Solstice is word that gets tossed around several times a year, and today marks the first day of summer for 2013—the Summer Solstice, if you will. But what does solstice actually mean, and where did this word and concept come from?
Simply put, a solstice is an astronomical event that happens twice a year, when the Sun reaches its highest or lowest point relative to “celestial equator” on the “celestial sphere.” The term itself comes from the Latin words sol (meaning ‘sun”) and sistere (meaning “to stand still”).
It was actually the ancient Greeks who were responsible for the concept of the “celestial sphere”, though they also believe that the stars rotated around the Earth over the course of a day—and that the Earth remained stationary. They thought that the stars were part of an invisible stationary surface, keeping them fixed and rotating. (This concept actually forms the basis of some modern astronomical thought, wherein the celestial sphere is acknowledged to exist, though only the stars rotate and not the sphere itself.)
Why is this important? In terms of the summer, it was through the acknowledgement of a Summer Solstice that some Greeks were able to make important and valuable contributions to science and our understanding of the Earth.
We know that one ancient Greek astronomer named Aristarchus observed a Summer Solstice in 280 B.C., and he was responsible for proposing the heliocentric hypothesis: the earth revolves around the sun, and not the other way around. Sadly for Aristarchus, the idea wasn’t popular during his lifetime, and it was only when Copernicus made the same claim 1800 years later that it gained any ground.
Another ancient Greek, a mathematician and geographer named Eratosthenes, was able to calculate the circumference of the Earth after observing the sun’s shadow at the Summer Solstice’s noon in Alexandria and Syene. By knowing the geographical distance between the two cities, he calculated a circumference of about 250,000 stadia—or 24,662 miles.
The Summer Solstice has traditionally had less of an “urgent” feel about it—we’re not heading into winter, dreading the loss of the sun—but it has still contributed to scientific advancement both in ancient and modern times, and thus has an important place in human history!