Why the summer solstice is the longest day of the year

The “sun standing still.” That’s what the Latin origin for solstice, solstitium, means.

Now the sun isn’t literally standing still, but in this twice-annual astronomical event, it almost seems to pause over the equator before the sun's rays reverse direction and head back south.

Sunday, June 21, will be the official start of astronomical summer and the day of the solstice this year, which typically happens around June 20 to 22. It’s the longest day of the year.

So, how does it work?

The Earth is tilted on its fixed axis by about 23.5 degrees. Because of this, different parts of the planet receive different amounts of sunlight throughout the year as the Earth orbits the sun.

(MORE: The summer solstice seen from space)

While the solstice is generally referred to as a day, it’s actually an exact moment the sun reaches its northernmost point or alignment, directly overhead at the Tropic of Cancer. This year, that’s 4:24  a.m. EST.

The Tropic of Cancer is an imaginary line of latitude circling the Earth at about 23.5° north of the equator. It spans three continents, passing through areas including Mexico, the Bahamas, the Sahara Desert, India and China.

And that name is actually outdated by about 2,000 years. When it was named, the sun appeared in the constellation Cancer during the June solstice. But because the Earth has a slow wobble of its axis — known as precession — the sun is actually now in the constellation Taurus during the June solstice.

This "wobble" takes about 26,000 years to complete a full cycle, meaning the sun will eventually line up perfectly with the Cancer constellation again in about 24,000 years.

Because the sun reaches its highest point in the sky, daylight lasts longer than on any other day of the year.

That’s because the Northern Hemisphere is tilted most directly toward the Sun, while at the winter solstice, it will be the longest night of the year. (That’s in the Northern Hemisphere; in the Southern Hemisphere, it’s swapped.)

And right in between the two, at the fall and spring equinox, days and nights are nearly equal.

This cycle affects the entirety of the seasons.

This tilt-induced change in daylight means the sun rises earlier and sets later in the summer months.

But the summer solstice also signals a slow and steady decline in this, as each day will gradually grow shorter until day and night are equal at the equinox and until nights are their longest at the winter solstice. And then back again, the winter solstice invites in light, with the gradual buildup of daylight hours toward summertime.

(MORE: Top sunrise-viewing destinations for the summer solstice)

In places above the Arctic Circle, known as the "land of the midnight sun," the sun doesn’t set at all, and some will experience 24-hour daylight.

In Fairbanks, Alaska, there’s a tradition since 1906 to play a baseball game starting around 10 p.m. on the solstice, since it never fully gets dark.

Also notable is not only the length of the day, but also the height of the sun. During the summer solstice, sunlight hits the ground at a steeper, higher angle, which is part of the reason for the warmer temperatures associated with summer. 

The most common solstice misconception is that distance from the sun is the cause of the effects we see on Earth. But actually, Earth is near its farthest point from the sun — known as aphelion — during summer. Seasons come from tilt and sun angle, not distance.

You may also be wondering why the hottest days come in late July and August if June 21 is the longest day. Both land and oceans take time to heat up and are a step behind the actual amount of sunlight, so peak warmth trails the peak daylight.

Culturally, the solstice has been celebrated and honored across time. 

One of the most historic physical representations is Stonehenge. Thousands gather at the prehistoric monument in England each year to watch the solstice sunrise align with the Heel Stone.

(MORE: The solstice family: a cosmic coincidence at Stonehenge)

Swedish Midsummer is also based around the summer solstice. This celebration features maypole dancing, flower crowns, pickled herring and bonfires. Other Nordic countries light bonfires, too, a tradition once meant to ward off evil spirits.

International Yoga Day also falls on June 21, chosen partly because the solstice carries special meaning in many cultures. Surya namaskar, also known as sun salutations, is a traditional sequence of poses created out of reverence for the sun to honor it as the source of energy and light for the world. Now it’s an integral part of many yoga routines.

Content writer Ada Wood enjoys exploring the stories that science and climate teach us about our natural world and how it influences the way we live in it.