Gaithersburg International Latitude Observatory, Gaithersburg, MD

gkat_099.pdf39° 8′ 12.00″ N, 77° 11′ 57.00″ W


An Innocuous White Building

Latitude—the number of degrees from the Equator toward either pole— seems like something that should be fairly fixed. But when measuring latitude by observations of the stars, the apparent latitude varies from day to day. This happens because the Earth does not spin uniformly on its axis—it wobbles.

The International Latitude Observatories were set up in 1899 to determine how much the Earth wobbles by observing a group of 12 stars. The observatories were located in Mizusawa, Japan; Tschardjui, Central Asia; Carloforte, Italy; and in the U.S. at Gaithersburg (Figure 99-1), Cincinnati, and Ukiah. All six observatories lay along the same line of latitude at 39º 8’ N.


Figure 99-1. The Gaitherbsurg Observatory; courtesy of Amy Fredericks (etacar11)

The apparent latitude varies because of the Earth’s wobble (see sidebar) and because of the refraction of light in the atmosphere. The wobble causes the pole to move as much as six meters from its average position on a daily basis. Determining the exact nature of the wobble was done ...

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