Winter Solstice: The Science Behind the Shortest Day of the Year (2023)

The winter solstice heralds the astronomical beginning of winter and marks the day with the fewest daylight hours of the year. But what is the science behind the shortest day and longest night?

OWinterThe solstice - and with it the four seasons - occurs because the Earth is tilted at an angle of about 23.5 degrees to the sun. Instead of rotating around a straight axis, our planet is "a little bit tilted", he said.Michael S. F. Kirk(opens in new tab), a research astrophysicist with the Division of Heliophysical Sciences at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

This tilt means that the northern and southern hemispheres receive different amounts of sunlight, and the amount of light each hemisphere receives varies throughout the year as our planet moves around the sun - which is why we have seasons. Much of the Northern Hemisphere gets little daylight during the winter months, while the Southern Hemisphere experiences the opposite - enjoying summer during the Northern Hemisphere's winter and enduring the winter while the Northern Hemisphere warms up in the summer.

But although the Northern Hemisphere's winter solstice is recognized as a full day, it occurs at a time when theNorth Poleis at its farthest inclination of 23.5 degrees from theson. That position puts the North Pole out of reach of the Sun and plunges it into complete darkness, Kirk said.

The winter solstice will happen in 202216:48 EST (21:48 UTC) on December 21 in the Northern Hemisphere.

In the southern hemisphere, this moment marks the summer solstice, or the day with the most hours of light this year, as the South Pole is tilted towards the sun and sunnier. “The winter [northern] solstice is when the North Pole is completely shrouded in darkness; the South Pole is totally bright - it's summer down there," Kirk told Live Science.

What happens to the sun at the winter solstice?

On the winter solstice in December, there are fewer hours of sunshine the further north you go in the northern hemisphere. People in this hemisphere may notice that the sun is not as high in the sky, even at noon.

On the equinox – the two days of the year when both hemispheres experience an equal amount of daylight and night – the sun appears directly overhead, 90 degrees above the equator around noon. But on the northern winter solstice, the midday Sun appears directly overhead at a lower latitude: the Tropic of Capricorn, which is about 23.5 degrees south of the equator and straddles Australia, Chile, the southern Brazil and northern South Africa. The Tropic of Capricorn is the southernmost degree of latitude where the sun can appear directly at noon,according to the Pacific Island Ocean Observing System(opens in new tab), a project at the University of Hawaii.

Because the Sun peaks at noon at such a southern latitude, at higher northern latitudes, the Sun "barely leaves the horizon and sets again," Kirk said.

Why does the date of the winter solstice vary?

Every year, the winter solstice falls on one of two days in the northern hemisphere: December 21st or 22nd. In the southern hemisphere, the winter solstice occurs on June 20 or 21.

The date varies because the Gregorian calendar has 365 days and an extra leap day is added every four years in February. In reality, Earth's orbit around the Sun takes 365.25 days,This was reported by NASA(opens in new tab). Due to this discrepancy, the winter solstice does not always occur on the same day.

distance from earth to sun

Some parts of the Northern Hemisphere get so cold in the winter that you might think the Earth is farther from the sun at that time. "Actually, it's the opposite," Kirk said. "In the northern hemisphere, winter occurs when we are closest to the sun."

On average, Earth is about 93 million miles (150 million kilometers) from the sun,with NASA(opens in new tab). Earth will be closer to the sun, or inperihelion no 11:17 am EST on January 4, 2023, when it will be 91,403,034 miles (147,098,924 km).of the sunaccording to the almanac(opens in new tab).

Earth will be farther from the sun, or inAfel, no 4:06 pm EDT on July 6, 2023 if it is 94,506,364 miles (152,093,250 km).away from our star.

How long does winter last?

Astronomically, or according to the solstices and equinoxes, winter begins at the winter solstice and ends at the vernal equinox. Therefore, winter in the northern hemisphere lasts from December 21st or 22nd to March 19th, 20th or 21st. As the Earth has an oval or elliptical orbit, the seasons are not the same length; Winter lasts an average of 89 days in the Northern Hemisphere and 93.6 days in the Southern Hemisphere.seatimeanddate.com(opens in new tab).

Meteorologically (looking at the weather), winter in the US lasts from December 1st to February 28th or 29th, which are typically cold months in much of the country.Laut Weather Underground(opens in new tab). According to this definition, winter lasts 89 or 90 days.

What does solstice mean?

A few days before and after the solstice, the sun's journey across the sky looks so similar that it seems to take the same path every day - hence the name 'solstice', which in Latin means 'sun standing still'.with NASA(opens in new tab). This isn't really the case; The sun's path is a little different these days, but it can be difficult to see without modern instruments.

Why isn't the winter solstice the coldest day?

If there is so little sunlight in the northern hemisphere during the winter solstice, why isn't it the coldest day of the year?

"The most intuitive way [to understand it] is that it takes time for everything to cool down," Kirk said. “The sun receives less radiation, less heat on earth. It takes a long time for land and oceans to radiate all that energy and cool down from all the lack of sunlight.”

After the land and oceans cool down, it can take weeks or months for them to warm up again. After the winter solstice, days in the northern hemisphere start to get longer again. But the sun still does not shine as brightly as in summer; Northern mid-latitude experienceabout 9 hours of daylightin the weeks following the solstice, compared to the approximately 15 hours of sunlight they receive daily around the summer solstice. Also, the northern hemisphere is still tilted away from the sun, making it cold.

When is the winter solstice?

Winter Solstice Celebrations

Many cultures recognized the winter solstice. The most famous prehistoric site associated with the solstice is atStonehengein England. As the sun sets on the shortest day of the year, the sun's rays focus on the central altar stone and battle stone of Stonehengemay have had spiritual meaning for peoplewho erected the monument.

EmMexicoOn the Yucatan Peninsula, the stone-walled ancient Mayan city of Tulum also has a structure that honors the solstices. As the sun rises at the winter and summer solstices, its rays shine through a small hole in the top of one of the stone buildings, creating a star-like effect.

"I think there's a deep connection between our lives as humans and the amount of daylight and seasonal patterns we experience," said Kirk.