What is the summer solstice, and why is June 21 the longest day?

The summer solstice is an astronomical event that marks the longest day of the year and the start of summer in the Northern Hemisphere.

It nearly always falls on June 21, but can also fall on June 20 or June 22 depending on the year and time zone you are in.

On that day, nearly 90 percent of the world’s population, who live above the equator in the Northern Hemisphere, will have the longest period of daylight in the year and also the shortest night.

Why is June 21 the longest day of the year?

The summer solstice occurs when the sun is at its highest position in the sky. This happens once every year because the Earth’s axis is tilted by about 23.5 degrees relative to its orbital plane around the Sun. This tilt is also what causes the changing seasons.

For several days or weeks before and after June 21, those living in the Arctic, including parts of Alaska, Canada, Greenland and Scandinavia, will have continuous daylight resulting in what is known as the midnight sun.

The first day of summer – in astronomical terms

The summer solstice also marks the first day of summer in astronomical terms for those living in the Northern Hemisphere.

Astronomers use the dates of the two solstices and two equinoxes to mark the seasons. An equinox occurs when the sun crosses the equator’s path and is positioned exactly between the Northern and Southern Hemispheres.

The astronomical seasons for the Northern Hemisphere are best approximated as:

• Spring (March 21)
• Summer (June 21)
• Autumn (September 21)
• Winter (December 21)

Meteorologists, on the other hand, use temperature cycles to divide the year into four seasons, each exactly three months long.

The meteorological seasons for the Northern Hemisphere are defined as:

• Spring (March 1)
• Summer (June 1)
• Autumn (September 1)
• Winter (December 1)

Shortest day south of the equator

June 21 is the shortest day of the year in the Southern Hemisphere, affecting about 10 percent of the world’s population.

For those living in countries such as Argentina, Australia, South Africa and New Zealand, June 21 marks the winter solstice or the first day of winter. In the Antarctic Circle, a Polar night will occur, where there will be zero hours of daylight.

What is the cultural significance of the summer solstice?

The summer solstice is celebrated in many cultures. Throughout history, different cultures have relied on celestial events in order to determine agricultural practices, such as when to plant and harvest crops, or in the development of different calendars around the world.

In Sweden, midsummer marks the midpoint of the growing season and traditions involve singing, dancing and bonfires.

Thousands of people also gather at Stonehenge, an ancient monument in Wiltshire, England, to witness the sunrise aligned with the stones.

Revellers attend summer solstice celebrations at Stonehenge near Amesbury, Britain, June 21, 2023. [Toby Melville / Reuters]