170 Encyclopedia of the Solar System
are essentially bottomless. Venus and Titan form one ter-
restrial subgroup that is characterized by a slowly rotating
planet, and interestingly, both exhibit a rapidly rotating at-
mosphere. Mars, Io, Triton, and Pluto form a second terres-
trial subgroup that is characterized by a thin atmosphere,
which in large measure is driven by vapor-pressure equilib-
rium with the atmosphere’s solid phase on the surface. Both
Io and Triton have active volcanic plumes. Earth’s weather
turns out to be the most unpredictable in the solar system.
Part of the reason is that its mountain ranges frustrate the
natural tendency for winds to settle into steady east–west
patterns, and a second reason is that its atmospheric ed-
dies, the fluctuating waves and storm systems that deviate
from the average, are nearly as big as the planet itself and
as a result strongly interfere with each other. [SeeVenus:
Atmosphere; Io: The Volcanic Moon; Triton; and
Pluto.]
Earth has many planetary attributes that are important
to the study of its atmosphere and oceans, and conversely
there are several ways in which its physically and chemically
active fluid envelope directly affects the solid planet. Earth
orbits the Sun at a distance of only 108 times the diameter
of the Sun. The warmth from the Sun that Earth receives at
this distance, together with a 30 K increase in surface tem-
perature resulting from the atmospheric greenhouse effect,
is exactly what is needed for H 2 O to appear in all three of its
phases. This property of the semimajor axis of Earth’s orbit is
the most important physical characteristic of the planet that
supports life. (One interesting consequence is that Earth is
the only planet in the solar system where one can ski.)
Orbiting the Sun at just over 100 Sun diameters is not
as close as it may sound; a good analogy is to view a bas-
ketball placed just past first base while standing at home
plate on a baseball diamond. For sunlight, the Sun-to-
Earth trip takes 499 s or 8.32 min. Earth’s semimajor axis,
a 3 = 1. 4960 × 1011 m=1 AU (astronomical unit), and or-
bital period,τ 3 = 365 .26 days=1 year, where the subscript
3 denotes the third planet out from the Sun, are used as con-
venient measures of distance and time. When the orbital
period of a body encircling the Sun,τ, is expressed in years,
and its semimajor axis,a, is expressed in AU, then Kepler’s
third law is simplyτ=a^3 /^2 , with a proportionality constant
of unity. [SeeSolar System Dynamics: Regular and
Chaotic Motion.]
1.1 Length of Day
The Earth’s rotation has an enormous effect on the motions
of its fluid envelope that accounts for the circular patterns
of large storms like hurricanes, the formation ofwestern
boundary currentslike the Gulf Stream, the intensity of
jet streams, the extent of the Hadley cell, and the nature
of fluid instabilities. All of these processes are thoroughly
discussed in Sections 2–5. Interestingly, the reverse is also
true: The Earth’s atmosphere and oceans have a measurable
effect on the planet’s rotation rate. For all applications but
the most demanding, the time Earth takes to turn once on
its axis, the length of its day, is adequately represented by a
constant value equal to 24 hours or 1440 minutes or 86,400
seconds. The standard second is the Syst `eme International
(SI) second, which is precisely 9,192,631,770 periods of the
radiation corresponding to the transition between two hy-
perfine levels of the ground state of the^133 Cs atom. When
the length of day is measured with high precision, it is found
that Earth’s rotation is not constant. The same is likely to
hold for any dynamically active planet. Information can be
obtained about the interior of a planet, and how its atmo-
sphere couples with its surface, from precise length-of-day
measurements. Earth is the only planet to date for which we
have achieved such accuracy, although we also have high-
precision measurements of the rotation rate of pulsars, the
spinning neutron stars often seen at the center of supernova
explosions.
The most stable pulsars lose only a few seconds every
million years and are the best-known timekeepers, even
better than atomic clocks. In contrast, the rotating Earth is
not an accurate clock. Seen from the ground, the positions
as a function of time of all objects in the sky are affected by
Earth’s variable rotation. Because the Moon moves across
the sky relatively rapidly and its position can be determined
with precision, the fact that Earth’s rotation is variable was
first realized when a series of theories that should have pre-
dicted the motion of the Moon failed to achieve their ex-
pected accuracy. In the 1920s and 1930s, it was established
that errors in the position of the Moon were similar to er-
rors in the positions of the inner planets, and by 1939, clocks
were accurate enough to reveal that Earth’s rotation rate has
both irregular and seasonal variations.
The quantity of interest is the planet’s three-dimensional
angular velocity vector as a function of time,(t). Since
the 1970s, time series of all three components of(t) have
been generated by using very long baseline interferometry
(VLBI) to accurately determine the positions of quasars
and laser ranging to accurately determine the positions of
man-made satellites and the Moon, the latter with corner
reflectors placed on the Moon by theApolloastronauts. [See
Planetary Exploration Missions.]
The theory of Earth’s variable rotation combines ideas
from geophysics, meteorology, oceanography, and astron-
omy. The physical causes fall into two categories: those that
change the planet’s moment of inertia (like a spinning skater
pulling in her arms) and those that torque the planet by ap-
plying stresses (like dragging a finger on a spinning globe).
Earth’s moment of inertia is changed periodically by tides
raised by the Moon and the Sun, which distort the solid
planet’s shape. Nonperiodic changes in the solid planet’s
shape occur because of fluctuating loads from the fluid com-
ponents of the planet, namely, the atmosphere, the oceans,
and, deep inside the planet, the liquid iron–nickel core. In
addition, shifts of mass from earthquakes and melting ice