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A swiftly tilting planet (Science Advent 21)

(Every day until December 25, I’m posting a science-related image or video and description.)

Day 21

The giant prehistoric stone rings known as Stonehenge were built in part as a solar calendar, with stones aligned with the rising and setting of the Sun and the brightest star Sirius on significant days of the year. Today marks one such day: the northern winter solstice, which is the shortest day of the year in the northern hemisphere. [Credit: moi]

On a plain near Salisbury in England stands an ancient set of giant stones. Known as Stonehenge to modern people, this monument dates back at least four thousand years. It remains as one of the most iconic human achievements in our species’ 100,000 year history, but the people who built it are long gone. Since they left no written language or pictorial art, we don’t know exactly how Stonehenge was built or for what purpose. (This explanation is almost certainly wrong.)

However, careful study of the position of the stones showed that on certain important days of the year, the Sun and the brightest star Sirius rose aligned with the monument. Today marks one of those days: the northern winter solstice, the shortest day of the year in the northern hemisphere. We don’t know the significance the solstice or any other astronomical event had in the lives of the builders of Stonehenge, but today we know the variation in the length of days—along with the yearly cycle of seasons—is due to the tilt of Earth’s axis relative to the plane of its orbit. The alignment of the stones with the rising of the Sun on these important days places Stonehenge with many other monuments constructed around the world over the millennia.

I visited Stonehenge in the fall of 1997, when I spent a semester studying in London. Since Stonehenge is such a familiar scene from innumerable photographs, many of my group seemed to feel there was no point in actually seeing it in person, and their reactions were apathetic. Surprisingly, I didn’t feel that way: even knowing what it looks like from practically every angle didn’t diminish the intellectual excitement of standing in the presence of an astronomical instrument constructed four thousand years ago. (The photo above was taken on that day using a cheap film camera with no focus ability. I dug out my pictures from the trip last night and scanned them in.) As I mentioned previously, seeing the Very Large Array (VLA) for the first time evoked similar emotions in me. The way the ancient people looked at the sky was no doubt very different from how we perceive it today, but from antiquity humans have looked up and wondered and asked.

[This post first appeared last year. I’m currently on the move for family holiday celebrations.]

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