Altitude: the arc of a vertical circle between the horizon and a point or body on the Celestial Sphere.
Assumed Position: geographical position in (assumed) Latitude and (assumed) Longitude chosen to facilitate
sight reduction.
Azimuth (Zn): the horizontal direction of a celestial body or point from a terrestrial point;
the arc of the horizon, or the angle at the Zenith between the north part of the celestial meridian
or principle vertical circle and a vertical circle through the body or point, measured from 000°
at the north part of the principle vertical circle clockwise through 360°.
Azimuth Angle (Z): the arc of the horizon, or angle at the Zenith, between the north part or the south part
of the celestial meridian, according to the elevated pole, and a vertical circle through the body or point,
measured from 0° at the north or south reference eastward or westward through 180° according to
whether the body is east or west of the local meridian. This is the azimuth value as tabulated in the
sight reduction tables.
BCRS: the Barycentric Celestial Reference System, is a barycentric system of space-time coordinates for the solar system within the framework of General Relativity. The metric tensor to be used in the system is specified by the IAU 2000 resolutions. For all practical applications, unless otherwise stated, the BCRS is assumed to be oriented according to the ICRS axis.
Celestial Equator: the primary great circle of the Celestial Sphere formed by the intersection of the
Celestial Sphere and the extended plane of the equator. Also called equinoctial.
Celestial Horizon: the circle of the Celestial Sphere formed by the intersection of the Celestial Sphere
and a plane though the center of the Earth and perpendicular to Zenith-Nadir line.
Celestial Meridian: a Great Circle on the Celestial Sphere through the Celestial Poles and the
Zenith. The Celestial Meridian usually refers to the upper branch, that half from pole to pole which passes
through the Zenith.
Celestial Poles: the north and south celestial poles are the two imaginary points in the sky where
the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars
called the celestial sphere. The north and south celestial poles appear permanently directly overhead to
an observer at the Earth's North Pole and South Pole respectively.
Declination (Dec): angular distance north or south of the celestial equator; the arc of an hour circle
between the celestial equator and a point on the Celestial Sphere, measured northward or southward from the
celestial equator through 90°, and labelled N or S to indicate the direction of measurement.
Ecliptic: the apparent annual path of the Sun among the stars; the intersection of the plane of the
Earth's orbit with the Celestial Sphere. this is a great circle of the Celestial Sphere inclined at an angle
of about 23°27' to the celestial equator.
Elevated Pole: the celestial pole above the observer's horizon, agreeing in name with the observer's latitude.
Estimated Position: geographical position in (estimated) Latitude and (estimated) Longitude obtained
from Dead Reckoning or other piloting techniques indicating the most probable position of the observer.
First Point of Aries: that point of intersection of the ecliptic and the celestial equator occupied
by the Sun as it changes from south to north declination on about March 21.
GCRS: the Geocentric Celestial Reference System, is a geocentric system of space-time coordinates within the framework of General Relativity. The metric tensor to be used in the system is specified by the IAU 2000 resolutions. The GCRS is defined such that its spatial coordinates are kinematically non-rotating with respect to the Barycentric Celestial Reference System.
Geographical Position (GP): the point where a line drawn from the center of a celestial body to the center
of the Earth passes the Earth's surface.
Great Circle: the intersection of a sphere and a plane through its center.
Greenwich Hour Angle (GHA): angular distance west of the Greenwich celestial meridian; the arc of the
celestial equator, or the angle at the celestial pole, between the upper branch of the Greenwich celestial
meridian and the hour circle of a point on the Celestial Sphere, measured westward from the Greenwich celestial
meridian through 360°.
Hour Circle: a great circle on the Celestial Sphere, through the celestial poles and a celestial body. Hour
Circles are perpendicular to the celestial equator.
ICRF: The International Celestial Reference Frame, is a set of extragalactic objects whose adopted positions and uncertainties realize the axis of the ICRS. The first such set was adopted in 1997 defining the ICRF-1 catalog. A second set was adopted in 2010 defining the ICRF-2 catalog.
ICRS: The International Celestial Reference System, is a time-independent kinematically non-rotating barycentric reference system adopted by the International Astronomical Union in 1997. Its axes are those of the International Celestial Reference Frame.
Intercept (Hd): the difference in minutes of arc between the computed and observed altitudes
(corrected sextant altitudes).
Line of Position (LoP): a line indicating a series of possible positions of an observer, determined by
observation or measurement.
Local Hour Angle (LHA): angular distance west of the local celestial meridian; the arc of the
celestial equator, or the angle at the celestial pole, between the upper branch of the local
celestial meridian and the hour circle of a celestial body or point on the Celestial Sphere,
measured westward from the local celestial meridian through 360°.
Meridian Angle (t): angular distance east or west of the local celestial meridian; the arc of the celestial
equator, or the angle at the celestial pole, between the upper branch of the local celestial meridian
and the hour circle of a celestial body, measured eastward or westward from the local celestial meridian
through 180° and labeled E or W to indicate the direction of measurement.
Nadir: that point on the Celestial Sphere 180° from the observer's Zenith.
Prime Meridian: the meridian of longitude 0°, used as origin for measurement of longitude. On the
Earth this is the Greenwich Meridian; on the Celestial Sphere this is the Celestial Greenwich Meridian.
Prime Vertical: the vertical circle through the east and west points of the horizon.
Prime Vertical Circle: the vertical circle through the north and south points of the horizon,
coinciding with the celestial meridian.
Sight Reduction: the process of deriving from a sight (observation of the altitude, and sometimes
also the Azimuth, of a celestial body) the information needed for establishing a Line of Position.
Small Circle: the intersection of a sphere, and a plane which does not pass through its center.
Vertical Circle: a great circle on the Celestial Sphere, through the Zenith and the Nadir.
Vertical Circles are perpendicular to the horizon.
Zenith: that point on the Celestial Sphere vertically overhead.
Zenith distance: angular distance from the Zenith; the arc of a Vertical Circle between the Zenith
and a point on the Celestial Sphere.