Tuesday, December 04, 2007

Grand Central


Ceiling of Main Concourse in Grand Central Terminal. Depicted on this vault are the ecliptic, celestial equator, Pisces, Triangulum, Ares, Taurus, Orion, Gemini and Cancer. Brighter stars are marked with electric lights. Legend has it that once a motorised Sun moved across that ecliptic. In a city where electric lights drown out the stars, this is a very pleasant diversion.

Once upon a time people thought that stars were fixed to an enormous vault rotating overhead. In some ways this was a useful assumption. The stars are so distant that their parallax is not easily noticed. In our neighbourhood one needs only their right ascension and declination. Many scientists, including this one, keep on the shelf a clear ball with stars drawn on the surface. Even for navigation in Earth, it is safe to treat the stars as if they were fixed to a sphere.

Sharp-eyed astronomers will notice that the constellations are depicted inside out. Inspired by medieval depictions, the artist chose to depict a celestial sphere as if seen from the outside. This is quite inaccurate, for we know that the stars are varying distances from Earth and one could never see such a view. Once again the clear ball on the shelf proves useful in deciphering this picture.

Holes in grillwork over the South windows allow sunlight to pass through, showing images of the Sun at our feet. This acts as a camera obscura, allowing one to track sunspots on the floor. One can even track the Sun's rotation from the sunspots. Galileo's observation of sunspots contradict old theories. It is a mistake to allow the assumption of a celestial sphere to become canon.

Once upon a time people thought that the speed of light was fixed, like the Earth. In some ways this was a useful assumption. The speed of light changes so slowly that the change is not easily noticed. In our neighbourhood in Space/Time one need only the present value of c. Many scientists, including this one, keep on the shelf a book where the speed of light is treated as a constant. Even for navigation in the solar system, it is safe to treat c as constant.

Holes in present understanding allow light to pass through, showing a changing c. Observations of high-redshift supernovae point to a past where values like c may not have been the same. We have reached the Moon and brought back rocks billions of years old, from a time when c was slightly different. Observations of the distant Universe contradict old theories. It is a mistake to allow the assumption of constant c to become canon.

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