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.

June 21, 2016

When the North Pole has its greatest slant toward the sun, left, summer begins in the Northern Hemisphere and winter begins in the Southern Hemisphere. Credit: WORLD BOOK map

Have you noticed that the days have been getting longer and longer (or the reverse, if you live in the Southern Hemisphere)? The increasing difference between day and night reached its peak yesterday at 22:34 Coordinated Universal Time. This moment is called the June solstice.

A solstice is one of the two moments each year when the sun is at either its northernmost or southernmost position in Earth’s sky. Earth’s axis of rotation is tilted at an angle of 23 degrees 27 minutes in relation to the plane of Earth’s orbit around the sun. As Earth orbits the sun, the angle of tilt points toward the same direction in space. Thus, the direction of this angle in relation to the sun changes throughout the year, causing seasons on Earth.

Solstices mark the beginnings of astronomical summer and winter. In the Northern Hemisphere, the June solstice marks the longest day of the year. In the Southern Hemisphere, yesterday was the shortest day of the year and signaled the arrival of astronomical winter. Now, the day and night will get closer to the same length until the next equinox in September. During an equinox, the tilt of Earth’s axis lies such that neither hemisphere is angled toward the sun. This means that each part of Earth receives the same amount of daylight, so day and night are roughly the same length.

Cultures around the world, but especially those in higher latitudes in the Northern Hemisphere, celebrate the June Solstice. For example, the people of Sweden celebrate Midsummer’s Eve and Midsummer’s Day around the time of the June Solstice. Solstices were also important to ancient people. Thousands of years ago, people living in what is now southwestern England built Stonehenge, a circle of large carved stones. Several features of Stonehenge align with the sun on solstices, suggesting that they were important to its builders.

Dec. 22, 2011

The arrival of the December solstice has marked the official start of winter in the Northern Hemisphere and of summer in the Southern Hemisphere. At the December solstice, the sun reaches its southernmost position on the horizon. At that point, Earth is tilted on its axis as far away from the sun as possible. The axis, the imaginary line around which Earth rotates, is not straight up and down but is tilted by about 23 degrees 7 minutes in relation to the plane of the planet’s orbit around the sun. In the Northern Hemisphere, the December solstice, also known there as the winter solstice, is the shortest day and longest night of the year. In the Southern Hemisphere, the December solstice, also known there as the summer solstice, is the longest day and shortest night of the year.

The solstice officially occurred at 5:30 Coordinated Universal Time (UTC) on Thursday, December 22, at the prime meridian at the Royal Greenwich Observatory in Greenwich, England. In the United States, 5:30 UTC was 12:30 a.m. in New York City (Eastern Standard Time) on Thursday and 9:30 p.m. in Los Angeles (Pacific Standard Time) on Wednesday, December 21. In Sydney, Australia, the December solstice began at 4:30 p.m. (Australian Eastern Daylight Time) on December 22.

The seasons change because the tilt of Earth's axis causes places on the planet to receive different amounts of sunlight during the year. Winter begins in the North Hemisphere when the North Pole has its greatest tilt away from the sun (right). At the same time, summer begins in the Southern Hemisphere, which has its greatest slant toward the sun. World Book illustration by Amie Zorn, Artisan-Chicago

The December solstice is one of two solstices that take place as Earth completes a full orbit of the sun. The June solstice occurs on June 20, 21, or 22, when the sun reaches its northernmost point on the horizon. It marks the start of summer in the Northern Hemisphere and the start of winter in the Southern Hemisphere.

As they have since prehistoric times, solstice celebrations were held in many places around the world. In the Northern Hemisphere, the December solstice has had particular significance as the day on which the sun is reborn and days begin to grow longer.

Additional World Book articles:

• Cahokia: Mysteries in the Mounds (a Special Report)
• Inca (Religion)
• Newgrange
• Reading the Sky: Early Places of Astronomy (a Special Report)
• Stonehenge