Section 2.1:
Earth-Sun Relationships

Learning Objective

Explain what causes the Sun angle and length of daylight to change throughout the year. Describe how these changes result in seasonal changes in temperature.

Section Content

The amount of solar energy received at any location varies with latitude, time of day, and season of the year. Contrasting images of polar bears on ice floes with palm trees along a tropical beach serve to illustrate the extremes. The unequal heating of Earth’s surface creates winds and drives ocean currents, which in turn transport heat from the tropics toward the poles in an unending attempt to balance energy inequalities.

The consequences of these processes are the phenomena we call weather. If the Sun were “turned off,” global winds and ocean currents would quickly cease. Yet as long as the Sun shines, winds will blow and weather will persist. So, to understand how the dynamic weather machine works, we must understand why different latitudes receive different quantities of solar energy and why the amount of solar energy received changes during the course of a year to produce the seasons (Figure 2.1).

Figure 2.1
An understanding of Earth–Sun relationships is basic to an understanding of the seasons

A. Clear, warm summer day in Chicago, Illinois. B. Cold winter scene in Chicago.

Tutorial Video - Understanding Seasons, Part 1 (Click to watch the video)

Tutorial Video - Understanding Seasons, Part 2 (Click to watch the video)

Earth’s Rotation and Orbit

Earth has two basic motions—its rotation (spin) on its axis and its annual orbit (or revolution) around the Sun. Rotation, the spinning of Earth on its axis, which is an imaginary line connecting the North Pole to the South Pole, takes 24 hours (1 day) and produces the cycle of day and night.

Earth also moves in a slightly elliptical orbit around the Sun that takes about 365¼ days (1 year). Because Earth’s orbit is not perfectly circular, the distance varies during the year (Figure 2.2). Each year, on about January 3, our planet is about 147 million kilometers (91.5 million miles) from the Sun, closer than at any other time—a position called perihelion. About 6 months later, on July 4, Earth is about 152 million kilometers (94.5 million miles) from the Sun, farther away than at any other time—a position called aphelion. The average distance between Earth and the Sun is about 150 million kilometers (93 million miles).

Figure 2.2
Earth’s slightly elliptical orbit around the Sun

Notice that the Earth is farthest from the Sun on July 4 (aphelion) and closest to the Sun on January 3 (perihelion).

Although Earth is closest to the Sun and receives up to 7 percent more energy in January than in July, this difference plays only a minor role in producing seasonal temperature variations, as evidenced by the fact that Earth is closest to the Sun during the Northern Hemisphere winter.

What Causes the Seasons?

If variations in the distance between the Sun and Earth do not cause seasonal temperature changes, what does? You have undoubtedly noticed that the length of daylight changes gradually throughout the year. This is more noticeable the further you get from the equator. In fact, at the North Pole, daylight is continuous from March 21 through August 21. This accounts for some of the differences in the temperatures experienced in summer versus winter: Longer daylight hours result in warmer days.

In addition, the angle (altitude) of the Sun above the horizon affects the amount of solar energy that reaches Earth’s surface. When the Sun is directly overhead (at a 90° angle), the solar rays are most concentrated and thus most intense. At lower Sun angles, the rays become more spread out and less intense. This explains why tropical areas, which experience consistently higher Sun angles throughout the year, are much warmer than polar regions, where the Sun angles are lower (Figure 2.3). You have probably experienced this when using a flashlight. If the flashlight beam strikes a surface at a 90° angle, a small intense spot is produced. By contrast, if the beam strikes at any other angle, the area illuminated is larger—but noticeably dimmer.

Figure 2.3
Changes in the angle of the Sun’s rays cause variations in the amount of solar energy that reaches Earth’s surface

The higher the angle, the more intense the solar radiation reaching the surface. Notice in the last image that the same amount of solar energy is spread over twice the distance.

