A "sight" in celestial navigation consists of measuring the body's altitude with a sextant and marking the time of that observation using a chronometer.  The celestial "line of position" (LOP), along which the observer is located, is a circle whose radius (called "zenith distance") is deduced from the sextant altitude measurement.  The instant of the observation, expressed in Universal Time (UT), specifies the body's "geographical position" (GP) which is the center of this LOP.  A crossing of this LOP with at least one other such LOP results in a celestial fix on the observer's position.

The GP is a set of two numbers: the declination (counterpart to latitude), and the Greenwich Hour Angle (GHA) which is analogous to longitude.  The declination is an angle ranging from -90º (South Pole), through 0º (Equator) to +90º (North Pole).  The GHA increases westward starting from 0º at the Prime (Greenwich) Meridian and ending at 360º upon reaching the Prime Meridian again after one full round-trip around the Earth.  The GP coordinates for the main navigation bodies are precomputed and published in almanacs for future use.

Stars differ from the bodies of the Solar System in that they have almost negligible proper motion relative to the Earth.  As a result, their declinations are nearly constant and their GHA evolution with UT is almost entirely due to Earth's rotation alone.  In other words, the stars are practically "fixed" to their very nearly constant positions on the celestial sphere.  This allows almanac publishers to save a lot of space using the following scheme.

Instead of tabulating the GHA of each navigation star in some increments of UT (like it is done for Sun, Moon, and planets), this value is computed by adding two numbers:

GHA_Star = GHA_Aries + SHA_Star

"Aries" (i.e. the point of vernal equinox) represents the chosen "Prime Meridian" on the celestial sphere and SHA (Sidereal Hour Angle) is the star's constant westward position relative to Aries.  Since SHA does not change with UT, almanacs can be made much more compact by tabulating each star's SHA just once and placing all UT dependence into the single column of GHA_Aries.  This arrangement is also used in our aries_stars.xls spreadsheet, as shown in the example below (all numbers are in degrees decimal):

For UT of January 1, 2012, 12h 00m 00s, we have GHA_Aries (280.56).

To get GHA_Acamar we add its SHA (315.31) to GHA_Aries (280.56), resulting in 595.87.  From this value we subtract 360.00 to bring the GHA to its conventional range and obtain GHA_Acamar (235.87).

To get GHA_Achernar we add its SHA (335.45) to the same GHA_Aries as above (280.56), resulting in 616.01.  From this value we subtract 360.00 to bring the GHA to its conventional range and obtain GHA_Achernar (256.01).