Shaping the Earth - The Science of Navigation - page 51

FIGURE 2.10. An LR-3 laser ranging retro-reflector on the moon. The retro-reflector is
the white object on the surface between astronaut Buzz Aldrin and the lunar module
Eagle . Photo by Neil Armstrong, July 20, 1969, courtesy of NASA.
lunar gravity, for example) and also drifts slowly and quickly (tectonic plate
shifts, for example). 18
The first useful radar (Chain Home, deployed in England just before the
Battle of Britain) appeared in 1940 and the first useful satellite (Sputnik),
in 1957. Lasers and computers emerged during the last half of the twen-
tieth century. It is the combinations of these devices that have brought
geodesy to its present high state; the best-known modern high-tech ap-
plication is the GPS system of satellites. Because the new geodesy has
shown us that the shape of our earth changes, geodesists have had to
redefine and refine their measurement of the passage of time. There are
18. A recent study by NASA and the University of Texas has shown that a lot of the
changes in the earth's surface are due to changes in climate, particularly during El Niño
years, when large bodies of water get shifted about. See, e.g., Adam (2002) and Leuliette et
al. (2002).