Summer solstice

Happy June solstice, everyone! If you’re in the Northern Hemisphere, the June solstice is your signal to celebrate summer. For us, this solstice marks the longest day of the year. Early dawns. Long days. Late sunsets. Short nights. The sun at its height each day, as it crosses the sky. Meanwhile, south of the equator, winter begins.


Planet Earth, as seen from the Sun, at the instant of June 2012 solstice: June 20 at 23:09 Universal Time (12:09 GMT) on the 21st .

What is a solstice? Ancient cultures knew that the Sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shifted in a regular way throughout the year.

They built monuments, such as Stonehenge, to follow the sun’s yearly progress.

Today, we know that the solstice is an astronomical event, caused by Earth’s tilt on its axis and its motion in orbit around the sun.

Because Earth doesn’t orbit upright. Instead, our world is tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly.

At the June solstice, Earth is positioned in its orbit so that our world’s North Pole is leaning most toward the Sun. As seen from Earth, the Sun is directly overhead at noon 23 1/2 degrees north of the equator, at an imaginary line encircling the globe known as the Tropic of Cancer – named after the constellation Cancer the Crab. This is as far north as the Sun ever gets.

For all of Earth’s creatures, nothing is so fundamental as the length of the day. After all, the Sun is the ultimate source of almost all light and warmth on Earth’s surface.

If you live in the Northern Hemisphere, you might notice the early dawns and late sunsets, and the high arc of the Sun across the sky each day. You might see how high the Sun appears in the sky at local noon. And be sure to look at your noontime shadow. Around the time of the solstice, it’s your shortest noontime shadow of the year.

For us in the modern world, the solstice is a time to recall the reverence and understanding that early people had for the sky. Some 5,000 years ago, people placed huge stones in a circle on a broad plain in what’s now England and aligned them with the June solstice sunrise.

We may never comprehend the full significance of Stonehenge. But we do know that knowledge of this sort wasn’t isolated to just one part of the world. Around the same time Stonehenge was being constructed in England, two great pyramids and then the Sphinx were built on Egyptian sands. If you stood at the Sphinx on the summer solstice and gazed toward the two pyramids, you’d see the sun set exactly between them.