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4.4 Reason for Seasons

One of the fundamental facts of life at Earth’s midlatitudes, where most of this book’s readers live, is that there are significant variations in the heat we receive from the Sun during the course of the year. We thus divide the year into seasons, each with its different amount of sunlight. The difference between seasons gets more pronounced the farther north or south from the equator we travel, and the seasons in the Southern Hemisphere are the opposite of what we find on the northern half of Earth. With these observed facts in mind, let us ask what causes the seasons.

Many people have believed that the seasons were the result of the changing distance between Earth and the Sun. This sounds reasonable at first: it should be colder when Earth is farther from the Sun. But the facts don’t bear out this hypothesis. Although Earth’s orbit around the Sun is an ellipse, its distance from the Sun varies by only about 3%. That’s not enough to cause significant variations in the Sun’s heating. To make matters worse for people in North America who hold this hypothesis, Earth is actually closest to the Sun in January, when the Northern Hemisphere is in the middle of winter. And if distance were the governing factor, why would the two hemispheres have opposite seasons? As we shall show, the seasons are actually caused by the 23.5° tilt of Earth’s axis.

4.4.1 The Seasons and Sunshine

Figure 4.8 shows Earth’s annual path around the Sun, with Earth’s axis tilted by 23.5°. Note that our axis continues to point the same direction in the sky throughout the year. As Earth travels around the Sun, in June the Northern Hemisphere “leans into” the Sun and is more directly illuminated. In December, the situation is reversed: the Southern Hemisphere leans into the Sun, and the Northern Hemisphere leans away. In September and March, Earth leans “sideways”—neither into the Sun nor away from it—so the two hemispheres are equally favored with sunshine.

Earth’s Seasons. This illustration shows the Earth at four positions along its orbit around the Sun, which is drawn in the center of the orbit indicated by circular arrows. At left, the Earth is shown at “Summer Solstice June 21”, and has its northern axis of rotation (tilted 23-degrees from vertical) pointing toward the Sun. At bottom center, the Earth is at “Autumnal Equinox September 21”, with the northern rotation axis pointing toward the right. At right, the Earth is shown at “Winter Solstice December 21”, with the northern axis of rotation pointing away from the Sun. Finally, at top, the Earth is shown at “Vernal Equinox March 21”, with the northern rotation axis pointing toward the right.
Figure 4.8: We see Earth at different seasons as it circles the Sun. In June, the Northern Hemisphere “leans into” the Sun, and those in the North experience summer and have longer days. In December, during winter in the Northern Hemisphere, the Southern Hemisphere “leans into” the Sun and is illuminated more directly. In spring and autumn, the two hemispheres receive more equal shares of sunlight. Note that the dates indicated for the solstices and equinoxes are approximate; depending on the year, they may occur a day or two earlier or later.
How does the Sun’s favoring one hemisphere translate into making it warmer for us down on the surface of Earth? There are two effects we need to consider. When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth’s surface (Figure 4.9). You can get a similar effect by shining a flashlight onto a wall. If you shine the flashlight straight on, you get an intense spot of light on the wall. But if you hold the flashlight at an angle (if the wall “leans out” of the beam), then the spot of light is more spread out. Like the straight-on light, the sunlight in June is more direct and intense in the Northern Hemisphere, and hence more effective at heating.
The Sun’s Rays in Summer and Winter. Panel (a), at left, illustrates how sunlight strikes the Earth’s surface in Summer. Five parallel yellow arrows, labeled “1 m2”, are drawn pointing downward at a 73-degree angle relative to the ground. Where the arrows strike the ground, a scale is drawn spanning the width of the arrows that reads “1.04 m2”. In panel (b), at right, illustrates how sunlight strikes the Earth’s surface in Winter. The five arrows are now drawn at 26-degrees relative to the ground. Where the arrows strike the ground, a scale is drawn spanning the width of the arrows that reads “2.24 m2”. Thus one square meter of sunlight falls on over twice the surface area in winter vs. summer.
Figure 4.9: (a) In summer, the Sun appears high in the sky and its rays hit Earth more directly, spreading out less. (b) In winter, the Sun is low in the sky and its rays spread out over a much wider area, becoming less effective at heating the ground.
The second effect has to do with the length of time the Sun spends above the horizon (Figure 4.10). Even if you’ve never thought about astronomy before, we’re sure you have observed that the hours of daylight increase in summer and decrease in winter. Let’s see why this happens.
The Sun’s Path in the Sky for Different Seasons. In each of these three illustrations, a beige ellipse represents the ground and horizon of an observer standing in the center, and is surrounded by a semi-transparent sphere representing the sky. North is to the left, and west is at the bottom of the horizon ellipse. A yellow line, labeled “North celestial pole”, is drawn from the feet of the observer toward the upper left. A yellow dashed ellipse, labeled “Celestial equator”, is drawn on the sky sphere so that it touches the horizon at the points labeled “W” (west) and “E” (east) and is tilted to be perpendicular to the celestial pole. The left-most illustration shows the “Sun’s path June 21”, indicated by a faint yellow ellipse. The Sun rises and sets above the celestial equator. The central illustration shows the “Sun’s path March 21 and Sept. 21”. The Sun rises and sets along the celestial equator. Finally, the right-most illustration shows the “Sun’s path Dec. 21”, indicated by a faint yellow ellipse. The Sun rises and sets below the celestial equator.
Figure 4.10: On June 21, the Sun rises north of east and sets north of west. For observers in the Northern Hemisphere of Earth, the Sun spends about 15 hours above the horizon in the United States, meaning more hours of daylight. On December 21, the Sun rises south of east and sets south of west. It spends 9 hours above the horizon in the United States, which means fewer hours of daylight and more hours of night in northern lands (and a strong need for people to hold celebrations to cheer themselves up). On March 21 and September 21, the Sun spends equal amounts of time above and below the horizon in both hemispheres.
An equivalent way to look at our path around the Sun each year is to pretend that the Sun moves around Earth (on a circle called the ecliptic). Because Earth’s axis is tilted, the ecliptic is tilted by about 23.5° relative to the celestial equator. As a result, where we see the Sun in the sky changes as the year wears on.

