**First Difference**-the difference between successive tabulations of a quantity.

**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 or about March 21. Also called VERNAL EQUINOX.

**Geographical Position (GP**)-the point where a line drawn from a celestial body to the Earth's center passes through the Earth's surface.

**Great Circle**-the intersection of a sphere and a plane through its center.

**Great-Circle Course**-the direction of the great circle through the point of departure and the destination, expressed as angular distance from a reference direction, usually north, to the direction of the great circle. The angle varies from point to point along the great circle. At the point of departure it is called INITIAL GREAT-CIRCLE COURSE.

**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-**on the celestial sphere, a great circle through the celestial poles and a celestial body or the vernal equinox. Hour circles are perpendicular to the celestial equator.

**Intercept (a)-**the difference in minutes of arc between the computed and observed altitudes (corrected sextant altitudes). It is labeled T (toward) or A (away) as the observed altitude is greater or smaller than the computed altitude; Hc greater than Ho, intercept is away (A); Ho greater than Hc, intercept is toward (T).

**Line of Position (LOP)-**a line indicating a series of possible positions of a craft, 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 1800, and labeled E or W to indicate the direction of measurement.

**Nadir (Na)-**that point on the celestial sphere 180 from the observer's zenith.

**Name-**the labels Nand S which are attached to latitude and declination are said to be of the same name when they are both N or S and contrary name when one is N and the other is S.

**Navigational Triangle**-the spherical triangle solved in computing altitude and azimuth and greatcircle sailing problems. The celestial triangle is formed on the celestial sphere by the great circles connecting the elevated pole, zenith of the assumed position of the observer, and a celestial body. The terrestrial triangle is formed on the Earth by the great circles connecting the pole and two places on the Earth: the assumed position of the observer and geographical position of the body for celestialobservations, and the point of departure and destination for great-circle sailing problems. The term' astronomical triangle applies to either the celestial or terrestrial triangle used for solving celestial observations.

**Polar Distance (p)-**angular distance from a celestial pole; the arc of an hour circle between a celestial pole, usually the elevated pole, and a point on the celestial sphere, measured from the celestial 'pole through 180°.

**Prime Meridian**-the meridian of longitude 0°, used as the origin for measurement of longitude.

**Prime Vertical**-the vertical circle through the east and west points of the horizon.

**Principal Vertical Circle**-the vertical circle through the north and south points of the horizon, coinciding with the celestial meridian.

**Respondent-**the vaiue in a table or diagram corresponding to the entering arguments. Second

**Second Difference**-the difference between successive first differences.

**Sidereal Hour Angle (SHA)-**angular distance west of the vernal equinox; the arc of the celestial equator, or the angle at the celestial pole, between the hour circle of the vernal equinox and the hour circle of a point on the celestial sphere, measured westward from the hour circle of the vernal equinox through 360.

**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**-on the celestial sphere, a great circle through the zenith and nadir. Vertical circles are perpendicular to the horizon.

**Zenith (Z)-**that point on the celestial sphere vertically overhead.

**Zenith Distance (z**)-angular distance from the zenith; the arc of a vertical circle between the zenith and a point on the celestial sphere.