What Causes the Seasons?

In many places on Earth, living things experience different temperatures and weather conditions that are caused by several factors. As human beings, we have divided these changes into four sections that occur at roughly the same time during the calendar year. These are the season: autumn (Fall), winter, spring and summer. Human beings have also assigned border lines to the Earth to better understand and keep track of the seasons better understand where we are on the surface of the planet.

For example, the imaginary line around the center of the earth, halfway between the North Pole and the South Pole, is called the equator. This is the dividing line between the northern hemisphere and the southern hemisphere. Depending on the relationship between the Sun and the Earth, the northern hemisphere experiences summer when the southern hemisphere has winter. The opposite is generally true. When it is warm in the north it is cool or cold in the south.

Why does this happen on a regular basis, year after year? Because the Earth doesn’t orbit the sun while “standing” straight up. The planet is tilted slightly on its axis (an imaginary line through the Earth from the North Pole to the South Pole). This slight tilt means that during summer in the northern half of the planet the Sun shines more directly on the surface than it does during the remainder of the year. During the winter season, the Sun shines on the northern hemisphere at an angle – the heat and light are dispersed and strike the surface less directly. For the “in between” seasons of autumn and spring the rays are not as direct as in summer but are a bit more direct than in winter.
Of course, this means that when the Earth orbits the Sun and the North Pole is tilted away from the Sun, the heat and light strike the southern half of the planet more directly. Living things in the south experience summer while those in the northern half experience the colder temperatures of winter.

Observations from space and studies of how the Sun’s light strikes the Earth show that the Earth is tilted a little more than 22 degrees off perpendicular. It’s interesting to note that the Sun and the Earth are closer to each other during the north’s winter but because the North Pole is tilted away from the Sun the light is less direct.

The effect of the seasons on living things is dramatic. Plants prosper during spring and summer, when the light of the Sun is a major factor in growth cycles of vegetation. Trees that lose their leaves in winter (deciduous) are responding to the changes in temperature and light, as well as the amount of time they normally thrive (biological clock). Animals respond to the changes as well, with many fur-bearing animals carrying lighter coats during the warmer months and heavier coats during the colder months. The orbiting of the Earth combined with the slight tilt provides the planet with the unique conditions that support life as we know it.