Museums & Galleries
Navigation Bar
Navigation Bar

    Related Items
  • Read a profile of the Library of Congress.
 
Analemma Dilemma

Friday, December 19, 1997

  What Am I?


Photo
Detail of an analemma, on a globe by Gilman Joslin, Boston, 1890, in the collection of the Library of Congress. (By Lawrence W. Jackson Jr./wp.com)
At noon in a perfect world, the sun would always be positioned 93 million miles directly over the equator, and the Earth, an unblemished sphere, would rotate evenly on a precisely vertical axis. The seasons would never change. Every day would last as long as every other. And we'd never have the equinoxes and solstices that mark the four quarters of the year.

As it happens, however, the Earth's axis is tilted and, according to Ruth Freitag, a senior science specialist at the Library of Congress, the "slightly eccentric ellipse" of the Earth's orbit around the sun led astronomers to come up with a consistent way to determine mean time, the time by which we all set our clocks. "The natural system is full of variables, and that's without even considering the irregularities of the Earth's rotation, which came to light in the late 19th century," says Freitag.

Thus we have the analemma, the somewhat mysterious looking figure-eight diagram on many globes and maps. The analemma charts where and when the sun will appear directly overhead in the "torrid zone," between the Tropic of Cancer and the Tropic of Capricorn. The curves of the analemma also mark the solstices and equinoxes. The winter solstice, occurring when the sun is at its southernmost position in the torrid zone, is shown on the most extreme point of an analemma's lower arc.

Photo
"Winter," depicted on an atlas by Joan Blaeu, Amsterdam, 1662-1665, in the collection of the Library of Congress. (By Lawrence W. Jackson Jr./wp.com)

"In the days before the radio, the analemma was also useful for correcting clocks," says author David Greenhood in his book "Mapping."

The days may be dark now but the horizon looks bright: Since the winter solstice marks the shortest day and the longest night of the year, the days will begin to stretch out from now until the summer solstice. Come February and March, when cold temperatures have you fearing that winter will never end, at least the sun will hang a little longer in the evening sky.

   
© Copyright 1997 The Washington Post Company

Back to the top
Navigation Bar
Navigation Bar