As the Earth rotates, half of the globe receives the sun's light while the other is bathed in shadow. But because Earth's axis tilts 23.5 degrees from the vertical, typically one hemisphere receives light for more time than the other, and therefore has a longer day. Twice a year, when the tilt is most extreme with respect to the sun, one hemisphere has its longest day of the year while the other has its longest night. This is called the solstice.
What causes the solstice?
We can thank the Earth's tilt for our planet's four seasons. The solar-facing hemisphere experiences summer — June to August in the Northern Hemisphere and December to February in the Southern Hemisphere. The hemisphere that tilts away from the sun receives less radiation and is colder. This alignment corresponds with winter, which falls in December to February in the Northern Hemisphere and June to August in the Southern Hemisphere.
If the Earth were not tilted, there would be no seasons because both hemispheres would receive equal light throughout the year. Surrounding the summer solstice in the Northern Hemisphere, parts of the Arctic Circle at the top of the Northern Hemisphere experience 24 hours of sunlight, while areas in the Antarctic, at the tip of the Southern Hemisphere, get no sunlight at all. The opposite is true during the Northern Hemisphere's winter solstice (24 hours of darkness in the Arctic Circle and 24 hours of light in the Antarctic).
When is the solstice?
Solstices mark the beginning of astronomical summer and winter and occur around June 21 and Dec. 21. A year is divided into four discrete seasons based on astronomical and meteorological cycles, but the two don't always have the same start and end dates for each season.
"People have used observable periodic natural phenomena to mark time for thousands of years," according to the National Oceanic and Atmospheric Administration's National Centers for Environmental Information. "The natural rotation of Earth around the sun forms the basis for the astronomical calendar, in which we define seasons with two solstices and two equinoxes." In comparison, the meteorological calendar breaks the year into four seasons based on annual temperature cycles.
The solstices don't land on the same calendar day every year because the astronomical year is 365.25 days long, but our calendar is either 365 or 366 days, so the solstices shift. Also, the Earth orbits the sun in a slight elliptical shape (rather than a perfect circle), which can add days to the astronomical seasons because the Earth's distance from the sun is not static. The Earth moves slightly faster the closer it is to the sun, when it is at its perihelion. As a result, winter in the Northern Hemisphere is a few days shorter than the summer and summer in the Southern Hemisphere is a few days shorter than winter.
Although we talk about a solstice day, the precise axial alignment occurs at a specific time, and the exact moment of the summer solstice — the longest day in the hemisphere — does not have to occur during daylight hours. The solstice occurs independently of which side of the Earth is facing the sun, which means it can occur even during the middle of the night, according to the U.K.'s Royal Museums Greenwich. The solstice occurs whenever the Earth's tilt is at its most extreme position with respect to the sun, which may or may not be when the hemisphere experiencing a summer solstice is facing the sun.
Why is it called a solstice?
Solstice is an amalgamation of the Latin words "sol" (sun) and "stare" (stand still), according to the Franklin Institute.
While the sun always rises in the east and sets in the west, it'll appear higher or lower in the sky over the year, depending on the season. Around the solstices, the sun reaches its apparent highest and lowest point in the sky. These correspond to midsummer and midwinter respectively, which are the turning points in the sun's journey. Once the sun reaches its zenith at the summer solstice, it will begin its journey toward the horizon, culminating in the winter solstice at its nadir. In the weeks before these solstice turning points, the sun appears to move very little, earning it the name "sun standstill."
If you were to map the sun's midday position every day for a year, it would make a lopsided figure eight, called an analemma. The point at which the curves of the figure eight intersect is the equinox, which is when day and night are roughly equal in length.
Who discovered the solstice?
Humans have been observing the sun's position in the sky for thousands of years. Solstices have inspired religious rituals throughout the world, and people have built monuments to celebrate the event.
At Stonehenge in the U.K., the sun rises behind the ancient entrance to a stone circle "and sunlight is channelled into the center of the monument," the BBC reported. Researchers believe that solstices have been celebrated at Stonehenge for thousands of years. The stone circle is particularly important to pagans and druids.
According to some ancient Greek calendars, the summer solstice heralded the beginning of the new year and marked the one-month countdown to the start of the Olympic Games, according to St Neots Museum in the U.K.
In the U.S., some Native American tribes living in the plains and Rocky Mountains perform a Sun Dance to celebrate the summer solstice.
In the Wiccan religion, people celebrate Yule at the winter solstice to herald the return of the sun and warmer months.