# Seasons

## The seasons are the result of Earth's axial tilt; describes solstices and equinoxes.

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Earth In Space

### Earth's Place in Space

One of eight planets, our Earth lies approximately 93,000,000 miles from the Sun, in what astronomers refer to as a habitable zone. A habitable zone is the distance from a star where the temperature on the surface of the planet is between the freezing point (0°C) and the boiling point (100°C). Three planets — Venus, Earth and Mars — fall within this habitable zone. Venus, with its thick atmosphere, is too hot, while Mars, with its thin atmosphere, is too cold. Had Venus developed a thinner atmosphere and Mars a thicker one, we may have had three planets that support life.

### Earth's Motions

Earth has two principle motions in space. These motions are responsible for changes in the amount of solar heating by the Sun. Because of these motions, Earth is heated unevenly; the amount of energy it receives from the Sun changes with latitude, time of day and season of the year. This unequal heating creates winds and drives ocean currents, which help create weather. If the Sun were "turned off", global winds and ocean currents would soon stop, and weather would cease.

#### Rotation

The first principle motion is rotation, the spinning of Earth about its axis. The axis is an imaginary line running between the poles. Our planet rotates once every 24 hours, producing a daily cycle of day and night.

Revolution

Revolution refers to the movement of Earth in its orbit around the Sun. Traveling at a speed of 70,000 mph, Earth takes approximately 365.25 days to orbit once around the Sun.

### Earth’s Seasons

We know that it is colder in the winter than it is in the summer. But why? The length of daylight and a gradual change in the angle of the noon sun affect the amount of energy Earth receives. Seasonal changes occur because Earth's position relative to the Sun is always changing as it travels along its orbit.

A common misconception is that the Sun is closer to Earth in the summer and farther away from it during the winter. In fact, Earth is farthest from the Sun, called perihelion, around July 4. Earth is closest to the Sun, called aphelion, around Jan 3.

In the diagram above, notice that Earth is closest to the Sun in the winter. [Figure1]

Instead, the seasons are caused by the 23.5o tilt of Earth’s axis of rotation relative to its plane of orbit around the Sun (Figure below). This tilt is "fixed", meaning that the axis always points to the same fixed location in space; our northern axis points almost directly to Polaris, the North Star.

Credit: Sam McCabe

The fixed position of Earth's axis causes one hemisphere to be tilted toward the Sun and one hemisphere to be tilted away from the Sun. This gives rise to the seasons. [Figure2]

The “fixed” tilt means that, during Earth's orbit around the Sun, different parts of Earth receive sunlight for different lengths of time. It also means that the angle at which sunlight strikes different parts of Earth's surface changes through the year. Sunlight striking the surface at an angle is “spread” across a wider area compared to sunlight striking perpendicular to Earth's surface. Areas that receive more scattered sunlight receive less energy from our Sun. All of these factors combine to give Earth its annual cycle of seasons.

The Sun's rays are more concentrated near the Equator, while at the poles, the rays are spread over a wider area, causing them to be less concentrated. [Figure3]

#### Northern Hemisphere Summer

During the summer, Earth's Northern Hemisphere is tilted toward the Sun. This causes the angle at which the Sun's rays hit the Earth to be fairly high, and as a result, more concentrated. More concentrated rays will cause higher temperatures. The Southern Hemisphere on the other hand, is pointed away from Earth at this time, causing the Sun's rays to strike at much lower angles. This causes the rays to be less concentrated and temperatures to drop; the Southern Hemisphere experiences winter at this time.

Notice the angle at which the Sun's rays strike Earth in the Northern Hemisphere during summer. Compare these angles to the Southern Hemisphere. [Figure4]

#### Solstices and Equinoxes

Solstices occur when Earth's axis is pointed directly toward our Sun. This happens twice a year during Earth's orbit. Near June 21 the north pole is tilted 23.5 degrees toward our Sun and the northern hemisphere experiences summer solstice, the longest day of the northern hemisphere year. On that same day, the southern hemisphere is tilted 23.5 degrees away from our Sun and the southern regions of Earth experience the shortest day of the year — the winter solstice.

The second solstice occurs on December 21 or 22 when the north pole is tilting 23.5 degrees away from our Sun and the south pole is inclined toward it. This is the shortest day of the year in the northern hemisphere — the northern hemisphere winter solstice.

Twice each year, during the equinoxes (“equal nights”), Earth's axis is not pointed toward our Sun, but is perpendicular to the incoming rays. During the equinoxes every location on our Earth (except the extreme poles) experiences 12 hours of daylight and 12 hours of darkness. The vernal (spring) equinox occurs in the northern hemisphere on March 21 or 22 (the fall equinox of the southern hemisphere). September 22 or 23 marks the northern hemisphere autumnal (fall) equinox.

[Figure5]

### Summary

• In the Northern Hemisphere, at summer solstice the Sun is closest to the north pole (around June 22) and at winter solstice, the Sun is closest to the south pole (around December 22). In the Southern Hemisphere, the names are changed.
• Over the course of a year, the amount of solar energy received by the Equator is greater than the amount received elsewhere.
• At equinox the Sun is directly over the Equator; autumnal equinox is around September 22 and spring equinox is around March 22 in the Northern Hemisphere.

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