Global Positioning System Reference
In-Depth Information
FIGURE 1.2.
Earth and moon.
The moon exerts a consider-
able gravitational influence
upon the earth, generating
tides. This picture was taken
by the satellite
Galileo
in
1990.
Photo courtesy of NASA.
complete revolution. It rotates, as we have seen, in the same way as the
earth; and this means that it takes somewhat longer than 24 hours to
reappear in a given part of the sky. In fact, if you look at the moon at
9:00 p.m. one evening, it will again pass through the same part of the sky at
9:50 p.m. the following evening. Our relatively large and dense satellite
exerts a considerable gravitational pull, and so the direction of this pull
also rotates about the earth once every 24 hours and 50 minutes. Because
the oceans are mostly fluid, they flow under the force of lunar gravity:
hence, tides. The earth-moon system is shown in figure 1.2.
5
In most parts of the world, the tides occur twice a day, with a period that
is half that of the moon, so that they peak every 12 hours and 25 minutes.
Why half? The simple explanation is shown in figure 1.3. The e√ect of the
lunar gravitational force is to pull the surface of the earth that is nearest to
it with a greater force than it pulls the center, and to pull the center with a
greater force than it pulls the far side of the earth. Thus, relative to the
center of the earth, that part of the earth's surface nearest the moon feels a
net attraction, and the part furthest away feels a net repulsion. The fluid
oceans are able to flow where the lunar force directs them; as a result,
the oceans bulge on the side nearest the moon and on the opposite side
(fig. 1.3). Another way of understanding this ''double bulge'' is to con-
5. Lunar and tide data are widely available. See, for example, Weast (1973). For the
mechanics of lunar orbit, earth rotation, and tides, and for the data discussed in this section,
see Kibble and Berkshire (1985, chap. 6), Lambeck (1980), and Williams (1997). The
National Oceanic and Atmospheric Administration (NOAA) also provides much useful
information about tides and the causes of tides on its website.
