Celestial Sphere

Vertical Circle of the Body

The vertical circle that passes through the body; similar to an hour circle.

Principal Vertical Circle

The vertical circle that passes through the north and south points of the horizon.

Prime Vertical Circle

The vertical circle through the east and west points of the horizon.

Prime vertical

The vertical circle which intersects the horizon at the east and west points.

Height Observed (Ho)

This is the Sextant observation (Hs) corrected.

Height Apparent (or Apparent Altitude) (Ha)

Total apparent altitude (Corrected for Index Error and Dip Angle).

Time Diagram

Used to approximate the positions of meridians and hour circles to compute the angular relationships of GHA, LHA, and SHA; the outer circle is the equinoctial the center is the South Pole;

The Four Navigational Planets

Venus (Inferior); Mars, Jupiter, and Saturn (all Superior)

Meridian angle (t)

the arc of the celestial equator or the angle at the pole measured between the local celestial meridian and the hour circle of the body, measured from 0° through 180° to the east or west.

Longitude

Angular distance east or west of the Greenwich Meridian.

Stars

At such extreme distances that they appear as "fixed;" classified by color and brightness; color indicates relative temperature - red cooler than yellow and yellow cooler than blue; each is assigned a number of magnitude - the smaller the number, the brighter the body; 57 are listed on the daily pages of the Nautical Almanac.

Azimuth (Zn)

The arc of the horizon or the angle at the zenith measured from the north point clockwise from 0° through 360° to the vertical circle of the body.

H. O. 229

A book of pre-computed altitudes and azimuth angles for an assumed position near the observer's DR position. The volumes tabulate the Hc and Az for whole degrees of LHA, latitude and declination, and are interpolated for declination to improve the accuracy of the LOP.

The Zodiac (also known as the Zodiacal Belt)

A circular band of the sky extending 8° on each side of the ecliptic. Divided into 12 section of 30°. The sun, moon, and planets all appear to move within this belt.

Celestial meridian of Greenwich

A great circle of the celestial sphere passing from pole to pole and over the Greenwich meridian. All celestial meridians remain fixed over a terrestrial point on earth.

Celestial meridian

A great circle on the celestial sphere that is perpendicular to the equinoctial and intersects the axis of the north and south celestial poles.

Hour Circle of Aries

A great circle passing from pole to pole and through the first point of Aries which is considered to be a body.

Equator

A plane perpendicular to the axis halfway between the poles.

Why must at least 3 LOPs be obtained to constitute a celestial fix?

Because it is possible for two circles to intersect at two points.

Height Computed (Hc)

Computed altitude for the same specific instant in time for a position in the observer's proximity.

Sides of the Navigational Triangle (also known as the Astronomical Triangle)

Consists of the observer's celestial meridian (colatitude); the hour circle of the body (polar distance); and the vertical circle of the body (coaltitude/zenith distance).

Find an LOP using Celestial Navigation

Draw a line perpendicular to the Azimuth Angle (Zn) will approximate the circle of equal altitude.

Meridians of Longitude

Great circles perpendicular to the equator intersecting at the poles. Measured east and west of reference meridian (Greenwich) 0°-180°

Celestial Sphere

Infinite size; earth is "fixed" at the center; the stars as "fixed" points of lights on the surface; the sun, moon, and planets appear to move east to west.

Altitude (90° - h)

Measured from the zenith of the observer to the body along the vertical circle of the body 0°-90°.

Angles in the Navigational Triangle (also known as the Astronomical Triangle)

Meridian angle (t), azimuth angle (Z or Az), and parallactic angle.

Parallels of Latitude

Small circles parallel to the equator and perpendicular to the axis of rotation. Measured north and south of the equator 0°-90°

Altitude (h)

The angle measured along the vertical circle from the horizon to the body; similar to Latitude and Declination.

Co-altitude (also known as the zenith distance) (Co-Alt)

The angle measured along the vertical circle from the zenith to the body.

Height Shot (Hs)

The angle taken with the sextant (sextant altitude)

Declination

The angular distance a body is north or south of the celestial equator. It is measured along the hour circle of the body from 0° through 90°.

Polar Distance (Co-Declination or Co-Dec)

The angular distance of a celestial body, measured along the hour circle of the body from the elevated pole (PElev) to the body.

Hour Angle

The angular measurement from a reference celestial meridian or hour circle of a reference body to the hour circle of the body from 0° to 360°.

Greenwich hour angle of Aries

The arc of the celestial equator or the angle at the pole measured between the celestial meridian of Greenwich and the hour circle of Aries westward from 0° thorough 360°.

Sidereal hour angle (SHA)

The arc of the celestial equator or the angle at the pole measured between the hour circle of Aries and the hour circle of the body westward from 0° through 360°.

Greenwich hour angle (GHA)

The arc of the celestial equator or the angle at the pole measured from the celestial meridian of Greenwich westward from 0° through 360° to the hour circle of the body.

Local hour angle (LHA)

The arc of the celestial equator or the angle at the pole measured from the observer's meridian westward from 0° through 360° to the hour circle of the body.

Azimuth angle (Az or Z)

The arc of the horizon or the angle at the zenith, measured between either the north point or south point from 0° through 180°, east or west, to the vertical circle of the body. It must have a prefix and a suffix. The prefix is determined by the observer's latitude; the suffix by the name of the meridian angle.

The Moon

The closest celestial body; revolves around the earth every 27.322 days.

Celestial poles

The earth's poles projected until they intersect the celestial sphere. The elevated pole takes the name of the latitude of the observer and its altitudes are numerically equal to the latitude.

Equinoctial (also known as the celestial equator)

The great circle of the celestial sphere which is everywhere 90° from the poles, or the earth's equator projected upon the celestial sphere.

Celestial horizon

The great circle of the celestial sphere which is everywhere 90° from the zenith and nadir.

Hour circle

The great circle which passes from pole to pole through the body and moves with the body.

The Sun

The nearest star - approximately 80,780,000 nautical miles from earth (1 AU); the brightest body in the sky; used as the basis of time.

Local celestial meridian

The observer's meridian projected upon the celestial sphere.

Vertical circle

The part of the celestial sphere which passes through the zenith, nadir and the body.

Ecliptic

The path the sun appears to take among the stars, inclined to the equinoctial at 23° 26'.

Nadir

The point on the celestial sphere directly below.

Zenith

The point on the celestial sphere directly overhead.

First point of Aries (the Vernal Equinox)

The point where the sun crosses the celestial equator as it moves from south to north. It occurs on or about March 21st and marks the first day of spring for an observer in northern latitudes.

Autumnal Equinox

The point where the sun crosses the celestial equator on its journey southward. This marks the first day of autumn for an observer in northern latitudes and occurs on or about September 23rd.

Winter Solstice

The point where the sun reaches its maximum southerly most declination. This occurs on or about December 21st and marks the first day of winter for an observer in northern latitudes.

Summer Solstice

The point where where the sun reaches its maximum northerly declination of about 23°26'. It occurs on or about the 21st of June and marks the first day of summer for an observer in northern latitudes.

Geographic Position of a Body (GP)

The position on the earth's surface a celestial body appears to have when viewed from the center of the earth or when the perpendicular light beam from the body is projected onto the earth's surface.

Axis of Rotation

A straight line through the center of the earth, about which the earth rotates. It intersects the surface of the earth at the North and South Poles.