Earth and Its Orbit - The Science of Navigation - page 11

12 hours instead of 12 hours, 25 minutes. The di√erent positions in the sky
of the sun and the moon mean that their combined influence on the tide
changes with time. When sun and moon are aligned, as in figure 1.3, the
tides are particularly strong. Such unusually high tides occur twice per
lunar cycle, when there is a full moon and a new moon, and are known as
spring tides . In between these times, at quarter and three-quarter moons
(when the moon appears as half a circle), the tides are particularly weak—
neap tides .
Thus, two components of celestial gravity act on our tides. The interplay
between them—due to their changing direction and their di√erent periods
—makes the behavior of our tides quite complex. Unfortunately, the situa-
tion is even more complicated than I have stated it to be. The moon's orbit
is not in the same plane as the earth's: it is inclined at 5.145\ to the ecliptic
plane (the plane of the earth's orbit about the sun). Also, the moon's orbit is
not quite circular; the shortest distance between the earth and its satellite
is about 364,000 km, and the longest distance is about 407,000 km. The
di√erence is big enough to see: at perigee (closest distance) the moon looks
noticeably larger than it does at apogee (furthest distance). Needless to say,
the gravitational force is significantly stronger at perigee than at apogee.
The situation is yet more complicated, because the tides are also influ-
enced by factors closer to home. For sure, the tides arise because of the
earth's rotation and the gravitational force of moon and sun, but the time
and the height of the tides at a given location on the earth are also greatly
influenced by local geography, both above and below sea level. Coastline
shape, the topography of the ocean floors, 7 ocean currents, wind speed and
direction, water flow from estuaries, and atmospheric pressure all influ-
ence the height of a tide and the time (relative to the phases of the moon)
when it peaks. The result is that each harbor, river mouth, or stretch of
coastline has its own tidal pattern, some with large changes in water level
due to tides, and some with virtually no tidal fluctuations. Sometimes the
semidiurnal (twice-daily) nature of tides is masked by other factors so that
the tides cycle only once per day or not at all. The types of tide that occur
on the coastlines of the world are shown in figure 1.4.
Thus, while most coastlines do experience twice-daily tides as the the-
ory says, in practice the details of tide height and precise timing depend
more on local factors. This makes detailed tide predictions di≈cult. An
7. Topography is the study of the shape of land; strictly speaking, here I should say
bathymetry , which is the study of ocean depths.