Page 449 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch16_405-432.indd Page 426 9/3/10 6:19 PM user-f465
tiL12214_ch16_405-432.indd Page 426 9/3/10 6:19 PM user-f465 /Users/user-f465/Desktop

a trough. The two tidal bulges, with the trough between, move When perigee coincides with a new or full moon, especially
slowly eastward following the Moon. Earth turns more rapidly high spring tides result.
on its axis, however, which forces the tidal bulge to stay in front The open basins of oceans, gulfs, and bays are all con-
of the Moon moving through its orbit. Thus, the tidal axis is not nected but have different shapes and sizes and have bordering
aligned with the Earth-Moon gravitational axis. land masses in all possible orientations to the westward-moving
The tides do not actually appear as alternating bulges that tidal bulges. Water in each basin responds differently to the tidal
move around Earth. There are a number of factors that influence forces, responding as periodic resonant oscillations that move
the making and moving of the bulges in complex interactions back and forth much as the water in a bowl shifts when carried.
that determine the timing and size of a tide at a given time in Thus, coastal regions on open seas may experience tides that
a given location. Some of these factors include (1) the rela- range between about 1 and 3 m (about 3 and 10 ft), but mostly
tive positions of Earth, Moon, and Sun; (2) the elliptical orbit enclosed basins such as the Gulf of Mexico have tides of less
of the Moon, which sometimes brings it closer to Earth; and than about 1/3 m (about 1 ft). The Gulf of Mexico, because of
(3) the size, shape, and depth of the basin holding the water. its size, depth, and limited connections with the open ocean,
The relative positions of Earth, Moon, and Sun determine responds only to the stronger tidal attractions and has only one
the size of a given tide because the Sun, as well as the Moon, high and one low tide per day. Even lakes and ponds respond to
produces a tide-raising force. The Sun is much more massive tidal attractions, but the result is too small to be noticed. Other
than the Moon, but it is so far away that its tide-raising force is basins, such as the Bay of Fundy in Nova Scotia, are funnel-shaped
about one-half that of the closer Moon. Thus, the Sun basically and undergo an unusually high tidal range. The Bay of Fundy has
modifies lunar tides rather than producing distinct tides of its experienced as much as a 15 m (about 50 ft) tidal range.
own. For example, Earth, Moon, and Sun are nearly in line dur- As the tidal bulges are pulled against a rotating Earth, fric-
ing the full and new moon phases. At these times, the lunar and tion between the moving water and the ocean basin tends to slow
solar tide-producing forces act together, producing tides that Earth’s rotation over time. This is a very small slowing effect
are unusually high and corresponding low tides that are un- of 1.4 milliseconds/day/century. Evidence for this slowing comes
usually low. The periods of these unusually high and low tides from a number of sources, including records of ancient solar
are called spring tides. Spring tides occur every two weeks and eclipses. The solar eclipses of 2,000 years ago occurred 3 hours ear-
have nothing to do with the spring season. When the Moon is in lier than would be expected by using today’s time but were on the
its quarter phases, the Sun and Moon are at right angles to each mark if a lengthening day is considered. Fossils of a certain species
other, and the solar tides occur between the lunar tides, causing of coral still living today provide further evidence of a lengthening
unusually less pronounced high and low tides called neap tides. day. This particular coral adds daily growth rings, and 500 million
The period of neap tides also occurs every two weeks. year old fossils show that the day was about 21 hours long at that
The size of the lunar-produced tidal bulge varies as the time. Finally, the Moon is moving away from Earth at a rate of
Moon’s distance from Earth changes. The Moon’s elliptical about 4 cm (about 1.5 in) per year. This movement out to a larger
orbit brings it closest to Earth at a point called perigee and orbit is a necessary condition to conserve angular momentum as
farthest from Earth at a point called apogee. At perigee, the Earth slows. As the Moon moves away from Earth, the length of
Moon is about 44,800 km (about 28,000 mi) closer to Earth the month increases. Some time in the distant future, both the day
than at apogee, so its gravitational attraction is much greater. and the month will be equal, about 50 of the present days long.

SUMMARY

Earth is an oblate spheroid that undergoes three basic motions: son. This begins on December 22 or 23 and is called the winter solstice.
(1) a yearly revolution around the Sun, (2) a daily rotation on its axis, On March 20 or 21, Earth is halfway between the solstices and has days
and (3) a slow wobble of its axis called precession. and nights of equal length, which is called the spring (or vernal) equinox.
As Earth makes its yearly revolution around the Sun, it maintains On September 22 or 23, the autumnal equinox, another period of equal
a generally constant inclination of its axis to the plane of the ecliptic of nights and days, identifies the beginning of the fall season.
66.5°, or 23.5° from a line perpendicular to the plane. In addition, Earth Precession is a slow wobbling of the axis as Earth spins. Precession
maintains a generally constant orientation of its axis to the stars, which is produced by the gravitational tugs of the Sun and Moon on Earth’s
always points in the same direction. The constant inclination and ori- equatorial bulge.
entation of the axis, together with Earth’s rotation and revolution, pro- Lines around Earth that are parallel to the equator are circles called
duce three effects: (1) days and nights that vary in length, (2) seasons parallels. The distance from the equator to a point on a parallel is called the
that change during the course of a year, and (3) climates that vary with latitude of that point. North and south arcs that intersect at the poles are
latitude. When Earth is at a place in its orbit so the axis points toward called meridians. The meridian that runs through the Greenwich Observa-
the Sun, the Northern Hemisphere experiences the longest days and the tory is a reference line called the prime meridian. The distance of a point
summer season. This begins on June 21 or 22, which is called the sum- east or west of the prime meridian is called the longitude of that point.
mer solstice. Six months later, the axis points away from the Sun, and the The event of time called noon is the instant the Sun appears to
Northern Hemisphere experiences the shortest days and the winter sea- move across the celestial meridian. The instant of noon at a particular

426 CHAPTER 16 Earth in Space 16-22