A Connecticut village in autumn is ablaze with the brilliant colors of turning leaves. Every year, many vacationers visit the state to enjoy its scenic countryside and to explore its many picturesque villages and historic sites.
Credit: © Corbis Bridge/Alamy Images

Fall means pumpkins, colorful leaves, and crisper weather. It also means the end of summer. What causes the seasons to change? The four seasons result from the tilt of Earth’s axis of rotation and the planet’s motion around the sun. The axis slants at an angle of 23 degrees 27 minutes in relation to the plane of Earth’s orbit. As a result, Earth’s Northern and Southern hemispheres tilt toward or away from the sun during different times of the year. Some people mistakenly believe that seasons result from Earth moving closer to or farther from the sun at different times of the year. But Earth orbits at nearly the same distance from the sun throughout the year. The small changes in distance that do occur have little effect on Earth’s weather.

Tomorrow, September 23rd, at 2:50 a.m. Eastern Time, the autumnal equinox marks the beginning of autumn in the Northern Hemisphere. In the Southern Hemisphere, where the seasons are reversed, the event is called the vernal equinox and marks the start of spring. The word vernal means of spring. An equinox is either of the two moments each year (the other is in March, again changing the seasons) when the sun is directly above Earth’s equator. On the days of the equinoxes, all places on Earth receive approximately 12 hours of sunlight. Today, the sun rose at 6:43 a.m. Eastern Time and will set at 6:53 p.m.—a total of 12 hours and 10 minutes of daylight. The term equinox comes from a Latin word meaning equal night. The equinoxes occur at different times of day each year on March 19, 20, or 21 and on September 22 or 23.

The time interval from the March equinox to the September equinox is longer than that between the September equinox and the next March equinox. This time difference results from the Earth’s elliptical (oval-shaped) orbit around the sun. Our planet moves faster in its orbit when it is closer to the sun. The distance between the Earth and the sun is shortest in January. Therefore, the Earth completes the semicircle from the September equinox to the March equinox faster than it does the opposite semicircle.

Astronomers also use the term equinox for either of two imaginary points where the sun’s apparent path among the stars crosses the celestial equator. The celestial equator is an imaginary line through the sky directly over the equator.

After the autumnal equinox, the weather cools and nights become longer than days, and days continue to shorten until the winter solstice. The weather then warms and daylight begins its recovery toward the summer solstice in June. The winter solstice is technically the shortest day of the year, and the day of the summer solstice enjoys the most sunlight.

September 22, 2017

Today, at 4:02 p.m. Eastern Time, the autumnal equinox marks the beginning of autumn in the Northern Hemisphere. In the Southern Hemisphere, where the seasons are reversed, the event is called the vernal equinox and marks the start of spring. The word vernal means of spring. An equinox is either of the two moments each year (the other is in March, again changing the seasons) when the sun is directly above Earth’s equator. On the days of the equinoxes, all places on Earth receive approximately 12 hours of sunlight. Today, the sun rose at 6:43 a.m. Eastern Time and will set at 6:52 p.m.—a total of 12 hours and 9 minutes of daylight. The term equinox comes from a Latin word meaning equal night. The equinoxes occur at different times of day each year on March 19, 20, or 21 and on September 22 or 23.

The equinoxes are the two moments of the year when the sun is directly above the equator. As Earth moves in its orbit around the sun, the position of the sun changes in relation to the equator, as shown by the dotted lines in this diagram. The sun appears north of the equator between the March equinox and the September equinox. It is south of the equator between the September equinox and the next March equinox. Credit: WORLD BOOK diagram

The time interval from the March equinox to the September equinox is longer than that between the September equinox and the next March equinox. This time difference results from the Earth’s elliptical (oval-shaped) orbit around the sun. Our planet moves faster in its orbit when it is closer to the sun. The distance between the Earth and the sun is shortest in January. Therefore, the Earth completes the semicircle from the September equinox to the March equinox faster than it does the opposite semicircle.

Astronomers also use the term equinox for either of two imaginary points where the sun’s apparent path among the stars crosses the celestial equator. The celestial equator is an imaginary line through the sky directly over the equator.

After the autumnal equinox, the weather cools and nights become longer than days, and days continue to shorten until the winter solstice. The weather then warms and daylight begins its recovery toward the summer solstice in June. The winter solstice is technically the shortest day of the year, and the day of the summer solstice enjoys the most sunlight.

March 20, 2012

The March equinox has marked the official start of spring in the Northern Hemisphere and of autumn in the Southern Hemisphere. At the March equinox, the sun is directly above the Earth’s equator. On the days of the equinox, all places on Earth receive approximately 12 hours of sunlight, though the exact amount of daylight in different locations varies by as much as several minutes. All places also have approximately 12 hours of darkness.

The equinox officially occurred at 5:14 Coordinated Universal Time (UTC) on Tuesday, March 20, at the prime meridian at the Royal Greenwich Observatory in Greenwich, England. In the United States, 5:14 UTC was 10:14 p.m. in Los Angeles (Pacific Standard Time) on Monday, March 19, and 1:14 a.m. in New York City (Eastern Daylight Time) on Tuesday. In Sydney, Australia, the March equinox began at 4:14 p.m. (Australian Eastern Daylight Time) on March 20.

The March equinox is one of two equinoxes that take place as Earth completes a full orbit of the sun. The March equinox occurs on March 19, 20, or 21. The September equinox takes place on September 22 or 23. This year’s spring equinox is the earliest since 1896. As a result, winter was only 88.994 days long.

The equinoxes are the two moments of the year when the sun is directly above the equator. As Earth moves in its orbit around the sun, the position of the sun changes in relation to the equator, as shown by the dotted lines in this diagram. (World Book diagram)

The positions of the two equinoctial points do not remain the same from year to year. They shift westward extremely slowly–about 1 degree every 70 years. This gradual movement of the points, called the precession of the equinoxes, is caused by a slight change in the direction of Earth’s axis of rotation. The change results mainly from the gravitational pull of the moon and the sun on Earth’s equatorial bulge.

In the United States, the start of spring has been accompanied by record high temperatures throughout much of the nation. On March 14, 400 locations around the United States reported record highs. Even the 68 °F (20 °C) temperature at Duluth, Minnesota, which was far below the 80 °F (27 °C) reported in Miami, Houston, and Chicago, was 35 Fahrenheit degrees (19 Centigrade degrees) above average.

Additional World Book articles:

• Reading the Sky: Early Places of Astronomy (a Special Report)
• Astronomy
• Season