11.3: How Does Earth’s Tilt Affect Surface Warming?

What does all this mean? As Earth orbits the Sun with its equatorial plane tilted at an angle, the Sun’s rays—which you can imagine as arrows—strike it at an angle. Some parts of Earth heat more at certain times of the year—those that receive more direct sunlight—and some parts heat less—where the Sun’s rays arrive at an angle. When the Northern Hemisphere is tilted toward the Sun and the intensity of sunlight is high, we experience summer. When the Northern Hemisphere is tilted away from the Sun and the intensity of sunlight is low, we experience winter. It’s this phenomenon—the tilt of Earth as it orbits the Sun—that creates the differences in heating that we notice as the seasons.

You can model this with a flashlight and a piece of paper. First, turn the flashlight on and hold it directly above the piece of paper. Draw a circle around the lighted region of the paper. Now tilt the flashlight so that the light strikes the paper at an angle. Look at the illuminated part of the paper. Has the circle changed shape? (Yes!) It should now look more elliptical. Draw an ellipse around the lighted portion of the paper. Now compare the circle and the ellipse. Which one covers a greater area of the paper? Hopefully, you can see that the ellipse covers a greater area. If you were as tiny as an insect and stood inside the circle, the light would appear brighter than when you stood inside the ellipse. Although the same total amount of light falls on the circle and the ellipse, the light in the ellipse is spread over a greater area. The light per unit area is greater in the circle than in the ellipse. Sunlight striking Earth’s surface at an angle will be less intense than sunlight striking Earth’s surface directly. It’s like splitting a pizza with more people. With more people, each person gets less pizza.

Earth’s tilt explains the change in daylength and the change in the sunrise and sunset points on the eastern and western horizons, respectively. An observer in the Northern Hemisphere sees the Sun low in the sky when the Northern Hemisphere is pointed away from the Sun. When the Northern Hemisphere is pointed toward the Sun, an observer sees the Sun high in the sky. The Northern Hemisphere observer also sees that the sunrise and sunset points are farther south in the winter and more northerly in the summer. The solar path across the sky in winter is lower than the path across the sky in summer. Put another way, the solar altitude angle, or simply, the noon sun angle—the position of the Sun above the equatorward (southern in Northern Hemisphere) horizon at noon—is lowest in winter and highest in summer. Stick a toothpick in your orange at the latitude of Southern California—about 30°N—to visualize how the sun angle changes as you orbit the Sun. With the sun higher in the sky from winter to summer, daylength is longer. The opposite is true from summer to winter. The seasonal changes in daylength, the sunrise and sunset points, the solar path, and the pattern of warming and cooling are explained simply as a result of Earth’s tilt relative to its plane of orbit around the Sun.

Now, before we proceed, it’s important that you recognize that the seasons of the Northern Hemisphere are not the same seasons for the Southern Hemisphere. In fact, they are exact opposites. When it’s summer in the Northern Hemisphere, it’s winter in the Southern Hemisphere. When it’s spring here, it’s fall there. Most of South America and all of Australia experience winter during our summer. Their beach days come in January instead of July. When the Southern Hemisphere is pointed toward the Sun, the Northern Hemisphere is pointed away. This causes the seasons of the two hemispheres of Earth to occur in different months of the year. That’s why some folks travel south (or north) during their winter. They can enjoy summer almost year round!

Solstices and Equinoxes

The times of year when the Sun is absolutely at its lowest or highest points in the sky are known as the solstices, meaning “when the sun is still.” These are the days when ancient people perceived that the Sun no longer moved lower or higher. We know these days as the winter solstice, the first day of winter and the shortest day (i.e., daylength is at its minimum) of the year, and the summer solstice, the first day of summer and the longest day (i.e., daylength is at its maximum) of the year.

What happens to daylength on the day following a solstice? Daylength on the day after the winter solstice is slightly longer, and the day after the summer solstice, it’s slightly shorter. In fact, daylength increases from the date of the winter solstice to the date of the summer solstice. On the other hand, daylength decreases from the date of the summer solstice to the date of the winter solstice. If you can remember the shortest day of the year (first day of winter, the winter solstice) and the longest day of the year (first day of summer, the summer solstice), then you can easily figure out what is happening to daylength at any time of the year.

