1 - NGA

stetronomlc deta-DMgnatfon of epoch by
year, month, day, and dacfmal fraction. For
ezsmpfe, the astronomic dale of Dacerrtrer 21,
1978, 18h UTC (univeraaf time morcfhalsd) is
1978 Dacarrtrar 21.75 UTC. The astronomic
date k also used In Omrrecfbn wilh the other
lime systems. me system commences every
cafendar year at Oh on Decatir 31 of the
previous year. This epoch is denoted by .faruary
0.0.
astronomic day—A mean mlar dey beginning
at mean noon, 12 hours feter than the beginning
of the cIvII day of the earns date, Aafmnomers
now genaralty use the civil day.
astronomic equator—The fine on the surface
of the Earth wfmse astmnomk fatituda at everv
point is O“. Due 10 the dafbcfion of the plumb’
iine, the aafronomk equator is not a pla”ne curve.
However, the verficafa a! all points on k are
parallel to one and the same plane, the pfane of
the celestial equator; that is, the zenith at every
point on tha eetmnomk equator lies in the
caleatial er$zalor. When lhe astronomic equalor
is corrected for station error, ii becomes the
geodetic equator. Also called terrestrial
aquator. See aiso geodafic aquator.
astronomic latltud+The angie between the
plumb fine and the plane of celestiai equalor.
Also defined as fha angla be fwean tfra piane of
the horizon and the axis of rotation of the Earth.
Astronomic kitnude appfies only to posltfons on
the Earth and is reckoned from the astronomic
equator (0°) notlh and south through 90°.
Aafmnomic Ietffude is the Iatffude whkh results
directly fmm obaervatbns of celestial fmdles,
urcorrecfed for deliecfion of the vertical.
astronomic lavellng—See aatrogeodetlc
ieveling.
eatronomlc iongltude-The angle between
the ptene of the celastlal meridfin end the plane
of an iniiial merfdiin, arbffrarffy chosen.
Astronomk bngffude la the longitude which
resuffs directly from obsewations on ceiesfial
toches, unmrrected for deflacfbn of the vertical.
aatronomlc merldlan plane-A plane that
cdmains the verrfml of the obsewer and is
~a#rl to the inalanrsnaas rotation 6x19 of the
aatronomlc meridian—A great circle 01 the
MIL-HDBK-850
calestial sphere Imersacfing the ncmh and muth
ceiestial poles. me local astronomk marldian is
[hat meridian whtih intersects the zenffh of fhe
poinl.
astronomic parallef-A fine on the surface ot
the Earth which hes the same eshncmdc
Ialiiude at every point. Because the deffactiin of
the veflid is not fhe SSmS at all pofms on tha
Earth, an astronomic peraflsl IS an irregular fine.
not Iyhrg in a single ptane. See afso
aetronomlc equator.
setronomlc poaltion-1. A pdim on the Earlh
whoaa coordhrates have been determined as a
reeuff of observations of caleslial bodies. The
expression is ueualfy used In connacibn with
positions on iand determined wffh greet erxasracy
for survey purposes. 2. A point on fhe Earth,
defined in terms of astronomic Iatituda and
fongllude.
astronomic refraction error-See
astronomic refraction.
astronomic reflection-Tha apparent
displacement of an object that resuffs hem fight
rays from a source outside Ihe atmosphere being
bent in paasing through the atmesphare. This
rasults in all objects appearfrq 10 be higher
above the horizon than they actuaffy are. The
magnitude of Ihk dbplacemem is greater whan
the object Is near the horizon and decreases to a
minimum assumed to be zero when the object is
al the zeniffr. Afao calied aatronomfc
refraction error; calestial refraction. See
afso atmosptserlc ref racllon; rafractlon.
astronomic station—A point on the Earth
whose position has been determined by
observations on cefestial bodies.
aafronom fc ●urva ylng—The caleafiaf
determination of latitude and bn@sde.
Separations ara cefculaled by mmputirrg
distances corresponding to measured angular
displacements afong the reference spheroid.
astronomic theodolita—See altazlmutft
Inatrumant.
aatronomlc tidal constltuant-See
constituent.
astronomic tfme-sofar time In a day
(astronomic day) that begins at noon. Aafronomic
Ime may be eilher apparem solar Ifme or mean
;olar time. Since 192S, civil tires is generalfy
2‘o