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Earth's tilt the reason for the season (and seasons)

Many people recognize today as one of only a few days left to wrap up their holiday shopping, but scientists and historians say the date is significant for another reason.

In a handout image, a view from space of how the sun falls on Earth during solstices and equinoxes captured by EUMETSAT's Meteosat-9. The scientific start of winter offers a moment to reflect on how we might not be here to witness the changing seasons without Earth’s particular tilt toward the sun. (Robert Simmon and EUMETSAT/NASA Earth Observatory via The New York Times)
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Many people recognize today as one of only a few days left to wrap up their holiday shopping, but scientists and historians say the date is significant for another reason.

This day marks the annual winter solstice, the time Earth's axial tilt of 23.4 degrees is farthest from the sun. It's that slant, some say, that is both the reason for the season and the reason for the seasons. More on that later.

First, and astronomically speaking, Sherry Fieber-Beyer, research assistant professor at UND's Department of Space Studies, says the winter solstice is the shortest day and longest night of the year in the Northern Hemisphere. According to meteorologist John Wheeler, Grand Forks will see eight hours and 23 minutes of daylight today.

The exact moment of the winter solstice is at 10:28 a.m., but the whole day generally gets bragging rights, she said.

"It's a good day for us. It's the first day of astronomical winter, that's for sure," Fieber-Beyer said. "We astronomers and scientists use the December solstice as the start of the winter season. Meteorologists, on the other hand, (say) winter began three weeks ago on Dec. 1."


Fieber-Beyer says it's important to note winter and summer solstices don't occur at the same time worldwide.

"The solstices are on opposite sides of the equator, so the winter solstice in the Northern Hemisphere is the summer solstice in the Southern Hemisphere and vice versa," she said.

The term "solstice" comes from the Latin word "solstitium," meaning the sun stands still.

The winter solstice in the Northern Hemisphere is when the sun's daily maximum elevation in the sky is at its lowest, Fieber-Beyer said.

"On this day, the sun reaches its southernmost position as seen from Earth, so what happens is the sun seems to stand still at the Tropic of Capricorn and then reverses its direction," she said. "It's also called the day the sun turns around."

No matter the definition, scientists agree it's that axial angle, or slant, at which the planet orbits the sun that prevents solar energy from being constant. It's the true reason for the seasons, plural: spring, summer, fall and winter. It also explains why the seasons occur at opposite times in the Northern and Southern hemispheres.

Season singular

Fieber-Beyer said many holidays and traditions throughout history were tied to the winter solstice, which falls between Dec. 20 and Dec. 23.


"It can happen anywhere on those four days," she said. "But the Dec. 23 solstices are rare. The last one was in 1903, and it won't happen again until 2303."

Christians worldwide celebrate Christmas on Dec. 25, and she said historians believe that specific date was chosen to offset pagan celebrations.

"Some believed that celebrating the birth of Jesus Christ was set in synchronization with the December solstice," she said. "They believed the days got longer because he's the true light of the world, and from that day on, the days began to have more daylight."

The ancient Inca Indians of South America commonly celebrated the winter solstice, too, in honor of their sun god. And Mayans in Mexico and Guatemala were known to perform a dangerous ritual known as the flying pole dance.

Five men would climb to the top of a 50-foot pole and as one of them beat a drum and played a flute, the other four would wrap a rope around a foot and hurl themselves to the ground.

"If they landed on their feet, they believed the sun god was pleased, and the days would start to get longer," Fieber-Beyer said.

The ritual, believed to stop droughts in ancient times, is performed yet today, but in modified form.

Related Topics: SCIENCE
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