Summer Solstice Lesson: Understanding the Longest Day

Lesson Overview

• What Does "Solstice" Mean?
• Why Is the Summer Solstice the Longest Day?
• Are All Seasons the Same Length?
• When Does the Summer Solstice Occur?
• Do Days Get Shorter After the Solstice?
• The Midnight Sun: 24 Hours of Daylight
• How Do People Celebrate the Summer Solstice?

Have you ever noticed how in late June the daylight seems to last forever? In some places, the Sun can shine for over 17 hours on this day! That's because of the summer solstice, often called the longest day of the year. 

The summer solstice marks the start of summer in the Northern Hemisphere, when the Sun's path across the sky is at its highest and daylight is longest. It's an event that blends both science and history.

Let's read on to explore what causes the summer solstice and discover the "why" behind its many fascinating facts.

What Does "Solstice" Mean?

• The word solstice comes from Latin: sol (sun) and stitium (standing still). So solstice basically means "Sun stands still."
  
• Around the solstice, the Sun's position at noontime seems to stop changing. For a few days, the midday Sun reaches nearly the same height in the sky, as if it has paused in place.
  
• There are two solstices each year: one around June 21 and one around December 21. In June, the summer solstice brings the longest day of the year in the Northern Hemisphere, and in December, the winter solstice brings the shortest day of the year.

Why Is the Summer Solstice the Longest Day?

The reason late June has the longest daylight is the Earth's tilt. Earth is tilted about 23.5° on its axis. By the time of the June solstice, the North Pole is tilted toward the Sun, and the Sun's rays hit the Northern Hemisphere more directly. Here's what that means for us:

• Sun High in the Sky: On the summer solstice, the Sun takes its highest path across the Northern Hemisphere sky. It rises earlier and sets later than on any other day. Because the Sun is following a longer, higher arc, we end up with more hours of daylight.
  
• Tropic of Cancer: The exact moment of the June solstice is when the Sun is directly overhead at noon at the Tropic of Cancer (an imaginary line about 23.5° North latitude). That is the farthest north the Sun ever gets directly overhead. If you stood on the Tropic of Cancer at that moment, your shadow would nearly disappear because the Sun is straight above!
  
  (In the Southern Hemisphere, June 21 is their winter solstice-the shortest day of the year.)

Every place north of the equator gets its longest daylight around June 21. The farther north you go, the longer the daylight on this day (places very far north even have the Sun up for 18, 20, or even 24 hours!).

Take This Quiz:

What do you know about Summer Solstice? Trivia Quiz

Are All Seasons the Same Length?

We often think each season lasts about three months, but they're not exactly equal. In fact, summer is the longest season (in days) for the Northern Hemisphere. Why? Because Earth's orbit around the Sun is slightly elliptical (oval-shaped) instead of a perfect circle, the Earth doesn't move at the same speed year-round. When Earth is closer to the Sun (around January), it moves faster; when it's farther (around July), it moves slower.

Northern Hemisphere summer happens during the part of the orbit when Earth is moving more slowly, so that season ends up lasting a bit longer. Northern winter occurs when Earth is moving faster, making that season shorter. 

Here is a comparison of the approximate lengths of each astronomical season in the Northern Hemisphere:

As you can see, summer lasts about 93.6 days, a few days longer than winter at ~89 days. These differences happen because of Earth's changing orbital speed.

When Does the Summer Solstice Occur?

If you look at a calendar, you'll usually see summer starting on June 21. However, the exact date (and time) of the solstice can vary slightly. Generally, the summer solstice falls on June 20, 21, or 22. It most often happens on June 21. Why can it change? Our calendar year (365 days, plus leap years) doesn't line up perfectly with Earth's orbit (365.24 days). This small mismatch causes the solstice date to drift over time.

• Mostly June 21: In most years, the solstice is on June 21 (often called the first day of summer).
  
• Sometimes June 20: Occasionally, leap years and time zones make it land on June 20.
  
• Rarely June 22: Very rarely, the solstice falls on June 22. The last time was 1975, and it won't happen again until 2203.

Do Days Get Shorter After the Solstice?

Up to the summer solstice, each day had a bit more daylight than the one before. After the solstice, the pattern flips – days start getting shorter again in the Northern Hemisphere. Right after June 21, the change is tiny (almost unnoticeable at first).

For example, around June 21 Anchorage, Alaska enjoys about 19 hours of daylight. On June 22, it has roughly 2 seconds less daylight than the day before. Just two seconds shorter! Such a small difference shows how "standstill" the Sun's path is at solstice time. As the weeks pass, those seconds turn to minutes and the daylight loss adds up. By August, you'll definitely notice earlier sunsets.

This gradual shrinking of daylight continues until the autumn equinox in September (when day and night are equal). After that, the days keep shortening, all the way to the winter solstice in December (the shortest day of the year).

The Midnight Sun: 24 Hours of Daylight

Have you heard of the "midnight sun"? In far northern regions, the summer solstice brings a time when the Sun never sets at all. This happens in places above the Arctic Circle (around 66.5° North latitude). On the summer solstice (and for some weeks around it), those areas have 24 hours of daylight - the Sun is even visible at midnight!

One example is Utqiaġvik, Alaska (formerly called Barrow), the northernmost city in the U.S. From mid-May until late July, the Sun never dips below the horizon there. On June 21, the Sun doesn't set in Utqiaġvik at all (meaning it loses 0 seconds of daylight that day!). In fact, in 2017 the Sun didn't set in Utqiaġvik until August 2. After that date, night slowly returns. This incredible midnight sun phenomenon is a direct result of Earth's tilt – because the North Pole is pointed toward the Sun in June, areas near the pole get sunlight around the clock.

(During winter, these far-north places experience the opposite - polar night, when the Sun stays below the horizon for a long period.)

Take This Quiz:

The Summer Solstice: Trivia Quiz!

How Do People Celebrate the Summer Solstice?

The summer solstice isn't just about astronomy – people around the world recognize and celebrate this longest day. Here are a few examples of solstice traditions:

• Ancient Celts: In parts of Europe, especially in Celtic regions, the summer solstice is celebrated as Midsummer. The ancient Celts (and many modern pagans) lit bonfires and held feasts to honor the Sun's peak. In the old Celtic calendar, the solstices and equinoxes were "lesser" festivals, and the midpoints between them (like Imbolc in February) were "greater" festivals. So while Midsummer was a notable celebration, Imbolc was not one of the solstice/equinox holidays.
  
• Ancient Greece: For the ancient Greeks, the summer solstice often marked the start of the new year in their calendar. It also kicked off a one-month countdown to the ancient Olympic Games (which included famous events like chariot races). In other words, about a month after the solstice, the big games would begin, and people saw the solstice as the kickoff for that festive period.
  
• Around the World: Many other cultures mark the solstice too. At Stonehenge in England, people gather at dawn on June 21 to witness the Sun rise in alignment with the stones. Ancient Romans held celebrations for the goddess Vesta around this time, and ancient Chinese ceremonies honored the Earth and yin (feminine energy) on the summer solstice. Numerous Indigenous peoples also have traditions - for example, some Native American communities perform sun dances or other ceremonies to celebrate the longest day of sunlight.