We also mark the days when the Sun crosses directly overhead at the equator. These are known as the equinoxes (equi = “equal”; nox = “night”), the times of the year when day and night are of equal length. The moment when the Sun is directly over the equator on its way into the Northern Hemisphere is called the Northern Hemisphere spring equinox or vernal equinox. Half a year later, as the Sun passes southward on its way into the Southern Hemisphere, the moment when it’s directly over the equator is called the Northern Hemisphere fall equinox or autumnal equinox. Of course, the seasons are reversed for the Southern Hemisphere. On the spring and fall equinoxes, you’ll find the Sun directly over the equator. That means if you stood on the equator at local noon on this day (when the Sun is directly overhead), you would barely see a shadow at your feet. “Look, Mom, no shadow!”

The Midnight Sun

Earth’s axial tilt dramatically affects daylength at the North and South Poles. When the Northern Hemisphere is tilted away from the Sun—during the Northern Hemisphere fall and winter—sunlight can’t reach the North Pole. This region experiences 24 hours of darkness or twilight for six months out of the year. Alternatively, when the Northern Hemisphere is pointed toward the Sun—during the Northern Hemisphere spring and summer—the northern polar region experiences 24 hours of sunlight. People refer to regions poleward of the Arctic and Antarctic Circles as the land of the midnight sun, places where the Sun appears 24 hours a day. The period when this occurs is referred to as the polar day, the six-month-long period of continual (or near-continual) sunlight in these regions. The six-month-long period of darkness in these regions is called the polar night. More than a novelty, polar day and polar night have profound effects on polar environments.

Lines of Latitude Mark the Seasonal Cycle

Astronomers and geographers have long noted the changing positions of the Sun in the sky over the course of a year. These positions have been given special importance by the establishment of names for latitudes of astronomical significance. The place where the Sun is directly overhead on the first day of summer in the Northern Hemisphere, 23.5°N, is called the Tropic of Cancer. Where the Sun is directly overhead on the first day of Northern Hemisphere winter, 23.5°S, we find the Tropic of Capricorn. The latitude above which no sunlight can reach following the fall equinox is known as the Arctic Circle, 66.5°N. At the other end of Earth, the latitude below which no sunlight falls following the Southern Hemisphere fall equinox, we find the Antarctic Circle, 66.5°S.

We should also note that regions of Earth that receive direct sunlight most times of the year stay warm, and those that receive less sunlight or variable sunlight experience colder temperatures. Because of these differences, we can divide Earth into three simplified climate zones, the average weather for a region of Earth based on the amount of sunlight they receive. The region of Earth bounded by the Tropic of Cancer and the Tropic of Capricorn is known as the tropical zone, or the tropics. This zone experiences the warmest weather on the planet because the Sun is mostly overhead most days of the year. The polar zones—regions north of the Arctic Circle and south of the Antarctic Circle—encompass the coldest regions of the planet. The region between the tropical and polar zones—the middle latitudes of Earth—are known as the temperate zones. Here, where most of the continental United States lives, the climate is moderate—not too hot and not too cold, on average.

The tropical, temperate, and polar climate zones have precise astronomical meaning. Their boundaries refer to extremes in the positions of the Sun (i.e., Tropic of Cancer and Tropic of Capricorn) or its rays (Arctic Circle and Antarctic Circle). The three climate zones also provide a general guide to the kind of climate one might experience. The tropical zone at low latitudes is generally warm, humid, and rather seasonless. The temperate zones at middle latitudes are mild, with the four classic seasons (winter, spring, summer, and fall). The polar zones at high latitudes are cold and dry, with two seasons: the cold season—polar summer and fall—and the colder season—polar winter and spring. Though other configurations of climate regions based on temperature and precipitation patterns (e.g., the Köppen classification) provide more detail and remain the ones preferred by meteorologists and climatologists, the three-climate-zone model serves well for understanding most ocean-related phenomena. The interested reader may consult a good introductory meteorology textbook for fancier models (e.g., Ahrens and Henson 2018).