In June, the Sun is north of the celestial equator and spends more time with those who live in the Northern Hemisphere. It rises high in the sky and is above the horizon in the United States for as long as 15 hours. Thus, the Sun not only heats us with more direct rays, but it also has more time to do it each day. (Notice in Figure 4.10 that the Northern Hemisphere’s gain is the Southern Hemisphere’s loss. There the June Sun is low in the sky, meaning fewer daylight hours. In Chile, for example, June is a colder, darker time of year.) In December, when the Sun is south of the celestial equator, the situation is reversed.

For Further Exploration

The University of Nebraska’s Seasons Simulation provides an overview of how Earth’s tilt combines with its orbital motion, illustrating how the angle of sunlight changes throughout the year.

Let’s look at what the Sun’s illumination on Earth looks like at some specific dates of the year, when these effects are at their maximum. On or about June 21 (the date we who live in the Northern Hemisphere call the summer solstice or sometimes the first day of summer), the Sun shines down most directly upon the Northern Hemisphere of Earth. It appears about 23° north of the equator, and thus, on that date, it passes through the zenith of places on Earth that are at 23° N latitude. The situation is shown in detail in Figure 4.11. To a person at 23° N (near Hawaii, for example), the Sun is directly overhead at noon. This latitude, where the Sun can appear at the zenith at noon on the first day of summer, is called the Tropic of Cancer.

We also see in Figure 4.11 that the Sun’s rays shine down all around the North Pole at the solstice. As Earth turns on its axis, the North Pole is continuously illuminated by the Sun; all places within 23° of the pole have sunshine for 24 hours. The Sun is as far north on this date as it can get; thus, 90° – 23° (or 67° N) is the southernmost latitude where the Sun can be seen for a full 24-hour period (sometimes called the “land of the midnight Sun”). That circle of latitude is called the Arctic Circle.

The Summer Solstice – June 21. The Earth is drawn with its axis of rotation, labeled “Earth axis”, pointing toward upper left. Sunlight is drawn as three red arrows coming from the left and striking the surface of the Earth. On the right-hand side of the figure, the five important circles of latitude are labeled. Starting from the bottom are: “Antarctic Circle”, “Tropic of Capricorn”, “Equator”, “Tropic of Cancer” and “Arctic Circle”.
Figure 4.11: Earth on June 21. This is the date of the summer solstice in the Northern Hemisphere. Note that as Earth turns on its axis (the line connecting the North and South Poles), the North Pole is in constant sunlight while the South Pole is veiled in 24 hours of darkness. The Sun is at the zenith for observers on the Tropic of Cancer.
Many early cultures scheduled special events around the summer solstice to celebrate the longest days and thank their gods for making the weather warm. This required people to keep track of the lengths of the days and the northward trek of the Sun in order to know the right day for the “party.” (You can do the same thing by watching for several weeks, from the same observation point, where the Sun rises or sets relative to a fixed landmark. In spring, the Sun will rise farther and farther north of east, and set farther and farther north of west, reaching the maximum around the summer solstice.)

