Here in the northern hemisphere (planet Earth) we just celebrated the summer solstice yesterday. In the southern hemisphere people celebrated winter solstice. Summer solstice is the occasion to celebrate if you like long, warm, sunny days and short nights; winter solstice usually means colder weather, shorter days, longer nights.

“Solstice” is another of those words that comes down from an ancient tongue (Latin, in this case). It’s two words jammed together: sol for “Sun” and sistere “to stand still.”

Does the Sun really stand still on this day? Well, that’s an interesting question. It depends on the frame of reference you’re using. The Sun is moving through space as part of the Milky Way Galaxy, which is, itself, moving through space as part of the Local Group of Galaxies, which is itself moving through space as part of a supercluster of galaxies, which is itself moving with the expansion of the universe, but possibly also affected by the gravitational pull of dark matter and the effect of dark energy.

Okay, that can get confusing really fast. So, let’s narrow it down a little, to just the motion of the Sun and Earth with respect to each other. The truth is, Earth rotates around the Sun and it also spins on its axis. We have day and night because Earth turns on its axis. We’re sitting on the surface of the planet, riding along as it turns on its axis. So, as the planet turns, things in the sky look like they’re moving across the sky. In reality, they’re more or less sitting still while our point of view is changing. It’s exactly like being on a merry-go-round as it spins around. Everything NOT on the merry-go-round isn’t moving, but it looks like it is.

So, Earth is spinning on its axis, which is tilted. That tilt, plus the apparent motion of the Sun across our sky each day, holds the key to understanding “solstice.” In northern hemisphere summer, the northern hemisphere is tilted toward the Sun. This lets a LOT more warmth and light from the Sun reach the northern hemisphere. At the same time, the southern hemisphere is receiving less light and warmth, and experiences winter. (Note: Earth is NOT closer to the Sun during this time.)

Solstice happens because as Earth makes its yearly trip around the Sun, spinning its axis, the perceived position of the Sun changes. NOT just the east-west motion we see every day, but also north-south. The Sun appears to move farther north in the sky from December to June, as the tilt of our axis brings more of the Northern Hemisphere into more sunlight. Then, from June to December, the Sun appears to move back to the south. It reaches its northernmost point in the sky on June 21, where that slow northward motion seems to stop for a day or two. Then, as the planet continues on its trip around the Sun, the tilt slowly changes, and the Sun appears to head south again.

In the southern hemisphere on June 21, the Sun is also at its northernmost point, which means that half of the planet is getting less warmth, less sunlight, and things get cold.

On December 20, the date of the other solstice when the Sun appears to “stand still” at its southernmost point in the sky, the northern hemisphere is getting less sunlight and has lower temperatures. In the southern hemisphere on the same day, it’s warmer and summer is in full bloom.

The ancients, who watched the sky pretty closely for a variety of reasons, noted that “stoppage” with the term “solstice.” Since it seemed to coincide with warm weather, a good growing season, and increased amounts of food, solstice time seemed a great time to celebrate.