A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Jonathan M Bresler
Date: 2014 Sep 16, 12:33 -0700
First a warning, like you are am newly learning this material, so there may be (will be) errors. Confident others on the list will correct and we'll both learn more.
For me is useful to compare new material to familiar items and do the old "compare and contrast" exercise.
To describe a location on earth we use terrestial coordinates. To describe a star's location we need celestial coordinates.
Terrestial coordinates are expressed in longitude and latitude. Celestial coordinates are expressed in GHA and declination.
Navigation uses a earth centered view of the universe. The sky is modelled as the inside of a large ball with the earth at its center. To continue the comparison, some terrestial "features" are geometrically projected into the sky. A line is drawn from the center of the earth through the north pole of the earth (true north, not magnetic) to the inside of the sphere, that's the celestial pole....the projection of the earth's north pole on the sphere. The south pole and the equator are projected onto the sphere as well forming the celestial south pole and the celestial equator. See http://www.astronomy.ohio-state.edu/~pogge/TeachRes/Artwork/Sky/CelestialSphere.gif
The earth's equator is equally distant from the north and south poles. The celestial equator is equally distant from the celestial north and south poles.
Locations on earth are described by longitude and latitude. Locations in the sky are described by GHA and declination.
Latitude is the angle between a location and the equator. Declination is the angle between the star and the celestial equator.
Longitude is the angle between a location and the Greenwich meridian. GHA is the angle between the star and the meridian of Greenwich projected onto the sky. See http://kmr.nada.kth.se/files/pointfocus/Celestial_Navigation/Celestial-navigation-1.1.jpg
Looking at today's page of a Nautical Alamanc (see http://navsoft.com/2014_Nautical_Almanac.pdf), we can see that the location, GHA and declination, of the Sun, the Moon, and the planets change constantly throughout the day. The motions are not constant, so each hour is listed separately for each of these bodies, Sun, Moon, Venus, Mars, Jupiter, Saturn. By contrast, the position of the stars in the sky are fixed in place for all practicable purposes. The angle in celestial coordinates cooresponding to longitude of each star is the GHA of that star. To list the GHA's of each of the 58(?) stars used for navigation each hour would require 58 more columns on each page. But the position of the stars is fixed. So a new reference point is created and the hourly GHA of the reference point is listed. That reference point is the "First Point of Aries". The GHA changes constantly. Each page of the Nautical Almanac covers three days. During three days the SHA may have moved enough that a new SHA is needed. Example: SHA of Acamar on 2014-09-16 is 315d 17.5'. The SHA of Acamar on 2014-09-19 is 315d 17.4'.
So what is this "First Point of Aries", this reference point that is used to describe star locations as SHA's which is added to the GHA of the "First Point of Aries" ?
The sun, the planets, and the constellations of the zodiac move along a path that is called the ecliptic. The ecliptic and the celestial equator intersect at two points, those are points at which the sun's path crosses the equator are the spring and the autumn equinox. The location of the spring equinox in the celestial sphere is the "First Point of Aries". See http://en.wikipedia.org/wiki/Astrological_sign#mediaviewer/File:Sign_cusps.png.