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year for directional values].
magnellc solar dally varlatlon—A periodic
variation of the Earths magnetic field that is in
phase with solar [bcal) lima.
maonotlc atrstlon-A rnonumenled station al
which a series of magnetic observations have
been msda, h usually consists of a bronze
marker se! in stone or concrete at which, in
addition to latitude and longitude, the magnetic
value Is irrdiiated.
magnetic storm—Sae magnatlc
dlalurbance.
magnetic aurvay-A survey conducted to
meaaure the strar?glh anct/or diraction of the
Earth’s magnetic field al spacific poinls on or
near the surface of the Earth.
magnetic tape—Ferrous-coated tape,
Selective polarl.zation of the suriace permits lhe
sequential storage of digital data. Commonly,
magnetic tape rafers to reel-to-reel tape as
opposed to tapa enclosed within cartridges.
magnatlc varletlon charl—A than depicting
isogonk fines, or lines 01 equal magnelic
declination, for a given time.
magnetic varlallon—(JCS)l. In navigation,
at a given place and time, the horizontal angle
betwean the true corfh and magnetic north
measured east or west according 10 whelher
magnetic north lies east or west of true north,
Also called vatlatlon. See also magnetic
declination. 2. In cartography, the annual
change In dlrecflon of the horizontal component
of the earfha magnelic fjeld.
magnatlam—The ability to attract magnetic
material, notably Iron and steel. Also called
terreatrlal magnetlam. See also blue
magnellam; geomagnetiam; horlzontaf
Intarmlty; red magnetism; vertical
Intenalty.
magnato-optlcaf disc—A mass storage
media which cunsists of a transparent rigid
substrate, which is coated with a recording layer
composed of rare earth and transition metals.
Wrffing isachieved by using a Iasertoraiee the
tempsratureof asmallaraaof the recording
layer. This terrporarily fowersthe coercivity (the
strength of the magnetic state), allowing the
direcfionot the magnetic field to be reversed by
a small external magnetic field applied by fhe
MIL-HDBK-850
153
drive’a magnetic head. Erasing is accomplished
by the same process, except theeppliad
megneticfiaJd isreveraed indirection. Reading
of magneto.opticai materiat can be patlonned
either magnetically or optktlfy.
magnetometer sunrey-A suwey wherein
the Earth’s magnetic field lsmapped by the use
of a magnetometer. See also magnetic
aurvay.
magnatometer—An Instrument used In the
smdyof geonzegnatlsm for measuring a
magnetic alement. See atso flux-gate
magnatomater; flux motar; nuclaar
praceselon magnetometer; optical
pumping magnetometer; theodo.
Iltefmagnetometer; varlometer.
magnification-(optics) The ratio of the size
of an object to the size of ifs magnified image.
Also called power of a lens. See also
angular magnlflcatlon; dlopter; letWfSl
magnlflcatlon; Iktear magnification;
Iongltudlnal magnitlcatlon;
unldlmenalonal magnification.
magnltylrrg power—The ratio of lhe apparent
length of a linear dimensional seen through an
optical instrument, and by the unaided eye.
Thus, an instrument witha magnify ing power of
three makes anobjecf appear lhrea timasas
high and three times aswida. Alac called
dlamatar. See also dlopter.
magnitude—l. Relative brightness of a
celestial body. Also called stellar magrsltuda.
2. The intensity of a shorf-parbd magnetic
Fluctuation, usuafy expressed in milfigausaesor
gammas. 3. Relating to amount, aize, or
greatness.
mehr scheme slatlon—A station through
whfch the main computations and adjustments
of thesurvay data arecarried andserve for the
continued extansionof the survey. Also cslled
primary station; prlnclpal atatlon.
malntalnablllty —Tha capacity for making
updates in an efficiant and cast-affetilve
manner to a data base or software.
major axla-The longest diameter ot an
ellipse or ellipsold.
major datum—See preferred datum.
major grid—The primary grid or grids on a