight hours; shorter days provide fewer.
• Total Solar Radiation: Seasonal energy received at Earth’s surface depends on the combination of sun angle and day length.

A final panel labeled “Important Latitudes” identifies the 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), along with their significance during solstices and equinoxes.

June Solstice (On or about June 21)

During this position in the orbit, the North Pole tips into the Sun while the South Pole tips away.

Subsolar Point and Tangent Rays: The Sun is directly overhead at 23.5⁰ N latitude (the Tropic of Cancer). Tangent rays strike precisely at the Arctic and Antarctic Circles (66.5⁰ N and 66.5⁰ S). 

Northern Hemisphere Situation (Summer Solstice): This marks the longest day of the year for places north of the Equator. The high sun angle brings peak summer heating. Regions poleward of the Arctic Circle (66.5^{​​​​​​​0} N) experience a 24-hour polar day.

Southern Hemisphere Situation (Winter Solstice): Conversely, this is the shortest day of the year south of the Equator, ushering in the astronomical winter. Regions poleward of the Antarctic Circle (66.5^{​​​​​​​0} S) are plunged into a 24-hour polar night.

September Equinox (On or about September 23)

As the Earth moves along its orbit, the poles reach a position where they point neither toward nor away from the Sun.

Subsolar Point and Tangent Rays: The Sun is directly overhead at the Equator (0^{​​​​​​​0}). Tangent rays strike exactly at the North and South Poles.

Northern Hemisphere Situation (Autumnal Equinox): This marks the transition into autumn, with sun angles and temperatures steadily decreasing.

Southern Hemisphere Situation (Vernal/Spring Equinox): This marks the arrival of spring, with sun angles and temperatures beginning to climb.

Global Day Length: Because the circle of illumination cuts every latitude line exactly in half, all places across both hemispheres experience an equal 12 hours of daylight and 12 hours of darkness.

December Solstice (On or about December 22)

The Earth's orientation is now completely reversed from June: the North Pole faces away from the Sun, and the South Pole faces directly into it.

Subsolar Point and Tangent Rays: The Sun lies directly over 23.5^{​​​​​​​0} S latitude (the Tropic of Capricorn). Tangent rays strike the Arctic and Antarctic circles once again.

Northern Hemisphere Situation (Winter Solstice): This brings the shortest day of the year and the lowest sun angles, marking the official start of winter. Regions poleward of the Arctic Circle (66.5^{​​​​​​​0} N) experience the 24-hour polar night.

Southern Hemisphere Situation (Summer Solstice): This is the longest day of the year for the Southern Hemisphere, bringing peak solar heating and the height of summer. Regions poleward of the Antarctic Circle (66.5^{​​​​​​​0} S) enjoy a 24-hour polar day.

March Equinox (On or about March 21)

The Earth reaches the opposite side of its orbit, returning to a balanced planetary orientation where the poles point neither toward nor away from the Sun.

Subsolar Point and Tangent Rays: The perpendicular rays of the Sun strike the Equator (0^{​​​​​​​0}) at noon, and tangent rays meet the surface at the North and South poles.

Northern Hemisphere Situation (Vernal/Spring Equinox): Transitioning out of winter, the hemisphere experiences the official arrival of spring.

Southern Hemisphere Situation (Autumnal Equinox): Transitioning out of summer, the hemisphere moves into autumn.

Global Day Length: Just like the September equinox, the circle of illumination bisects all latitudes, giving every location on Earth an equal 12-hour day and 12-hour night.

Summary of Annual Energy Distribution Over the course of a year, the Sun's rays are only ever perpendicular (90^{​​​​​​​0}) to the surface between the Tropics of Cancer and Capricorn. Consequently, locations between 23.5^{​​​​​​​0} N and 23.5^{​​​​​​​0} S experience two days each year when the Sun is directly overhead at noon. In this tropical zone, the noon sun angle remains consistently high throughout the year. In contrast, a much larger range in annual sun angles occurs poleward of these latitudes in both the Northern and Southern Hemispheres. Because the variation in sun angle increases toward the poles, the mid- and high-latitude regions experience the greatest seasonal fluctuations in surface heating.