Global Positioning System Reference
In-Depth Information
FIGURE 1.3.
Moon, sun, and tides. Lunar gravity causes the world's oceans to bulge
on the far side as well as the near side, so that tides usually peak twice per day. The
sun's gravity also contributes to the tides, though about half as much as the moon's;
as a result, tides are particularly high when sun and moon align, as shown here.
sider the centrifugal force that acts on both earth and moon as they spin
around one another, like two skaters holding hands. The centrifugal force
throws the oceans outward, away from the moon; on the near side, gravity
overrides this centrifugal repulsion and pulls the oceans inward, toward
the moon.
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The double bulge immediately explains why high tides occur twice a day
(approximately) instead of once per day. From the point of view of the
Man on the Moon, the earth's oceans are shaped like an egg, with the
long axis pointing toward him, and with the solid earth rotating beneath
these oceans.
This is the standard explanation for one of the prominent features of the
tides, but it is by no means the whole story. There are many additional
factors that influence the tides. The most important of the celestial influ-
ences, apart from the moon, is the sun. The e√ect of solar gravity on our
oceans is about half that of lunar gravity, but the period is slightly di√erent:
6. These two views—of gravity relative to the center of the earth and of gravity plus
centrifugal force—are exactly the same to a physicist but are expressed from di√erent
frames of reference. The tidal e√ect of the moon's gravity is also felt by the solid earth as well
as by the oceans. However, the solid parts of our planet cannot flow so easily as the oceans,
though the earth's crust is distorted 10-50 cm as a consequence of lunar gravity. Undoubt-
edly it is this distortion that is responsible for the strong correlation between earthquake
events and lunar phase: at a given latitude and longitude, earthquakes happen much more
commonly when the tidal force is strong than when it is weak
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