**Altitude**-the arc of a vertical circle between the horizon and a point or body on the celestial sphere. Altitude as measured by a.sextant is called sextant altitude (hs). Sextant altitude corrected only for inaccuracies in the reading (instrument, index, and personal errors, as applicable) and inaccuracies in the reference level (principally dip) is called apparent altitude (ha). After all corrections are applied, it is called corrected sextant altitude or observed altitude (Ho). An altitude taken directly from a table is called a tabular or tabulated altitude (ht). Tabular altitude as interpolated for declination, latitude, and LHA increments as required is called computed altitude (Hc).

**Altitude Difference (d)-**the first difference between successive tabulations of altitude in a latitude column of these tables.

**Argument-**one of the values used for entering a table or diagram.

**Assumed**(or Chosen) Latitude (aL), Assumed (or Chosen) Longitude (a)-geographical coordinates assumed to facilitate sight reduction.

**Assumed Position (AP)-**a point at which an observer is assumed to be located.

**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 principal vertical circle and a vertical circle through the body or point, measured from 000 at the north part of the principal vertical circle clockwise through 360°.

**Azimuth Angle (Z)-**the arc of the horizon, or the angle at the zenith, between the north part or 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. It is prefixed N or S to agree with the latitude and suffixed E or W to agree with the meridian angle.

**Celestial Equator**-the primary great circle of the celestial sphere, everywhere 90° from the celestial poles; the intersection of the extended plane of the equator and the celestial sphere. Also called EQUINOCTIAL.

**Celestial Horizon**-that circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through the center of the Earth and perpendicular to zenith-nadir line.

**Celestial Meridian**-on the celestial sphere, a great circle through the celestial poles and the zenith. The expression usually refers to the upper branch, that half from pole to pole which passes through the zenith.

**Course Angle**-course measured from 0° at the reference direction clockwise or counterclockwise through 180°. It is labeled with the reference direction as a prefix and the direction of measurement from the reference direction as a suffix. Course angle S21°E is 21 ° east of south, or true course 159°.

**Course Line**-the graphic representation of a ship's course.

**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 labeled N or S (+ or -) to indicate the direction of measurement.

**Declination Increment (Dec. Inc.)-**in sight reduction, the excess of the actual declination of a celestial body over the integral declination argument.

**Double-Second Difference (DSD)-**the sum of successive second differences. Because second differences are not tabulated in these tables, the DSD can be formed most readily by subtracting, algebraically, the first difference immediately above the tabular -altitude difference (d) corresponding to the entering arguments from the first difference immediately below. The result will always be a negative value.

**Ecliptic-**the apparent armual 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**(Pn or Ps)-the celestial pole above the observer's horizon, agreeing in name with the observer's latitude.