Celestial Navigation Vocabulary Test 1

SHA

Sidereal Hour Angle. The angular distance of the Hour Circle of the Star measured westward from the Hour Circle of Aries

Vertical Circles

A great circle on the celestial sphere passing through the observer's zenith and nadir, perpendicular to the plane of the celestial horizon

Hour Circle

A great circle through the celestial poles and a point or body on the celestial sphere. Similar to a Celestial Meridian however it rotates along with the Celestial Sphere

Celestial Horizon

A plane passing through the center of the earth, perpendicular to a line passing through the observer's position and the earth center.

Principle Vertical Circle

Coincides with the projected terrestrial meridian passing through their position

GHA

Greenwich Hour Angle. Angular distance of a celestial object measured west of the Prime Celestial Equator (Greenwich)

Azimuth

Horizontal angle measured along the celestial horizon in a clockwise direction from 000°T to 360°T from the Principal Vertical circle to the vertical circle passing through a given point or body on the celestial sphere

LHA

Local Hour Angle. Angular distance of a celestial object measured west of the local celestial meridian through 360.

Nadir

Point on the celestial sphere vertically below the observer

Zenith

Point on the celestial sphere vertically overhead of the observer

Geographic Position (GP)

Position of a Celestial Object corresponding to its placement directly below on the Terrestrial Sphere

Meridian (t) Angle

The Angular distance of a celestial object measured east of the local celestial meridian. (the opposite of LHA)

Celestial Triangle

The Terrestrial, celestial, and the horizon coordinate systems are combined on the celestial sphere, forming the Celestial Triangle. The Celestial Triangle is the basis of celestial navigation.

Meridian Angle

The angle measured East or West from the observer's celestial meridian to the hour circle of the body. LHA is the Meridian Angle measured Westward through 360. t is the Meridian Angle measured Eastward

GHA of Aries

The angular distance of the Hour Circle of Aries measured westward from the Celestial Prime Meridian

Celestial Meridian

The arc of a Great Circle through the poles of the celestial sphere. Remains fixed in relation to the Terrestrial Sphere

Zenith Nadir Axis

The axis used for the Horizon System. It is a direct line which connects through the earth from the observer's Zenith to their Nadir

Navigation Triangle

The corresponding triangle to the Celestial Triangle located on the Terrestrial Sphere. This triangle is used to simplify calculations for position on the Terrestrial Sphere. Meridian Angle and Zenith Angle are also the same in this triangle as they are in the Celestial Triangle

Hour Angle

Angular measurement from the Prime Celestial Meridian.. Similar to Longitude on Terrestrial Sphere

Celestial North/South Poles

Are located at an extension of the Terrestrial North and South Poles

Altitude

Angular distance of a point on the celestial sphere above a designated reference horizon, measured along the vertical circle passing through point

Azimuth Angle

Abbreviated Z. The angle at the zenith between the projected celestial meridian of the observer and the vertical circle passing through the body. Measured 0° through 180°, East or West. Used to find the Celestial Body's True Bearing.

Hour Circle of Aries

An Hour Circle used as a reference point for the Celestial Sphere. SHA of all stars are measured from this reference point.

Celestial Equator

An extension of the Terrestrial Equator onto the Celestial Sphere

Prime Celestial Meridian

An extension of the Terrestrial Prime Meridian (Greenwich) onto the Celestial Sphere

Celestial Sphere

An imaginary sphere of infinite radius with the Earth at its center

Declination

Angular distance from the celestial equator. Similar to Latitude on Terrestrial Sphere