Scientific Explanation of the Fall Equinox

The fall equinox, also known as the autumnal equinox, marks the beginning of autumn in the Northern Hemisphere. This point in the Earth's rotation occurs around September 22 each year. Equal amounts of daylight and darkness occur on the day of the equinox. After the fall equinox, each day will receive less light and more darkness until the winter solstice.

Axis Tilt

Earth rotates around the sun in an elliptical orbit rather than in a perfect circle. We know that this orbit takes about 365 days, or one Earth year, to complete. The Earth also rotates on its polar axis, an imaginary line drawn through the planet from pole to pole. This rotation takes 24 hours to complete.

As Earth rotates on its axis and orbits the sun, it maintains a slight tilt of about 23.5 degrees. The tilt does not vary. Earth's northern pole is always pointing toward Polaris, the North Star. This tilt is responsible for the fall equinox as well as the spring equinox, summer solstice and winter solstice.

The Sun's Role

As Earth moves in this configuration, it receives the sun's light, heat and energy in different amounts at different points along the journey. At the summer solstice, the northern hemisphere is facing toward the sun, so it receives more warmth than the Southern Hemisphere, which faces further away from the sun.

During an equinox, the sun appears to be almost directly over the Earth's equator. At the fall equinox, the sun appears to move north to south over the equator. After the fall equinox, the Earth's tilt causes the Northern Hemisphere to face further away from the sun. The days get progressively cooler as less energy warms the Earth.

Weather

While the fall equinox is determined by the position of the sun relative to Earth and visible as the amount of available daylight, neither equinox nor either solstice marks the highest, lowest or most moderate temperature points. Weather patterns begin to change at the fall equinox, but weather is complex and dependent on more factors than sunlight including, for instance, how heat is absorbed by the ground and the oceans.

Two Hemispheres

Since the Earth's tilt affects the northern half of the globe differently than the southern half, seasons are reversed. When fall begins in the Northern Hemisphere, spring is arriving in the Southern Hemisphere. Winter in the south is summer in the north. As the fall equinox begins in the Northern Hemisphere, the spring or vernal equinox is taking place in the Southern Hemisphere.

Considerations

At the equator, no vast changes in seasons occur. The equator's position relative to the sun does not change much. There is a rainy season and a dry season but, even though the equator figures into the fall equinox because of where the sun appears, no real climate changes occur.

There is always some variation as to the start date of the fall equinox from year to year. This is a result of slight variations in the how the Earth's tilt matches up with the sun.

References

• Wolfram Research: Autumnal Equinox
• NASA: Seasons of the Year
• infoplease: Fall Equinox