DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

epoch An arbitrary fixed instant of time used
as a chronological reference for calendars, celestial
reference systems, star catalogs, or orbital
motions. Prior to 1984, star catalog coordinates
were commonly referred to the mean
equator and equinox of the beginning of the
Besselian year (see year). Thereafter, the Julian
year (365.25 days) has been used.
equant A point in the Ptolemaic description
of orbits about which the center of the epicycle
moved at constant angular velocity as it moved
along the deferent. The equant was equidistant
on the opposite side of the center of deferent
from the Earth. See deferent, epicycle.
equation of continuity Any of a class of
equationsthatexpressthefactthatsomequantity
(mass, charge, energy, etc.) cannot be created or
destroyed. Such equations typically specify that
the rate of increase of the quantity in a given region
of space equals the net current of the quantity
flowing into the region. Thus, the change
of a property ɛ (e.g., mass) inside a volume element
results from the convergence of a flux
C(ɛ) across the boundaries of the volume and
from sources and sinks S(ɛ) inside the volume:
∂ɛ
∂t +∇·C(ɛ)=S(ɛ).
The most common application is the conservation
of mass
∂ρ
∂t
=−∇ ·(ρu) =−∇ ·j
with ρ being the density and j=ρu the mass
current. Using the total derivative instead of the
partial one, this can be written as
dρ
dt
∂ρ
= s+u·∇ρ=−ρ ·∇u.
∂t
equation of state An equation relating the
pressure, volume, and temperature (or some
othersetofthermodynamicalvariables)ofasubstance
or system.
equation of state (cosmic string) Foranaccurate
macroscopic description of a vacuum vortex
defect as a cosmic string, one needs the string
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equatorial anomaly
equations of state, which give both the string
energy per unit length U and its tension T (in
general both are variable), and also the relation
between them. The simplest example of equation
of state is T =U= const, corresponding
to the Goto–Nambu string.
Also conducting cosmic strings can be described
by fitting analytical expressions to the
numerical solutions of the classical equations
of motion for the microscopic fields. The best
analytical expression available for the Witten
model is known as the Carter–Peter equation
of state. See Carter–Peter model, conducting
string, energy per unit length (cosmic string),
Goto–Nambu string, tension (cosmic string),
wiggle (cosmic string).
equation of state of pure water Potential
densityofpurewaterisafunctionoftemperature
only. For actual values see Chen and Millero
(1986).
equation of time The hour angle of the true
(observed) sun minus the hour angle of the (fictitious)
mean sun of mean solar time.
equator The locus of a point traveling around
the globe that is equidistant from the north and
south poles forms the equator.
equator, geomagnetic (1.) The line around
the Earth, along which the vertical component
of the magnetic field is zero. Approximately
equidistant from both magnetic poles, but distorted
from that shape by non-dipole harmonics
of the Earth’s magnetic field.
(2.) A term sometimes applied to the equatorial
surface of the magnetosphere.
equatorial anomaly Although processes in
the ionospheric F region are complex, it is still
reasonable to expect the maximum daytime ionization
to fall roughly near to the location of
the overhead sun. However, in the equatorial
region, and most noticeable near equinox,
the daytime maximum in ionization splits forming
crests 20 ◦ to 30 ◦ to the north and south of
the dip equator, leaving an ionization trough
at the dip equator. This is called the equatorial
anomaly. It is formed by a process analogous
to a fountain, ionization near the dip equa-
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