Today is a very special day: It’s the longest day of the year, or the “summer solstice.” Often the day just drifts by without even being noticed, but for me, growing up in southwest England, I always associated solstice with Stonehenge and Druids.

Since studying astronomy at school, I soon realized that the summer solstice — which occurs on June 20 or 21 each year — was more than just a date to worship the sun; it’s a subtle astronomical phenomenon that underpins all life on Earth.

ANALYSIS: Bones Hint at Stonehenge Solstice Feast

The science is fairly straightforward: The Earth’s axis is tilted by about 23.5 degrees from its orbital plane. In other words, our planet spins around its axis like a spinning top, but the North Pole is always “off vertical” by 23.5 degrees.

As the Earth orbits the sun, the North Pole is pointing preferentially toward the sun during the summer in the Northern Hemisphere (now), but points away six months later on Dec. 20 or 21. For the northern hemisphere, when the North Pole is tilted toward the sun, it’s the summer solstice; when pointing away, it’s the winter solstice. For the Southern Hemisphere, the opposite is true (i.e. when it’s the summer solstice in Canada, it is the winter solstice in Australia).

The summer solstice is the longest day of the year, while the winter solstice is the shortest day of the year.

Also, the summer solstice is the day that the sun can be seen at the highest, northernmost point in the sky. At the summer solstice, the sun will be directly above the Tropic of Cancer (Northern Hemisphere) or the Tropic of Capricorn (Southern hemisphere). If you are located between the Tropics, the sun will pass overhead (i.e. the zenith) in the run-up to summer solstice. At these latitudes, the sun will be directly overhead at different times of the year, depending where you are.

The tilt of the Earth relative to the sun’s rays during the Northern Hemisphere’s summer solstice. Note that the Tropic of Cancer sees the sun directly overhead (Przemyslaw “Blueshade” Idzkiewicz).

Summer solstice is also a reminder about how lucky we are to be living on a planet with a tilt.

If the Earth spun vertically — with no tilt relative to its orbital plane around the sun — the equator would always have the sun directly overhead and the poles would be in perpetual twilight.

Every day would be a summer solstice for the equator and winter solstice for the poles. There would be no seasons, and the world as we know it would be a very different place: a lifeless desert around the equator and frozen poles equally as hostile to life.

I suspect there would be some pretty nasty weather in between; hot and cold air forming a huge cell of violent circulating air dominating the hemispheres.

So, if I were inclined to celebrate any time of the year, summer solstice would probably be it. But I wouldn’t necessarily worship the sun; I’d be giving thanks for our “tilted world.”

Leading image: The sun over the U.K. at precisely 11:28 Universal Time or 12:28 p.m. British Summer Time on June 21, 2010, the moment when the sun reached its most northerly point in the sky (Ian O’Neill/Discovery News).