Science First Look

Solstice! Why Northern Hemisphere gets first official day of winter.

The winter solstice was once a high point on the astronomical calendar. Today, it is significant for the extra minutes (and eventually hours) of daylight it returns to Northern Hemisphere dwellers.

Rebecca Smith poses for a photograph during winter solstice with her Irish Wolfhound dog called Amazing Grace at the 5000 year old stone age tomb of Newgrange (not in view) in the Boyne Valley at sunrise in Newgrange, Ireland, December 21, 2016.
Clodagh Kilcoyne/REUTERS
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Daylight lovers in half the world can rejoice! While winter days in the Northern Hemisphere will get only colder in the coming months, sunset will occur later and later each day after Wednesday morning's solstice.

Although the winter solstice is merely a date on the calendar to most modern humans, even those who are happy to begin marking time towards the long, warm days of summer, this date’s historical celestial significance makes it remarkable in itself.

Astronomically, the December solstice occurs as the sun reaches its southernmost point in the sky, which happened this year at 5:44 a.m., Eastern Standard Time, on Wednesday. Yet while this day marks the beginning of astronomical winter, scientists say that nothing much will change, weather-wise, at least in the short term.

In fact, it is not until late January that residents of the Northern Hemisphere will likely experience the coldest day of the season.

“There’s not a good answer for why people say that December 21 is the beginning of winter,” the National Oceanic and Atmospheric Administration’s Anthony Arguez said in 2015, according to The Christian Science Monitor. “There’s nothing magical that says that winter has to happen after the solstice.”

Wednesday is a remarkably short day by any terms. Washington, D.C., will experience fewer than nine and a half hours of sunlight, and more northern cities such as New York and Montreal will see even less.

The Boston Globe reports that while the number of daylight hours lost between the summer and winter solstices varies by region, residents of some of the largest US cities, including Chicago and New York, can expect to lose six to seven hours of daylight from summer to winter.

But for those north of the equator, things will only get brighter from here. From now on, the Earth will tilt slowly on its axis towards the sun, until the longest day of the year occurs on Wednesday, June 21, 2017, also known as the summer solstice in the Northern Hemisphere.

Yet due to the Earth’s tilt, as the Northern Hemisphere tilts away from the sun, the Southern Hemisphere tilts towards it, making Wednesday the Southern Hemisphere’s summer solstice. From now on, days will get marginally shorter for residents of São Paulo or Sydney.

While some individuals still celebrate both summer and winter solstices, as well as their vernal and autumnal equinox cousins, the solstice was far more important to our ancient ancestors.

Some of the world’s most awe-inspiring ancient sites were constructed with the celestial calendar in mind, from England’s Stonehenge to the Goseck Circle in Germany to pyramids in Mexico and other parts of Central America. Ancient humans used these sites to keep track of the seasons, keeping track of the rhythms of the seasons and daily life.

In 2014, Noelle Swan wrote for The Christian Science Monitor:

The great sandstone monoliths at Stonehenge in southern England are said to mark the autumn and spring equinoxes, as well as the summer and winter solstices, the longest and shortest days of the year respectively.

Many historians believe the massive rock formation was erected under the supervision of astronomer-priests as an astrological calendar to alert ancient people of the optimal time to begin planting, harvesting, and breeding cattle.

On Wednesday, however, modern humans look forward, not to begin planting or breeding their cows, but rather to more daylight and commuting before nightfall. 

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