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latitude. See also atrtronomlc equator;
celeetlal ●quator; flctltlous squator;
galsctlc equator; geodetic aqusstor;
geomagnetic aquator; grid aquator;
lunar caleatlal ●quator; rnagnatic
●quator; obllque ●quStOr; Iranaversa
equator.
eqularr@ar ●plral-see rhumb Ilne.
equlangulator-An opticsl instrument,
em@Yi~ a 60° prism, used to determine
astrmnomc fetifude and bngifude by equal
affiiudes 01 heavenly b@es when the tima of
the prime meridian (Greenwich) is known at the
p~ce oi observation.
equlgeopota;tlal eurtece-See
equlpotentlel eurtace.
MIL-HDBK:850
equlllbrlum ●ptrarold-The shape !hat the
Earth woufd attain ii il were entirely covered by a
Ikteless ocean of constant deplh. See also
geold.
aqulllbrlum theory-A hypothesis which
assumes an ideal Earth which has no
mntinental barriers and is unilormly wvered with
waler of mnsiderabie depth. 11‘also ‘assumes
that the waler responds instantly to the lideproducing
tortes of the Moon and Sun to form a
surface in equilibrium and moves around the
Earth whhout viscosity or friction. See also
geold.
equlllbrlum—A state of balance between
forces. A body is said 10 fM in equilibrium when
the vector eum of all tortes acting upon 11is
zero.
aqUlnOCtlal colure-The hour circle through
the equinoxes.
equlnoctial day—See sidereal day.
equlnoctlal polrrt-See equinox.
equlnocflal ayatam of coordlnalae- See
celestial equator eyatem of coordlrsatea.
aqulnoctlal yeer—See tropical year.
equlnoctlal—See calestlal equstor.
equhrox-One of the two points of intersection
ot the ecfipt”c and the celestial equator,
occupied by the Sun when its declination is OO.
86
Also called equlnoctlal point. Sae also
autumnal equlrsox; mean aquhtox;
vernal aqulnox.
equlpotenllal ●urtace-A surtaoe having lhe
same ptenlisl of gravity et every point, Also
called level surface. See also gaold; OSOP;
geopoterrllal aurlace.
aqulecalar—A surface along whbh a scalar
quantify has a constant vafue.
equivalent focal lenglh—(JCS) The
distance measured along the optical axis of the
lens trom the rear nodal point 10 the plane of
best average definition over the entire tiefd used
in a camera. See also focal length.
equivalent map projection—See equalarea
map projection.
aquhralent scale-The relationship which a
small distance on a grephic bears to the
mrresporrding distance on the Earth, expressed
as an equivalence, such as 1 inch (on the
graphic) equals 1 mile (on the ground). Also
called verbal scale.
equlvelenl vertical photograph—A
theoretically, truly vertical photograph taken al
the same camera station with a camara whose
local length is equal to lhal 01 a camara taking a
mrresponding tilted photograph.
erasable opllcal disc (EOD)—A class of
mmpact discs which allow users to rerecomf data
onlo the disk.
erect Image-An Image that appears upri9hi,
or in lhe same ralatlve position as the object.
arecllng telescope-An observer sees
objacts right side up when Iooklng through an
erecting telescope and upside down whan
twking through an inverting telescope. The
eyepiece In lhe optical syslem of an erecting
teleswpe usually has four lenses, and the
eyepiece in the opticel system of an inverling
telescope has two lenses. See also Inverflng
tefeacope.
erratic error—An error caused by an
incomplete element in an inatrumenf, such as
backlash in a gear train. Sea also Instrument
arror.
error budgel—A correlated set of the