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Celestial Navigation
Pole Star
Latitude by Altitude
The Pole Star so named because it points due north, is
actually away from the astronomical pole by about 57’ and describes a small
circle (clockwise) around the Pole every day, the azimuth changes from
002˚ to 358˚. During the day (24h) the pole star bears north only on
two occasions when it crosses the meridian above and below the pole.
The Pole Star is on the meridian at Upper Transit its
Local Hour Angle is 0˚. At the lower transit it is 180˚, when it is
at right angles to the meridian due WEST of the pole it is 90˚ and when it
is at right angles to the meridian due EAST of the pole it is 270˚.
In the figures above we see the LHA of the Pole Star
(angle ZPX) as 050˚ and 130˚.
AX is the true altitude of the pole star and ZX is the
zenith distance. NM is made equal to AX and is equal to the true altitude.
When this done we see that the correction to be
applied to NM to get the Latitude would be MP.
This correction MP would be applied to the true
altitude if the LHA were between 90˚ and 270˚, and it would be
subtracted if the LHA were between 0˚ to 90˚ or between 270˚ to
360˚.
Using the
Almanac for obtaining the corrections:
To obtain the observed Latitude is quite simple. The
following steps are taken:
Correct the observed altitude to True altitude
Subtract 1˚ from this True Altitude
Add the three corrections: a0, a1, a2
The above converts the True altitude into the
Latitude.
Entering
the Tables:
LHA (Aries) found by:
LHA (Aries) = GHA (Aries) +/- Longitude [Longitude
WEST (-) and Longitude EAST (+)]
First Correction a0: This is given in the Almanac for
every degree of Local Hour Angle (Aries) and abreast of the degree line.
Second Correction a1:
This is given under the nearest ten degrees of LHA (Aries) and abreast
the nearest Latitude. The altitude of the star gives the approx Latitude.
Third Correction a2: This is given under the nearest
ten degrees of LHA (Aries) and abreast of the Month of observation.
a0 is
computed in the Almanac using mean values for Latitude and the star’s SHA and
Declination, then 58.8’ is added.
a1 is the
adjustment for observer’s Latitude and then 0.6’ is added.
a2 is the
adjustment for changes in declination and SHA and then 0.6’ is added
a0 + a1 + a2 = 1˚ so that,
Latitude = True Altitude – 1˚ + a0 + a1 + a2
EXTRACTS FROM THE NAUTICAL ALMANAC
