3. The theory behind celestial navigation


The basic theory behind celestial navigation is finding our unknown position from a known position. If we have some information we can deduce the rest.


Below are several basic rules on which the practice of celestial navigation is build upon:

  1. Celestial navigation is based on the pre-Copernican concept. Its view of the heavens is that the Earth is the unmoving centre, around which the sun, moon, stars and planets revolve.

Ptolemyís pre-Copernican concept


  1. Space and time are a continuum. In celestial navigation, time=distance. Longitude is measured in degrees, and each 15o is represented by 1 hour. (Since 360o = 24 hour) i.e. One hour of the rotation of the Earth corresponds to 15o angle of the Earthís rotation. One second of time is equal to roughly ľ mile at the equator.


  1. The three coordinate systems:

(It is essential that we are familiar with some of the common terms used in celestial navigation.)


Earthís coordinate system :

Equator: is the only great circle whose plane passes through the centre of the earth and is perpendicular to the polar axis. It is used as a reference point for setting latitude lines, thereby given the value 0o .


Latitude lines: runs parallel to the equator. It circles the Earth to the North Pole from 0 to 90 degrees for the North Latitude and from 0 to 90 degrees to the South Pole for South Latitude.


Longitude lines: runs perpendicular to the equator. They are vertical circles, beginning at 0 degrees at the Greenwich Meridian running through Greenwich, England, and circling 180 degrees to the east for East Longitude and 180 degrees to the west for West longitude. Each degree is further subdivided into 60 minutes, and each minutes into 60 seconds (3600 seconds per degree)


With this grid system, we can pinpoint the location of anything on Earth by giving its latitude and longitude.


Celestial Bodiesí coordinate system:

Celestial Equator: It is the imaginary great circle above the earthís terrestrial equator and is located half way between the celestial poles.


Declination: It is the angular distance of the celestial body measured north or south from the celestial equator (0 degrees). It is analogous to earthís latitude.


Right ascension or hour circles: It is the angle between the meridian of the vernal equinox and the meridian of the star and is measured eastward from the vernal equinox. It is analogous to the Ďlongitude linesí on the celestial equator. However, right ascension is measured starting from 0 to 360 degrees, or in hours from 0 to 24 hours (rather than 180 degrees east or west as in the case of longitude.)


First Point of Aries:

Also known as the vernal equinox. It is also referred as the 0 degree hour circle (or celestial meridian). It is the base point for calculating right ascension.


Observerís coordinate system:

Zenith: the point directly over the observerís head


Nadir: the point beneath the feet of the observer.


Horizon: The equator of the observer, perpendicular to the zenith.


Altitude: The Ďlatitudeí of the celestial body, measuring from the horizon up to the zenith of an observer (90 degrees). The angle is measured with a sextant or other navigational instruments.


Azimuth: The number of degrees along the horizon of a celestial body and it corresponds to the compass direction. It starts due North and increases clockwise to 90o East, then 180o South and then 270o West and finally back to 0o North.


Observerís meridian: the imaginary line running from observerís zenith due north or south to the observerís horizon.


  1. Hour Angles:


Geographical Position (GP) : The point on Earthís surface which passes through the centre of the Earth, through its surface, and into the centre of the celestial body. This information is in the Almanac for every day, hour and minute of the year.


Greenwich Hour Angle (GHA): The angle measured at the pole between the meridian passing through Greenwich and the meridian passing through the celestial body.


Local Hour Angle (LHA):

The angle measured at the celestial pole between the meridian passing through the observerís meridian and the meridian passing through the celestial body.


Sidereal Hour Angle (SHA):

The angle measured at the celestial pole between the meridian passing through the first point of Aeries and the meridian passing through the celestial body, measured westwards.


The Almanac gives the GHA of everything but the stars because that would take up too much room, instead it gives the SHA, which we can use to convert, and worksheets help us figure the LHA by using our longitude.


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