When is the summer solstice, the longest day of the year in the northern hemisphere? Find out what it is, and its meaning

• The 2023 summer solstice happens on Wednesday 21 June at 10:58 a.m. ET.
• It is the longest day of the year and marks the arrival of the warmer months.
• It is due to Earth's tilted axis as it orbits around the sun. Here's how it works. 

The summer solstice is almost here. The highly-anticipated event will take place at 10:58 a.m. ET on Wednesday 21st of June.

For people who live in the northern hemisphere, it will be the longest day of the year. It also signals the arrival of summer and a gradual advance toward fall, the start of which is marked by an equinox.

Two things drive this all-important seasonal shuffle: Earth's tilted axis and the planet's orbit around the sun.

How the summer solstice works

The summer solstice is the point when the sun will stay closest to the horizon all day. 

While the Earth rotates around its axis once a day and orbits the sun once every 365 days, approximately, its axis is not perfectly perpendicular to the orbit around the sun. Instead, the axis is tilted by about 23.45 degrees, which means different parts of the world receive more or less sunlight, depending on the time of year.

On the summer solstice, the northern hemisphere is tilted toward the sun, where the sun rises earlier and sets later, meaning countries will get the longest day of sunlight they will get all year. 

This is what the change of seasons looks like, as seen by a satellite.

How Earth's axis and orbit drive the seasons

Our planet's orbit is elliptical, and its center of gravity is slightly offset from the sun.

This means the time it takes to cycle through the seasons isn't perfectly divvied up:

As the graphic above shows, it takes 89 days after the December solstice for Earth to reach the March equinox. That's when the most direct rays of the sun have slipped back up to the equator.

Another 92 days and 19 hours later, it will be the June solstice. At that point, the sun's most direct rays reach the Tropic of Cancer, summer starts for the northern hemisphere, and winter begins for those south of the equator.

Then it takes 93 days and 14 hours for the sun's zenith to get back to the equator and kick off the September equinox, followed by 89 days and 19 hours to complete the cycle with the December solstice.

During each of these phases, certain regions of Earth's surface get more sunlight, and energy gets stored or sapped from water sources, leading to the creation of seasonal temperatures and weather variations.

Why it's so celebrated

Cultures around the world have historically celebrated the solstice as a time when crops mature and start bearing fruit.

The day has been injected with religious meaning. Celtic, Slavic, and Germanic people, for instance, light bonfires intended to boost the sun's power. 

In 16th-century China, the emperor would make offerings to the gods at the Temple of the Earth during the solstice, according to English Heritage.

Some traditions linked to the solstice have carried on until modern times. Sweden celebrates midsummer, a gathering of friends and family where some people will adorn traditional clothing and floral wreaths and dance around a maypole. The tradition us thought to date back to a pagan festival celebrating fertility and the triumph of light over dark.

People who follow druidic traditions still mark the event to this day. One notable example of this is a yearly pilgrimage to the neolithic stone circle Stonehenge in the UK.