Global Positioning System Reference
In-Depth Information
GMT
Between the year of its adoption in 1884 and the year it was superseded, 1972,
Greenwich Mean Time was the universal clock by which the world ticked.
GMT is time referenced to the orbit of the earth around the sun. The earth's
variable speed in its orbit, and the inclination of its rotation axis, mean that
the apparent motion of the sun around the earth is not constant: it changes
speed. Because of this change of speed, a clock that was simply proportional
to the apparent position of the sun would tick at a variable rate. To have a solar
clock that ticked at a constant rate, GMT was based on an average sun—a
yearly average of the sun's apparent movement about the earth. (This aver-
aging is expressed in the ''mean'' of Greenwich Mean Time.) The averaging
process implies that, in practice, there is a difference between actual noon
(the time at which the sun is highest in the sky) and 12:00 p.m. GMT of up to 16
minutes.
Universal Time (UT) took over from GMT in 1972, although in England and
some other places UT is still called GMT. The difference between UT and GMT
is never more than a second. There are several versions of UT. UTC keeps
time according to an atomic clock, which is entirely independent of the solar
orbit. UT0 takes into account the polar motion of the earth, which is useful for
precise geodetic work; and UT1 measures time by the rotation of earth with
respect to distant celestial objects called quasars. For more details on the
subject of universal time, I refer you to the technical literature—but be warned
that it will take you a considerable amount of time to read it.*
* See, for example, Landes (2000), McCarthy (1991), and McCarthy and Seidelmann
(2009). NASA also has a website that explains Universal Time.
65\ 13% 55& distant from Draco.'' (I am making up this simple example—
these numbers are fictional.) The lunar distances from a number of promi-
nent stars, against the time at Greenwich when these distances occur, are
compiled in an almanac. A mariner observes certain lunar distances, con-
sults his almanac to find the time in Greenwich, estimates local time, and
then calculates his longitude from the time di√erence.
Such is the theory. In practice, the lunar distance method was fraught
with di≈culties; and at sea, it could not be made as accurate as the chro-
nometer method. Not for lack of trying. The Greenwich observatory had
been set up to make astronomical measurements that could be used for
