Behind the Forecast: Why our seasons change

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Published: Sep. 11, 2020 at 8:03 AM EDT
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LOUISVILLE, Ky. (WAVE) - The Earth’s tilt is the reason for our seasons. Throughout the year, each part of our planet receives different amounts of sunshine. That uneven heating causes our astronomical seasons.

The North Pole tilts towards the sun during the summer, and the South Pole tilts towards the sun during the Northern Hemisphere’s winter. The planet’s tilt does not change throughout a year. The northern axis is always pointing towards Polaris, also known as the North Star.

Scientists believe that an enormous object smacked into the Earth a long time ago, causing it to rotate with a slight lean and not with its axis straight up. The Earth’s tilt on its axis varies from 22 degrees to 24.5 degrees; on average, the angle is 23.5 degrees.

The change of seasons is based on the tilt of Earth on it's axis.
The change of seasons is based on the tilt of Earth on it's axis.(NASA)

Earth’s orbit is elliptical, a bit of a lop-sided circle, which causes the planet to be closer to the sun at different times of the year. The nearest point to the sun is called the Perihelion and occurs in January. The Aphelion occurs when the Earth and the sun are farthest away from each other; this occurs in July.

As we orbit the sun, Earth’s axis always points in the same direction, so different parts of the planet get varying amounts of the sun’s direct rays.

Due to the tilt of the Earth, the Northern Hemisphere’s summer occurs when the Earth is farther away from the sun. More of the sun’s direct rays beam down on the Northern Hemisphere during their summer months. The Northern Hemisphere is in winter when the South Pole tilts towards the sun.

The Northern Hemisphere is more directly in the path of the sun’s rays around the summer solstice (near June 21st). During the summer solstice, the Earth is tilted enough that the sun is directly over the Tropic of Cancer, which is at 23.5° north latitude. More of the sun’s energy reaches the ground because it has less of the atmosphere to travel through. This high sun angle leads to long days.

For the autumnal equinox, which is usually around September 21, the Earth and sun are positioned in a way that places the sun directly over the equator. The sun’s energy is distributed almost equally across the northern and southern hemispheres. The same thing happens for the spring equinox around March 21. On both equinoxes, there are 12 hours of daylight and night.

On December 21, the winter solstice, the sun is directly over the Tropic of Capricorn at 23.5 degrees south latitude. In this situation, the Southern Hemisphere receives direct sunlight, producing longer days and hotter temperatures.

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