Tuesday, December 14, 2010

Erasing stars

My daughter Maureen was recently in Egypt, visiting sites ancient and modern from Alexandria to Lake Nasser. Here are a couple of her photos (click to enlarge) of ceilings in the Temple of Hatchetsup, at Luxor, about 180 km north of Aswan on the Nile. Yellow stars on a blue background -- the sky goddess Nut who arches her star-spangled body over the earth from horizon to horizon. Among those stars is Canopus, the second brightest in the sky, after Sirius, and visible in the south from the latitude of Luxor or Aswan. It is not a star we can see from New England, or indeed from most of the United States.

Aswan is perhaps best known today as the site of the Aswan High Dam. As the Egyptian town of Syene it played a pivotal role in he history of science -- it provided the key to the first measurement of the Earth's diameter by Eratosthenes of Alexandria, in the 3rd century B.C.E.

According to the story, there was a deep well in Syene that at noon on midsummer day one could see the Sun reflected in the water at the bottom of the well. This meant that the Sun was directly overhead at Syene in midsummer; that is to say, Syene was on the Tropic of Cancer, the line that marks the Sun's northernmost excursion during the course of the year. Eratosthenes knew that the Sun was not overhead at Alexandria at the same time, and he knew why; the Earth is a sphere. He measured the Sun's shadow at Alexandria, and from the known distance to Syene (measured by surveyors of the Pharaoh), and a bit of geometry, worked out the diameter of the Earth. I tell this story in all of its details in Walking Zero: Discovering Cosmic Space and Time Along the Prime Meridian.

The sky goddess Nut and Alexandrian astronomers of a startlingly modern bent lived in what was perhaps then, as now, an uneasy coexistence.

In a few days I will be off to our island, which happens to lie, like Aswan, exactly on the Tropic of Cancer. Canopus is prominent in our winter sky, low in the south. And the sky goddess Nut, whose dark star-spangled body was one of the reasons we chose the island, is losing her luster to artificial light.

More of Mo's photos here.