The Earth's tilt and the seasons

Many of us in Walla Walla think the best times to live here are spring and fall. Cold, snowy winters like the one we’ve just had and summers when we endure extended heat waves encourages such thinking.

Winters all over the Earth are colder and summers are warmer. Why is this?

The major source of energy that warms the Earth is the sun. It is almost always warmer in the daytime than at night, so the temperature differences must be caused by variations in the amount of solar energy that reaches the Earth’s surface during different times of the year.

Let’s look at some possible causes for this solar energy variation.

We know that the distance between the Earth and the sun changes during the year. It might seem logical to believe that the Earth is closer to the sun during the summer and farther from it during the winter. However, if this were the case, summer and winter would occur at the same time in both the Northern and Southern Hemispheres. They do not; when it is winter in the United States, it is summer in Australia. Another major problem with the distance theory, at least in the Northern Hemisphere, is that the Earth is closest to the sun in January.

Then what about clouds? Do they contribute to the solar energy variation? Not significantly. The average cloud cover has only a small effect on the seasonal temperatures. Skies can be cloudy in the winter and in the summer, and most of the radiant energy from the sun passes through the clouds.

There are two major factors in the seasonal temperature changes: the sun angle and duration of daylight. Let’s first look at sun angle.

A measure of the amount of solar energy reaching the Earth’s surface for a given area is called insolation. Insolation is greatest when the sun is directly overhead, 90 degrees above the horizon. As the sun gets closer to the horizon, the energy per unit area is less.

For example, at 90 degrees a one-mile-wide sunbeam covers one mile; but at 30 degrees above the horizon, the one-mile-wide sunbeam covers two miles. Since the energy in the sunbeam is the same in both cases, at a sun angle of 30 degrees the insolation is one-half that of 90 degrees.

The other major factor in determining the temperature is the duration of daylight. The total daily energy from the sun is a combination of intensity and duration. Even if the sun angle were the same in winter and summer (which it is not), winters would be colder because of the shorter amount of daylight. Thus, a location on Earth where the sun shines more directly and longer will have warmer temperatures than a location with fewer hours of daylight.

It is the tilt of the Earth’s rotational axis in relation to the plane of its orbit around the sun that causes the seasonal changes in both sun angle and daylight duration. On a spring or fall equinox, the sun will be directly overhead at noon if you are living on the Earth’s equator. The sun angle will be 90 degrees and you will be receiving maximum energy from the sun. The duration of daylight will also be at its maximum of 12 hours.

But the Earth presents a spherical surface to the sun, so the equator is the only latitude where the sun will be directly overhead at noon on the equinox. At latitudes north or south of the equator, the sun angle will be less than 90 degrees. At the North or South poles the sun angle would be zero, and the sun would be on the horizon.

On about June 21 each year, the Earth’s orbit bring it to a point where the North Pole is inclined toward the sun by 23.5 degrees: the summer solstice. This occurs in the summer for the Northern Hemisphere and in the winter for the Southern Hemisphere.

On this day the insolation is greater for northern latitudes and less for southern latitudes than it was on the equinox. The daylight duration is also longer for the northern hemisphere and shorter for the southern hemisphere. With greater insolation and duration, the Northern Hemisphere gets warmer; with less insolation and shorter duration, the Southern Hemisphere gets colder.

If you were on the equator on the summer solstice, the sun would not be overhead at noon, but 23.5 degrees to the north. The sun would be directly overhead at 23.5 degrees north latitude, which we call the Tropic of Cancer.

Six months later, on the winter solstice at the equator at noon, the sun would be 23.5 degrees to the south. It would be overhead at 23.5 degrees south latitude, which we call the Tropic of Capricorn.

The only places on Earth where the sun is directly overhead at noon are between the Tropic of Cancer and the Tropic of Capricorn. It is never overhead at noon in Walla Walla, since our latitude is about 46 degrees north.

Because the North Pole is tipped by 23.5 degrees toward the sun on the summer solstice, the area around the pole within 23.5 degrees is in total sun and the sun does not set that day. The circle of latitude 66.5 degrees north is called the Arctic Circle. All points on or north of the Arctic Circle will experience no darkness on the summer solstice. The same is true for the South Pole on the winter solstice.

Now you know why summers are hot and winters are cold.

Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at marty.scott@wallawalla.edu

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