Celestial Navigation

Local Apparent Noon (LAN)

When the sun crosses your meridian.

Time Zone

a range of longitudes where a common standard time is used. Every 15 degrees 1 hour time change is observed. West of primer meridian= ZD -8

Alidade

a sighting device or pointer for determining directions or measuring angles, used in surveying and (formerly) astronomy.

Time Diagram

a sketch to represent relations between various hour angles. Characteristics: Viewed from above the celestial South Pole, circle represents celestial equator, Easterly direction is clockwise, Westerly Direction is counter clockwise, Celestial meridian and hour circles are radial lines.

Chronometer

a timepiece with a nearly constant rate, set approximately to Greenwich mean time and not reset as the craft changes time zones.

GHA (Greenwich Hour Angle)

angular measure of a celestial body from the Greenwich meridian along the celestial equator= longitude on celestial sphere (measured westward).

Azimuth Circle

azimuth circle can be use to measure the azimuth or amplitude of celestial bodies to determine compass error.

Elevated Pole

celestial pole that corresponds to the earth pole that is in the same hemisphere as observer.

deviation

deflection of the needle of a compass caused by local magnetic influences (as in a ship)

Equation of Time (EoT)

discrepancy between real sun and imaginary sun . Gives offset in minutes applied to mean solar time, as may be determined by a chronometer to calculate apparent solar time; specifically at the suns apparent passage at the local meridian. (Apparent time=mean time+(Equation of time)

Ecliptic

great circle on the celestial sphere representing the suns apparent path during the year.

Navigational Triangle

is a spherical triangle, made up of great circles, used for solving celestial observations.

Local Apparent Time (LAT)

local time based on sun. Sun time.

Lower Branch

lower half of the celestial meridian including nadir.

Terrestrial Triangle

on earth, this triangle is referred to as terrestrial triangle.

Celestial Triangle

on the celestial sphere, this triangle is called celestial triangle.

Nadir

point in the sky directly opposite the zenith and directly beneath the observer.

Equinoctial (Celestial Equator)

projection into space of the earths equator onto the celestial sphere. An imaginary circle equidistant from the celestial poles.

Geographical Position

projection of celestial body on earth directly below itself

Chronometer Error

the amount by which chronometer time differs from the correct time to which it was set, usually Greenwich Mean Time.

Chronometer Rate

the amount gained or lost by a chronometer in a unit of time. It is usually expressed in seconds per 24 hours, to an accuracy of 0.1sec, and labeled gaining or losing. Rate of chronometer error.

Meridian Angle

the angle, measured Eastward or Westward through 180 degrees, between the celestial meridian of an observer and the hour circle of a celestial body.

SHA (Sidereal Hour Angle)

the angle, measured westward through 360 degrees, between the hour circle passing through the vernal equinox and the hour circle of a celestial body.

Variation

the angular difference between true north and magnetic north at a particular place.

Zenith distance

the angular distance between the zenith and the celestial body measured along the vertical circle through the body (Zenith distance=90-Altitude above celestial horizon)

Altitude

the apparent height of a celestial object above the horizon; measured as an angle.

Azimuth

the arc of the horizon measured clockwise from the south point, in astronomy, or from the north point, in navigation, to the point where a vertical circle through a given heavenly body intersects the horizon.

Amplitude

the arc of the horizon measured from the east or west point to the point where a vertical circle through a heavenly body would intersect the horizon.

Compass Error

the difference between compass heading and true heading expressed as the algebraic sum of variation and deviation.

RA (Right Ascension)

the distance of a point East of the first point of Aries, measured along the celestial equator and expressed in hours, minutes and seconds.

Celestial Meridian

the great circle passing through the celestial poles, as well as the zenith and nadir of an observers location. Consequently it contains also the North and South points on the horizon and is perpendicular to the celestial equator.

Visible Horizon

the line where earth and sky appear to meet.

Zenith

the point on the celestial sphere vertically above a given position or observer.

Upper branch

the upper half of the celestial meridian including the zenith.

Depressed Pole

celestial pole that corresponds to the earth pole that is not in the same hemisphere as observer.

Celestial Horizon

great circle on the celestial sphere perpendicular with the zenith and nadir.

Declination

the angular distance of a point North or South of the celestial equator.

Prime Celestial Meridian

the prime meridian projected onto the celestial sphere. GHA is measured to the west from the prime celestial meridian.

First point of Aries (Vernal Equinox)

when the sun crosses the celestial equator (equinoctial) from the south.

Colatitude

90 degrees - Altitude (Zenith Distance)

Co-Declination

90 degrees - Declination

Co-Latitude

90 degrees - Latitude

Hour circle

A great circle on the celestial sphere passing through the celestial poles and containing a point on the celestial sphere , as a star or the vernal equinox.

Local Celestial Meridian

A great circle that runs from North to South and goes straight over your head through the zenith.

LHA (Local Hour Angle)

It is the angle between the meridian of your Assumed Position and the meridian of the geographical position of the celestial body

Time Tick

Radio signal often used to determine chronometer error.

Local Mean Time (LMT)

Solar time as measured by the position of the mean sun with respect to an observer's local meridian. Like apparent time, local mean time differs continuously with the observer's longitude and is not standardized over a time zone. However, a day as measured by local mean time does not vary in length throughout the year-it is always 24 hours.

Chronometer Correction

The amount that must be added algebraically to the chronometer time to obtain the correct time. Chronometer correction is numerically equal to the chronometer error, but of opposite sign.

Meridian Angle (T)

The angular distance East or West of the local celestial meridian and the Hour circle of a celestial body.

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 degrees at the North or South reference Eastward or Westward through 180 degrees 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 Two agree with the meridian angle.

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 of a point, measured from 000 degrees at the North part of the principal vertical circle clockwise through 360 degrees.

Meridian Passage (MERPASS)

The passage of a celestial body across an observer's meridian.

Sight Reduction

The process of solving celestial observations is referred to as sight reduction.