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

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