The angle at which the Sun strikes a location varies seasonally. For example, for someone living in Chicago, Illinois, the Sun is highest in the sky at noon on June 21–22. (Comparisons of seasonal changes in Sun angles are based on noon solar time because that is when the Sun is highest in the sky.) But as summer gives way to autumn, the noon Sun gradually appears lower in the sky, and sunset occurs earlier each evening as daylight hours decrease. The lowest noon Sun angle and earliest sunset in Chicago occur on December 21–22.

Although Chicago and other midlatitude cities in the Northern Hemisphere experience their shortest day and lowest Sun angle in late December, their lowest average temperatures are usually experienced a few weeks later, in January. The reason for this temperature lag will be discussed in Chapter 3.

Sun angle also determines the path that solar rays take as they pass through the atmosphere (Figure 2.4). When the Sun is directly overhead, the rays strike the atmosphere at a 90° angle and travel the shortest possible route to Earth’s surface. Rays entering the atmosphere at a 30° angle must travel twice this distance before reaching the surface, whereas rays entering at a 5° angle travel a distance roughly equivalent to 11 atmospheres. The longer the path the rays must travel, the greater the chance that sunlight will be dispersed (scattered) or absorbed by Earth’s atmosphere, which reduces the intensity of sunlight reaching the surface. Changes in sun angle explain why noon is the brightest part of a clear day and why light dims as sunset approaches.

Figure 2.4
The amount of atmosphere sunlight must traverse before reaching the Earth’s surface affects its intensity

Rays striking Earth at a low angle (near the poles) must traverse more of the atmosphere than rays striking at a high angle (around the equator) and thus are subject to greater depletion by reflection, scattering, and absorption.

Daily Changes in Earth’s Orientation to the Sun

What causes fluctuations in Sun angle and length of daylight over the course of a year? Variations occur because Earth’s orientation to the Sun continually changes. Earth’s axis (the imaginary line through the poles around which Earth rotates) is not perpendicular to the plane of its orbit around the Sun—called the plane of the ecliptic. Instead, the axis is tilted 23½° from the plane of the ecliptic, called the inclination of the axis. If the axis were not inclined, Earth would lack seasons. Because the axis is always pointed in the same direction (toward the North Star), the orientation of Earth’s axis to the Sun’s rays is constantly changing (Figure 2.5).

Figure 2.5
Earth–Sun relationships

Animation Video - Earth-Sun Relationships (Click to watch the video)

For example, on one day in June each year, Earth’s position in orbit is such that the Northern Hemisphere is inclined or “leaning” 23½° toward the Sun (left in Figure 2.5). Six months later, in December, when Earth has moved to the opposite side of its orbit, the Northern Hemisphere “leans” 23½° away from the Sun (Figure 2.5, right). On days between these extremes, the “lean” of Earth’s axis is less than 23½° relative to the rays of the Sun. This change in orientation causes the spot where the Sun’s rays are vertical (striking the atmosphere at a 90° angle) to make an annual migration from 23½° north of the equator to 23½° south of the equator. In turn, this migration causes the angle of the noon Sun to vary 47° (23½° + 23½°) for all midlatitude locations during a year. New York City, for instance, has a maximum noon Sun angle of 73½° when the Sun’s vertical rays have reached their farthest northward location in June, and a minimum noon Sun angle of 26½° 6 months later—a difference of 47° (Figure 2.6). By contrast, a city on the equator will experience an annual migration of half that amount, 23½°. Box 2.1 explains how the angle of the noon Sun can be calculated for a given latitude.

Figure 2.6
Characteristics of the solstices and equinoxes

Tutorial Video - Solstices and Equinoxes (Click to watch the video)

Box 2.1

Calculating the Noon Sun Angle

Because Earth is roughly spherical, the only locations that receive vertical (90°) rays from the Sun are located along one particular line of latitude on any given day. As we move either north or south of this location, the Sun’s rays strike at decreasing angles. Thus, the closer a place is situated to the latitude receiving the vertical rays of the Sun, the higher will be its noon Sun, and the more concentrated will be the radiation it receives.

