Scoping Out the Sky

For everyday folks and presidents, too, the Naval Observatory is a fascinating place to study the stars

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Malarial Foggy Bottom, near the Potomac, was not the best spot for viewing the heavens but was still better than 17th and G streets, where the first observatory was erected in 1830.

We checked out the 12-inch scope in the present observatory's dome, a classic refractor built in 1895. This is the one the public usually gets to look through.

"This was used extensively until the '50s," Chester said, "when it was replaced by a camera that photographed the moon against the stars. You need the star background to determine the exact orbit of the moon, which, for instance, was kind of vital for the Apollo program. You wouldn't want to find yourself a mile off your calculations as you came in for a landing. After Apollo was finished the scope was dismantled." Later the staff rebuilt the scope, which Chester says is hard to beat for clarity of detail.

"It doesn't have the aperture of the 26-inch telescope, of course, but on any given night, sky conditions favor this over the larger one for a sharp picture of, say, the surface of Mars. It's the optics." The lenses for both scopes were made by Alvan Clark and Sons, and apparently Clark improved his skill in the 22 years between these jobs.

"Clark really had a knack for working glass," Chester added. "He would test a lens in his workshop, sight a star with it and throw it out of focus so he could see where the defects were. Then he would put some optical rouge on his thumb and actually feel where the error was, the tiny bump in the surface, and polish it away."

On the nights when it's too cloudy to see the stars, visitors examine the 26-inch telescope instead, an impressive sight at 30 feet long. To reach it, they must ride Washington's largest elevator: the entire floor rises and falls inside the observatory's 40-foot-wide dome. It's cold in the dome in winter, and warm in summer, because air currents that distort the view are created if the temperature is not the same inside and out.

This scope is the one with which Asaph Hall discovered the moons of Mars, Deimos and Phobos, in 1877. It is so precise that it is still used for measuring binary, or double, stars. Many measurements for the Washington Double Star Catalogue - a database that goes back over a century - were made with this instrument.

Not so long ago this data was collected the hard way, with repeated observations and notes penciled in by hand. Now, with a camera that videotapes a star area 30 times a second, and computers that interpret the data, we can fix the parameters of binary stars and determine orbit and mass in a matter of minutes. It is this function - finding the position of objects in the sky with maximum exactness, plus determining time with great precision - that is the task of the Naval Observatory.

We looked at the little meridian transit scope with a 6-inch aperture. This one is fixed on a north-south axis and measures the east-to-west movement of the stars as they cross one specific meridian. "It's probably the most accurate anywhere, said Chester, and was used for a century, until it was superseded two years ago by more sophisticated instruments."

The observatory has compiled the world's largest star catalogue, with half a billion entries contained on ten CD-ROMs. Knowing the position of stars is especially useful if, for example, you're monitoring a new celestial satellite for which you haven't been given the orbit. You need a bunch of star positions as the background against which you can work out its path.


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