11.2: Why Do Seasons Occur?

Day and night length and sunrise and sunset points change because of Earth’s position as it orbits the Sun. To understand Earth’s orbit, we need to review a few astronomy terms and concepts. But before we start, let’s clear up one very common misconception about the seasons.

Earth’s tilt (explained below) causes the seasons. As you learned in Chapter 12, the Sun heats Earth’s surface (with shortwave radiation). Earth’s surface, in turn, heats our atmosphere (with longwave radiation). Because Earth is tilted with respect to the incoming solar radiation, some places receive more intense sunlight than others. Some times of the year, a given location will receive direct sunlight. Other times of the year, it will receive less direct sunlight. The result is intense heating some times of the year and less intense heating effect other times of the year. Those changes in heating we know as the seasons.

Much of this chapter is devoted to exploring the details of the seasonal cycle so don’t worry if it’s not clear to you just yet. But if the tilt model of Earth’s season doesn’t satisfy you, ask yourself this: Why does the Northern Hemisphere enjoy summer in June while folks in the Southern Hemisphere experience winter in June. Let’s find out why.

Earth’s Rotation

As I’m sure you know by now, Earth turns like a disco ball in outer space, in what is called Earth’s rotation. The imaginary line around which it rotates is called Earth’s axis of rotation, or simply Earth’s axis. Grab a spherical piece of fruit—an orange works nicely—and carefully jab a pencil through the middle of it. (Styrofoam balls work for this too, but not everyone keeps Styrofoam balls in their house.) Hold the top end of the pencil–orange physical model of Earth so that the pencil is vertical. Then twirl the orange. Voilà! You have created a model of how Earth rotates on its axis. Now add this detail: Look down on your model from above the North Pole (where the North Pole is oriented up) and rotate it counterclockwise. Your model now correctly represents the direction of rotation of Earth around its axis. The Sun rises in the east because the Earth rotates towards the east. That means the Sun rises first on the east coast of the US and even earlier in the UK.

Earth’s Tilt

Next, using a marker, draw an equator at the halfway point between the top and bottom of the orange (i.e., midway between the North and South Poles). Tilt the pencil to a vertical position (straight up and down). The equator you just drew should be horizontal. Now—and this is the most critical part of your demonstration—try a 45-degree tilt (symbolized as 45°), halfway between vertical and horizontal. (Review degrees and angles on the internet if this terminology is unfamiliar to you.) Then try a 22.5° angle, halfway between vertical and 45°. This is close to the angle of Earth’s axial tilt, the angle between vertical and the axis of rotation of a planet. (Astronomers officially call this obliquity, but that word is harder to remember, so we’re sticking with tilt.) Earth’s axial tilt is roughly 23.5° in modern times. (To be precise, it’s 23.43681°, but most textbooks and websites round it off to 23.5, apparently because it’s easier to remember, but what could be easier to remember than 2-3-4?)

Earth’s Orbit

Now we need to see what happens when Earth orbits the Sun, what’s called Earth’s orbital revolution or Earth’s orbit. For this, you will need a model of the Sun—a lamp, a grapefruit, a chair in the middle of the room, anything that your body can orbit. Start with Earth held so that its axis of rotation is at a 23.5° angle with the North Pole pointed away from your sun. Hold it firm in that position while you walk in a circle around your imaginary sun. When you have returned to your starting position, note the position of your Earth. It should be the same: at a 23.5° angle pointed away from your sun. As you orbited your sun holding your tilted Earth, you traced an orbital plane, the horizontal plane in which Earth’s orbit occurs. The equator also can be considered as a plane, called the equatorial plane. Viewed in this way, the equatorial plane makes a 23.5° angle with the orbital plane. Alternatively (just looking at it a different way), Earth’s tilt represents the angle between Earth’s axis of rotation and a vertical line drawn perpendicular to the orbital plane. Earth’s tilt is maintained throughout its orbit around the Sun, 365.25 days a year.

Now, if you want to be really accurate, your simulation of Earth’s orbit would take a slightly elliptical shape rather than a perfect circle. As it turns out, the distance between Earth and the Sun varies during Earth’s annual orbit around the Sun. You may recall this distance as the astronomical unit, or AU. By definition, the Earth–Sun distance is 1 AU, about 93 million miles. Although the AU is considered a constant, we know the Earth is slightly closer to the Sun in January. At this location, known as perihelion—when a planet is closest to its star—Earth is positioned about 91.4 million miles away from the Sun. In July, at aphelion—when a planet is farthest from its star—Earth is slightly farther away, about 94.5 million miles (e.g., NASA 2001). This should put to rest any notion that the Earth–Sun distance causes the seasons. The Sun is closer to the Earth in January—when the Northern Hemisphere experiences winter—and farther from the Earth in July during Northern Hemisphere summer.