A place located 1° away (either north or south) receives an 89° angle; a place 2° away, an 88° angle; and so forth. To calculate the noon Sun angle, simply find the number of degrees of latitude separating the location you want to know about from the latitude that is receiving the vertical rays of the Sun. Then subtract that value from 90°. The example in Figure 2.A illustrates how to calculate the noon Sun angle for a city located at 40° north latitude on December 22 (winter solstice).

Figure 2.A
Calculating the noon Sun angle

Recall that on any given day, only one latitude receives vertical (90°) rays of the Sun. On December 22, the Sun is directly overhead at 23½° south. In this example, the number of degrees of latitude separating 40° N from the location of the Sun’s vertical rays is 63½°. Subtracting this from 90° gives you a noon Sun angle of 26½°.

Solstices and Equinoxes

Based on Earth’s position in orbit and the annual migration of the vertical rays of the Sun, 4 days each year are especially significant. On June 21 or 22, the vertical rays of the Sun strike 23½° north latitude (23½° north of the equator), a line of latitude known as the Tropic of Cancer (Figure 2.5). For people living in the Northern Hemisphere, June 21 or 22 is known as the summer solstice, the first “official” day of summer (Box 2.2).

Box 2.2

When Are the Seasons?

Have you ever been caught in a snowstorm around Thanksgiving, even though winter does not begin until December 21? Or have you endured several consecutive days of 100° temperatures although summer has not “officially” started? The idea of dividing the year into four seasons originated from the Earth–Sun relationships discussed in this chapter (Table 2.A). This astronomical definition of the seasons defines winter in the Northern Hemisphere as the period from the winter solstice (December 21–22) to the spring equinox (March 21–22). This is also the definition used most widely by the news media, yet it is not unusual for portions of the United States and Canada to have significant snowfalls weeks before the “official” start of winter.

Table 2.A
Occurrence of the Seasons in the Northern Hemisphere

Because the weather phenomena we normally associate with each season do not coincide well with the astronomical seasons, meteorologists prefer to divide the year into four 3-month periods based primarily on temperature. Thus, winter is defined as December, January, and February, the three coldest months of the year in the Northern Hemisphere (Figure 2.B). Summer is defined as the three warmest months, June, July, and August. Spring and autumn are the transition periods between these two seasons. Inasmuch as these four 3-month periods better reflect the temperatures and weather that we associate with the respective climatological seasons, this definition of the seasons is more useful for meteorological discussions.

Figure 2.B
Mean monthly temperatures for a midlatitude city in the central United States

Notice how well the three warmest and three coldest months align with the occurrence of summer and winter seasons, respectively. The astronomical seasons began about 21 days after the climatological seasons. Therefore, based on the astronomical seasons, winterlike conditions can occur long before the designated “first day of winter.”

Six months later, on December 21 or 22, Earth tilts (or leans) in the opposite direction, so the Sun’s vertical rays strike at 23½° south latitude. (Recall that Earth’s axis always points in the same direction; it is Earth’s changing position relative to the Sun that causes the apparent change in tilt.) This line of latitude is known as the Tropic of Capricorn. In the Northern Hemisphere, December 21 or 22 is the winter solstice, the first day of winter. However, on this same day, the Southern Hemisphere is experiencing its summer solstice.

The equinoxes occur midway between the solstices. September 22 or 23 is the date of the fall (autumnal) equinox in the Northern Hemisphere, and March 21 or 22 is the date of the spring (vernal) equinox. On these dates, the vertical rays of the Sun strike the equator (0° latitude) because Earth’s position is such that its axis is tilted neither toward nor away from the Sun.

The length of daylight versus darkness is also determined by the position of Earth relative to the Sun’s rays. The length of daylight on the summer solstice in the Northern Hemisphere, June 21, is greater than the length of night. This fact can be established by examining Figure 2.6, which illustrates the circle of illumination—that is, the boundary separating the dark half of Earth from the lighted half. The length of daylight is established by comparing the fraction of a line of latitude that is on the “day” side of the circle of illumination with the fraction on the “night” side. Notice that on June 21, all locations in the Northern Hemisphere experience longer periods of daylight than darkness (Table 2.1). By contrast, during the Northern Hemisphere winter solstice in December, the length of darkness exceeds the length of daylight at all locations in the hemisphere.

