The summer solstice on Thursday signals the end of spring in the Northern Hemisphere and the start of a brand-new season, one that promises more warmth and more sunlight. To mark the transition into summer 2024, the astronomical event serves as a kind of grand opening: everywhere above the equator, it will be the longest day of the year.

At Stonehenge, an prehistoric monument of massive stones that is now a protected heritage site in southern England, historians believe ancient people built a ceremonial circular structure from enormous sarsen stones with a specific intention to honor and celebrate the solstice. 

Mysteriously erected around 2,500 B.C.E., Stonehenge is thought to be a spiritual or ritual ground of some sort, although the true reasons why people conceived of the idea to build it, and what they may have used it for, are still unknown. The stones were raised and meticulously arranged in the late Neolithic, or Stone Age, period —a time when creating such a monument would have been a brilliant feat of advanced construction and engineering. 

To a person standing in the center of Stonehenge, the layout is oriented so that the stones frame, with precision, the sunrise on the summer solstice and the sunset on the winter solstice. Thousands of people flock to the site each year on both solstices to witness the phenomenon for themselves.

Just a day before this year's summer solstice, climate protesters sprayed Stonehenge with orange paint to make a statement against fossil fuels. The organization that manages the Stonehenge site, English Heritage, told CBS News the incident was "extremely upsetting and our curators are investigating the extent of the damage," but that the monument remained open to the public.

What is the summer solstice?

The solstice is technically the kickoff to summer in the Northern Hemisphere, with its inverse, the winter solstice, simultaneously ushering in winter across the Southern Hemisphere. 

It occurs when Earth, which is tilted 23.5 degrees on its axis, reaches the maximum point at which the planet's northern half is oriented towards the sun since the summer solstice last came around. Because Earth is constantly rotating on its axis and simultaneously orbiting the sun, this tilt offers each hemisphere the chance to bathe in its longest extent of daylight only one per year.

Seasons exist on Earth because of the 23.5-degree tilt. As the planet is spinning and traveling its orbital path over roughly 365 days, Earth's slanted axis means the angles at which different parts of its surface face the sun shift throughout the year. During the Northern Hemisphere's summer solstice, which usually falls on either June 20 or June 21 — it's June 20 this year — those cosmic mechanics nudge the top of the planet forward the sun. At the North Pole, six months of daylight commence, while at the South Pole, it means six months of darkness instead.

Conditions reverse six months later, when Earth arrives at a point in its orbit where the axis tips it back so that the South Pole is nearest the sun. On that day, typically Dec. 21 or 22, the winter solstice in the Northern Hemisphere brings the shortest stretch of daylight, while the Southern Hemisphere begins its summer. 

Crowds gathered at Stonehenge in December to mark the winter solstice as well.

Why is June 20 the longest day of 2024?

The North Pole is never as skewed toward the sun as it is during the summer solstice. That pronounced tilt exposes a larger section of the Northern Hemisphere to sunlight at one time than it does at any other point in Earth's revolution. From the perspective of a person on the ground, that exposure creates the longest period of daylight in 24 hours to occur all year.  

This year, the North Pole reaches its most extreme tilt at 4:51 p.m. EDT on June 20, according to the National Oceanic and Atmospheric Administration. The sun is then directly over the Tropic of Cancer, a longitudinal line at wraps horizontally around the circumference of the Earth at 23.5 degrees above the equator. The line runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India and southern China.

On the other end of the cycle, when the North Pole is tilted as far as possible away from the sun, the section of the Northern Hemisphere that sunlight can reach is as small as it can be. That's why the winter solstice is the shortest day of the year for people above the equator, and the longest day of the year for people below.

How summer solstice has been celebrated throughout history

The summer solstice is an occasion that civilizations have observed and celebrated for millennia. Historians believe the Neolithic people who constructed Stonehenge were part of a broader ancient culture in northern and central Europe that, experts say, did seem to acknowledge the solstice and changing seasons as they related to agriculture and, potentially, the timing of crop cycles. 

The British nonprofit National Trust writes in an overview of ancient solstice traditions that the event "was typically marked by Celtic, Slavic and Germanic people by lighting bonfires, intended to boost the sun's strength for the remainder of the crop season and ensure a healthy harvest." Other Neolithic stone circles somewhat like Stonehenge also appear to have been built with the solstices in mind, according to the organization.

These days, people mark the solstice with their own take on the traditional ceremonies of ancient times. Festivals and bonfires are common in communities around the world. But there are myriad ways in which modern-day people acknowledge Earth's seasonal transit. Depending on where it takes place, a celebration of the summer solstice can look like a baseball game at midnight in Fairbanks, Alaska, or an all-day mass yoga gathering in Times Square.

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• Summer Solstice

Emily Mae Czachor

Emily Mae Czachor is a reporter and news editor at CBSNews.com. She covers breaking news, often focusing on crime and extreme weather. Emily Mae has previously written for outlets including the Los Angeles Times, BuzzFeed and Newsweek.

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