DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

E
Eady, E.T. (1915–1966) English mathematician
and meteorologist. Worked virtually alone
in developing the theory of baroclinic instability
during World War II.
Eady model A baroclinic geophysical fluid
dynamic model that clearly illustrates the baroclinic
instability process.
EAL See International Atomic Time.
Earth Third planet in the solar system from
the sun, orbital semimajor axis 1.496 × 10 8 km,
eccentricity 0.0167, polar inclination to the
ecliptic 23 ◦ .45; Mass 5.9736 × 10 24 kg, radius
6126 km. Home of the human race. A planet
with active plate tectonics, and frequently (by
geologic standards) reversing magnetic field.
The Earth has a dense but transparent atmosphere
(approximately 1.013 bar at the surface).
The atmosphere consists of 77% nitrogen, 21%
oxygen, with traces of argon, carbon dioxide,
and water and produces a greenhouse effect contributing
about 40 ◦ C to its average temperature
of approximately 14 ◦ C. Oceans cover 71% of
the Earth’s surface, which acts as a thermal
reservoir, and CO2 buffer. Earth is the only
planet with a known active biosphere, extending
from as much as 1 km into the Earth to 30 km
above it. Photosynthetic organisms produce copious
amounts of oxygen (O2) and remove CO2
from the atmosphere, using energy from the sun
(G2 dwarf).
earth ellipsoid and flattening factor The
figure of the Earth is approximately an oblate
spheroid, symmetrical about its North-South
axis. A spheroid is a special case of an ellipsoid,
with two of its three principal axes (A,B,C)
equal, viz: A = B > C. The flattening factor
f = (A − C)/A, now approximately 1/298,
slowly decreases, as tidal dissipation slows the
Earth’s rotation. The “weighted mean axis” is
(2A+C)/3. The “GCT Sphere” is used in map-
earthquake
ping transformations that do not accommodate
oblateness. Most models were determined by
fitting a spheroid to surveys. Ground-based surveying
methods depend on the plumb bob or
its equivalent to find the local vertical, yielding
not the spheroid, but the geoid. Some models
were determined dynamically, by measuring
the Earth’s gravity field through the analysis
of spacecraft orbits. Among these, cases
such as GEM-9 and GEM-10, with identical
axes, were derived from different parent models,
whose gravity fields differ in detail.
Earth Orbiter (EO-1) A spacecraft belonging
to the New Millennium Program, intended
to validate revolutionary technologies
for future land imaging missions. Launched
November 21, 2000, with three advanced land
imaging instruments that collect multispectral
and hyperspectral scenes in coordination with
the Enhanced Thematic Mapper (ETM+) on
Landsat-7. EO-1 flies in a 705-km circular, sunsynchronous
orbit at a 98.7 ◦ inclination allowing
it to match within 1 min the Landsat-7 orbit
and collect identical images for later comparison.
earth orientation parameters (EOP) Coordinates
of the Earth’s rotational pole as measured
along the Greenwich meridian and the
meridian 90 ◦ west. They are determined geodetically
by very long baseline radio interferometry
or satellite or lunar laser ranging.
earthquake The sudden movement of the
ground caused by the release of stress along a
fracture (or fault) within the lithosphere. Earthquakes
usually occur along plate boundaries,
although intraplate earthquakes do occur and
probably represent the release of excess stress.
When too much stress builds up within the
planet’s lithosphere, the material will fracture
and elastic waves carry the resulting energy
throughout the planet. The actual location of
the initial slip along the fault is called the focus
of the earthquake; it is usually located at
some depth below the surface. The location on
the surface directly above the focus is called the
epicenter of the earthquake. Energy released by
an earthquake is measured in terms of magnitude
on the Richter scale — an increase of one magni-
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