You Will Not Know the Summer From the Winter

Check this out … the Breathing Globe. It's a year of seasonal transformations on our planet, including the December solstice. John Nelson created this blitheness, using images from the NASA Visible Earth team. Read more about the animation via John Nelson.

For us in the Northern Hemisphere, the December solstice marks the longest nights and shortest days of the twelvemonth. Meanwhile, the Southern Hemisphere is having brusk nights and long days. The 2021 December solstice moment – when the sunday reaches its southernmost signal in the heaven – will happen on Tuesday, December 21, 2021, at 15:59 UTC (nine:59 a.grand. CST; interpret UTC to your time).

No thing where you live on Earth's globe – no matter what time the solstice happens for you – it'south your indicate to gloat seasonal alter.

View larger. | Ian Hennes in Medicine Hat, Alberta, Canada, created this solargraph between a June solstice and a December solstice. It shows the path of the sun during that time flow. Thank you, Ian!

On the solar day of the December solstice, the sunday takes its farthest pass south on the globe.

An blitheness of Earth as it orbits, with points marking both equinoxes and solstices along with relevant data. Image via James O'Donoghue/ Business Insider.

What is a solstice?

The earliest people on Earth knew that the lord's day's path across the sky, the length of daylight, and the location of the sunrise and sunset all shifted in a regular way throughout the yr. They congenital monuments such as Stonehenge in England and at Machu Picchu in Peru to follow the sunday's yearly progress.

But today, nosotros see the solstice differently. We can picture it from the vantage point of space, and we know that the solstice is an astronomical event. It'south caused by the tilt of Earth's axis and past its orbital motion around the dominicus.

Earth doesn't orbit upright. Instead, it's tilted on its axis past 23 1/ii degrees. Through the yr, this tilt causes World's Northern and Southern Hemispheres merchandise places in receiving the sun's light and warmth most directly. It's this tilt, not our distance from the sun, that causes wintertime and summer. In fact, we're closest to – not farthest from – the lord's day at the plough of every new year. But we in the Northern Hemisphere are moving into wintertime. That's because the Northern Hemisphere leans uttermost abroad from the lord's day for the year around this time.

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The December solstice

At the December solstice, Earth is positioned so the sun stays below the North Pole's horizon. As seen from the latitude 23 1/2 degrees s of the equator, at the imaginary line encircling the globe known every bit the Tropic of Capricorn, the sun shines directly overhead at apex. This is as far south equally the sun e'er gets, and all locations south of the equator have day lengths greater than 12 hours.

Meanwhile, all locations north of the equator accept twenty-four hour period lengths shorter than 12 hours.

For us on the northern office of Earth, the shortest 24-hour interval comes at the solstice. After the wintertime solstice, the days will get longer, and the nights shorter.

It's a seasonal shift that nearly everyone notices.

Earth has seasons because our earth is tilted on its axis with respect to our orbit around the sun. Prototype via NASA/ kudzuacres.com.

Where should I look to meet signs of the December solstice in nature?

Everywhere.

For all of Earth'due south creatures, zippo is so key equally the length of daylight. Afterwards all, the lord's day is the ultimate source of all light and warmth on Earth.

In the Northern Hemisphere, y'all'll notice late dawns and early sunsets, the low arc of the lord's day across the sky each day, and how low the sun appears in the sky at local apex. Look at your noontime shadow, too. Around the time of the December solstice, information technology'south your longest noontime shadow of the year.

In the Southern Hemisphere, information technology's opposite. Dawn comes early, dusk comes late, the sun is high, and it's your shortest noontime shadow of the yr.

Around the fourth dimension of the winter solstice, watch for late dawns, early on sunsets, and the low arc of the sun across the sky each day. Notice your noontime shadow, the longest of the year. Photo via Serge Arsenie/ Flickr.

Meanwhile, at the summer solstice, noontime shadows are brusk. Photo via the Slam Summer Beach Volleyball festival in Australia.

Why doesn't the earliest sunset come on the shortest solar day?

The December solstice marks the shortest mean solar day of the year in the Northern Hemisphere and longest solar day in the Southern Hemisphere. Merely the earliest sunset – or primeval sunrise if y'all're south of the equator – happens earlier the December solstice.

Instead of focusing on the fourth dimension of sunset or sunrise, the fundamental is in what is called truthful solar apex, which is the time of twenty-four hour period that the sun reaches its highest point in its journey beyond your sky.

In early December, true solar noon comes most 10 minutes earlier past the clock than information technology does at the solstice effectually December 21. With truthful noon coming subsequently on the solstice, so volition the sunrise and sunset times.

Information technology'southward this discrepancy between clock time and lord's day time that causes the Northern Hemisphere's primeval sunset and the Southern Hemisphere's earliest sunrise to precede the December solstice.

This happens primarily because of the tilt of the Earth's axis. A secondary only another contributing factor to this discrepancy betwixt clock noon and dominicus noon comes from the Earth's elliptical – oblong – orbit around the sun. Earth's orbit is not a perfect circle, and the closer nosotros are to the sun, the faster nosotros move in our orbit.

Our closest point to the sunday – or perihelion – comes in early on January. So nosotros are moving fastest in orbit around now, slightly faster than our average speed of about 19 miles per second (xxx km per 2d). The discrepancy between sun fourth dimension and clock time is greater around the Dec solstice than the June solstice because we're nearer the sun at this time of year.

Solstice sunsets, showing the sun'south position on the local horizon at December 2015 (left) and June 2016 (right) solstices from Mutare, Zimbabwe. Image via Peter Lowenstein.

Does latitude bear upon the earliest sunset?

Yes! The precise date of the earliest sunset depends on your latitude. At mid-northern latitudes, it comes in early Dec each year. At northern temperate latitudes further north – such as in Canada and Alaska – the yr's earliest sunset comes effectually mid-December. Close to the Arctic Circumvolve, the earliest sunset and the December solstice occur on or almost the same day.

By the way, the latest sunrise doesn't come on the solstice either. From mid-northern latitudes, the latest sunrise comes in early on Jan.

The exact dates vary, merely the sequence is always the same: earliest dusk in early Dec, shortest day on the solstice around December 22, latest sunrise in early Jan.

And then the wheel continues.

Bottom line: The 2021 December solstice takes place on Tuesday, Dec 21, at fifteen:59 UTC (9:59 a.m. CST; translate UTC to your time). It marks the Northern Hemisphere'southward shortest mean solar day (first day of winter) and Southern Hemisphere's longest solar day (first day of summer). Happy solstice to all!

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Deborah Byrd

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About the Author:

Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves every bit Editor-in-Chief of this website. She has won a milky way of awards from the dissemination and scientific discipline communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a strength for skillful in the globe and a vital tool for the 21st century. "Existence an EarthSky editor is like hosting a big global party for absurd nature-lovers," she says.

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Source: https://earthsky.org/astronomy-essentials/everything-you-need-to-know-december-solstice/

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