Now look at the South Pole in Figure 4.811. On June 21, all places within 23° of the South Pole—that is, south of what we call the Antarctic Circle—do not see the Sun at all for 24 hours.

The situation is reversed 6 months later, about December 21 (the date of the winter solstice, or the first day of winter in the Northern Hemisphere), as shown in Figure 4.12. Now it is the Arctic Circle that has the 24-hour night and the Antarctic Circle that has the midnight Sun. At latitude 23° S, called the Tropic of Capricorn, the Sun passes through the zenith at noon. Days are longer in the Southern Hemisphere and shorter in the north. In the United States and Southern Europe, there may be only 9 or 10 hours of sunshine during the day. It is winter in the Northern Hemisphere and summer in the Southern Hemisphere.

The Winter Solstice – December 21. The Earth is drawn with its axis of rotation, labeled “Earth axis”, pointing toward upper right. Sunlight is drawn as three red arrows coming from the left and striking the surface of the Earth. On the right-hand side of the figure, the five important circles of latitude are labeled. Starting from the bottom are: “Antarctic Circle”, “Tropic of Capricorn”, “Equator”, “Tropic of Cancer” and “Arctic Circle”.
Figure 4.12: Earth on December 21. This is the date of the winter solstice in the Northern Hemisphere. Now the North Pole is in darkness for 24 hours and the South Pole is illuminated. The Sun is at the zenith for observers on the Tropic of Capricorn and thus is low in the sky for the residents of the Northern Hemisphere.

Many cultures that developed some distance north of the equator have a celebration around December 21 to help people deal with the depressing lack of sunlight and the often dangerously cold temperatures. Originally, this was often a time for huddling with family and friends, for sharing the reserves of food and drink, and for rituals asking the gods to return the light and heat and turn the cycle of the seasons around. Many cultures constructed elaborate devices for anticipating when the shortest day of the year was coming. Stonehenge in England, built long before the invention of writing, is probably one such device. In our own time, we continue the winter solstice tradition with various holiday celebrations around that December date.

Halfway between the solstices, on about March 21 and September 21, the Sun is on the celestial equator. From Earth, it appears above our planet’s equator and favors neither hemisphere. Every place on Earth then receives roughly 12 hours of sunshine and 12 hours of night. The points where the Sun crosses the celestial equator are called the vernal (spring) and autumnal (fall) equinoxes.

4.4.2 The Seasons at Different Latitudes

The seasonal effects are different at different latitudes on Earth. Near the equator, for instance, all seasons are much the same. Every day of the year, the Sun is up half the time, so there are approximately 12 hours of sunshine and 12 hours of night. Local residents define the seasons by the amount of rain (wet season and dry season) rather than by the amount of sunlight. As we travel north or south, the seasons become more pronounced, until we reach extreme cases in the Arctic and Antarctic.

At the North Pole, all celestial objects that are north of the celestial equator are always above the horizon and, as Earth turns, circle around parallel to it. The Sun is north of the celestial equator from about March 21 to September 21, so at the North Pole, the Sun rises when it reaches the vernal equinox and sets when it reaches the autumnal equinox. Each year there are 6 months of sunshine at each pole, followed by 6 months of darkness.

Food for Thought

As we have discussed above, seasons are primarily controlled by the axial tilt of a planet as it orbits around the Sun. The eccentricity of a planet’s orbit also plays a role, but it’s effect is typically secondary and relatively minor, contributing to the length of the seasons only. All planets in the solar system have some form of “seasons”, but their nature differs greatly due to differences in orbital characteristics (e.g., axial tilt, orbital period, eccentricity, etc.).

Think about this: Mars has an axial tilt of ~25º, Uranus has a tilt of ~98º, and Mercury has a tilt of ~2º. Based on this information alone, what do you think seasons are like on each of these planets? Do planets with greater axial tilts have more or less extreme seasonal variation?

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