When is the first day of winter? December solstice 2018 explained.

Shortening days and falling temperatures
are heralding the arrival of winter across the Northern
Hemisphere.

This year, the northern
winter solstice^[1]
falls on December 21 at 5:23 p.m. ET (22:25 UT). The solstice
happens at the same moment around the world, though its local time
depends on which of the 24 times zones you call home. And south of
the Equator, the December solstice marks the onset of
summer.

For people in the north, is the year’s shortest day also the
official beginning of winter? Why does the solstice occur anyway,
and how have people observed it over the ages? Read on for
everything you need to know about the December solstice.

The reason for the solstice—and the seasons—is that Earth is
tilted with respect to the sun at an average of 23.5 degrees. This
means the Northern and Southern Hemispheres receive unequal amounts
of sunlight over a year as we orbit our star.

This astronomical event appears on calendars as the first day of
winter—but meteorologists have already gotten a head start on the
season. By the time the solstice rolls around, climate scientists
have been observing winter for nearly a month, says Greg Hammer of
NOAA’s National Centers for Environmental Information.

“Meteorological winter in the Northern Hemisphere is always the
months of December, January, and February, because those tend to be
the coldest months of the year,” he says. “It’s entirely based on
the annual temperature cycle, rather than astronomically
based.”

Considering sunlight’s enormous influence on Earth’s climate,
why isn’t the darkest part of the year also the coldest?

“Basically, it takes a while for water and landmasses to cool
down from all the heat they have absorbed” during summer, Hammer
says. “So, the minimum of daylight isn’t followed by the coldest
temperatures until about a month later. It varies from place to
place, but around the third week in January tends to have the
coldest temperatures of the year for most of the U.S.”

Meteorological winter more closely mirrors our civic calendars
and the way most people actually think about the seasons, Hammer
adds. “We think of winter being the coldest time and summer being
the warmest time, with spring and fall being transitions.”

And for climate scientists, following meteorological winter
offers important simplicity: “It just makes it easier in terms of
calculating statistics and making comparisons from one season to
another,” Hammer says. “The timing of the solstice can vary a bit.
This way, we are comparing apples to apples when speaking
seasonally, like noting that a certain winter was the coldest on
record in a certain area.”

Most of us see the year’s earliest sunset a week or two before
the solstice. That’s because the sun and our human clocks don’t
keep exactly the same time.

We’ve organized our days into 24-hour segments, but Earth
doesn’t spin on its axis that precisely. While the time from noon
to noon is always exactly 24 hours, the time varies between solar
noons, the moment each day when the sun reaches its highest point
in the sky. As we move through the year, the chronological time of
solar noon shifts seasonally—and so do each day’s sunrises and
sunsets.

During December, solar noons can happen some 30 seconds after we
would mark a 24-hour cycle. While the shortest amount of total
daylight falls on the solstice, that day’s sunset is actually a few
minutes later on our clocks than it was earlier in the month.

In Washington, D.C., for example, sunset on the solstice will be
at 4:49 p.m. ET—three minutes later than it was back on December
9.

Closer to the Equator, the year’s earliest sunset occurs in
November. To see it coincide more closely with the solstice, simply
head toward the Arctic. Seasonal shifts in the sun’s path across
the sky are most extreme closer to the Poles, which makes sunset
times at high latitudes match the solstice^[2]
schedule more closely.

It is possible to see the effects of the solstice by noting what
happens in the skies overhead, as well as the changes in sunlight
over time.

For viewers in the north, the sun’s arc across the sky has been
steadily dropping lower and becoming shorter since June. At the
northern winter solstice, it reaches its lowest possible arc—so low
that in the few days surrounding the solstice, it appears to rise
and set in the same place. That phenomenon produced the Latin
origins of the word “solstice,” which means “sun stands still.”

The sun’s low angle means that your noontime shadow is the
longest it will be for the entire year during the winter
solstice.

In the ancient world, people built a number of monuments^[3]
to commemorate the solstice. One example is Newgrange^[4], a huge Stone Age tomb
mound built in the Irish countryside around 3200 B.C., about a
thousand years before Stonehenge. A tunnel facing the solstice
sunrise runs to a main chamber, where the dead may have once been
placed. A small window bathes the chamber in solstice light for 17
minutes

The Paracas people of Peru, who lived around 800 to 100 B.C.,
crisscrossed the desert with lines of earth and rock called
geoglyphs that connect ceremonial mounds with the place where the
winter solstice sun sets on the horizon. The famed Nas^[5]ca
Lines^[6]—awe-inspiring monkeys,
lizards, and other giant figures etched into the earth by a
subsequent Peruvian culture around A.D. 1 to 700—also feature
alignments with the winter solstice.

Ancient Egypt’s sprawling Temple of Karnak was constructed in
alignment with the winter solstice at Luxor more than 4,000 years
ago. Similar alignments can be seen at Angkor Wat in Cambodia and
Machu Picchu in Peru.

For more than two billion Christians, the solstice has long been
overshadowed by Christmas. But to historian David
Gwynn^[7] of the University of
London, the proximity of the two events may not be an accident.

According to Gwynn, one theory holds that Christmas was set on
December 25 to replace a Roman holiday, which had roots in the
pagan cult of Sol Invictus (“the unconquered sun”).

Other solstice traditions color today’s winter holiday
celebrations. Scandinavians once celebrated Juul, or Yule, a
multiday feast marking the sun god’s return. In Britain, Druids
observed the solstice by cutting
mistletoe^[8].

Some ancient solstice celebrations continue in the present day.
Iran’s Yalda festival marks the day when Mithra, an angel of light,
was thought to have been born. The tradition was adopted into
Zoroastrianism and is still observed by staying up late and
savoring treats like watermelon and pomegranate.

China’s Dōngzhì festival marks the time when winter’s darkness
begins to give way to light. Families observe this time by enjoying
special foods, such as glutinous rice balls known as tang
yuan.