Table 2.1
Length of Daylight

The seasonal changes in length of daylight and the Sun’s angle are the primary causes of the month-to-month variations in temperature observed at most locations. This is illustrated in Figure 2.7, which depicts the daily paths of the Sun over the seasons of the year for a location at 40° north latitude—New York City, for example. Notice that the length of daylight is longest during the summer solstice, whereas the opposite is true for the winter solstice. (Compare the length of the Sun’s path [yellow line] for each of these days.) New York City has about 15 hours of daylight on June 21, compared to 9 hours on December 21. In addition, the maximum Sun angle is 73½°on June 21, and only 26½°on December 21.

Figure 2.7
Daily paths of the Sun

The various paths of the Sun for a place located at 40° north latitude at three different times of the year.

Also note from Table 2.1 that the farther north a location is from the equator on June 21, the longer the period of daylight it will experience. On this date, places located on or north of the Arctic Circle (66½° north latitude), experience the midnight Sun, a natural phenomenon in which the Sun is visible at midnight (Figure 2.8A). In fact, on this date the Sun does not set for a period that ranges from 1 day at the Arctic Circle, to about 4 months at 80° north latitude, and 6 months at the pole. Figure 2.8B shows the path of the Sun as seen by an observer located at 80° north latitude during the summer solstice. Notice that the Sun does not drop below the horizon at any time during the entire day. The Antarctic Circle, which is the corresponding latitude in the Southern Hemisphere, experiences the opposite situation—total darkness on June 21.

Figure 2.8
Midnight Sun

A. Multiple exposures (taken on the same day) of the Sun as it appears before midnight (left portion of the image) and after midnight in midsummer, at about 80° north latitude. B. Illustration of the path of the Sun at the same location. Notice that the Sun never sets and only gets close to the horizon at midnight.

Video - Net Radiation at the Top of the Atmosphere (Click to watch the video)

Figure 2.9 summarizes the characteristics of the solstices and equinoxes for a location in the Northern Hemisphere. When you examine Figure 2.9, you will see why a midlatitude location is warmest in the summer, when the days are longest and the angle of the Sun above the horizon is highest. Near the winter solstice, the reverse occurs: The days are shortest, and the Sun angle is lowest. During an equinox (meaning “equal night”), the length of daylight is 12 hours everywhere on Earth because the circle of illumination passes directly through the poles, thus dividing the lines of latitude in half.

Figure 2.9
Characteristics of the solstices and equinoxes for the Northern Hemisphere

All locations situated at the same latitude have identical Sun angles and lengths of daylight. If the Earth–Sun relationships were the only controls of temperature, we would expect these places to have identical temperatures as well. This is not the case. Other factors, such as a location’s elevation or its proximity to a large body of water, also influence local temperature and will be addressed in Chapter 3.

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

Briefly explain the primary causes of the seasons.

The primary cause of the seasons at any location on Earth involves both the changing angle of the solar rays over the course of the year and the changing length of the daylight hours.

What is the significance of the Tropic of Cancer and the Tropic of Capricorn?

The “tropics” stretches from 23.5°N to 23.5°S and experiences the noon Sun directly overhead (90°) at least 1 day a year. The northernmost boundary of this region (23.5°N) is the Tropic of Cancer, where the noon Sun reaches 90° around June 21–22. The southern boundary is set by the Tropic of Capricorn, and the vertical rays of the noon Sun will be found there around December 21–22.

After examining this table to the right, write a general statement that relates the season, latitude, and the length of daylight.

Tropical regions experience little variation between seasons due to the high noon Sun angle throughout the year and uniform length of daylight/darkness hours, but the further one moves from the equator, the more extreme the variation between seasonal noon Sun angles and seasonal length of daylight hours.