DETERMINATION OF LATITUDE BY LOCAL APPARENT NOON (LAN)
Since the latitude of a position may be determined by finding the distance between the Equinoctial and the zenith, one needs to know only the declination and zenith distance
co-latitude) of a body to determine latitude. The procedure involved has been used by mariners
for many centuries because of its simplicity. Before the discovery of the Sumner line, and prior to the Harrison chronometer, longitude was most difficult to compute. Accordingly, early mariners seized upon the technique of "latitude or parallel sailing," by which they traveled
North or south to the known latitude of their destination, then east or west as appropriate, often using the meridian sight as their only celestial computation. The meridian sight as described is applicable to all celestial bodies, although in practice it is primarily used with the sun.
ALTITUDE BY MERIDIAN SIGHT
When the altitude of a celestial body is measured as it transits the meridian, we think of the observation, and the subsequent solution for a line of position, as a "meridian sight". This sight includes observations of bodies on the lower branch of the meridian (lower transit) as well on the upper branch (upper transit). Circumpolar stars may be observed on either branch of the celestial meridian. In practice, however, bodies are seldom observed on the lower branch, the sun is normally the only body observed. In polar latitudes, when the declination of the sun corresponds in name to the latitude of the observer, the sun may be observed when in
lower transit, but generally, meridian sights of the sun are made when it is in upper transit
(LAN).
The meridian sight is important for the following reasons:
A. It provides a celestial LOP without using trionometry
B. The intersection of the LOP, obtained at LAN, and advanced morning sun lines, establishes
a celestial running fix.
C. It is practically independent of time.
D. The knowledge of the approximate position is unnecessary.
E. The LOP is a latitude line, and is useful in latitude or parallel sailing.
F. A latitude observation is obtained when the celestial body is either bearing due north or
south of the observer. When, reduced, such an observation yields a LOP extending in an east or west direction. This is termed a latitude line.
To observe a body when on the meridian we must first determine the time of local transit. This may be accomplished by one of the three following methods:
FOLLOWING TO MAXIMUM ALTITUDE
The oldest and most common method of determining meridian altitude of the sun is known as following to maximum altitude. It is recommended because of its adaptability to various conditions, and because its use develops an insight into how the altitude varies near the time of apparent noon.
AT approximately 10 minutes before watch time of LAN, the observer contacts the sun's lower limb, with the horizon in the sextant. The observer then swings the sextant from side to side, and adjusts it until the sun, seen moving in an are, just touches the horizon at the lowest part of the arc. This procedure is called "swinging the arc."
As the sun continues rising, a widening space appears between its lower limb and the horizon. Adjusting the tangent screw, the observer keeps this space closed and maintains the sun in contact with the horizon. The change in altitude becomes slower and slower, until the sun "hangs." While it is hanging, the observer swings the sextant to make certain of accurate contact with the horizon. This observation is continued until the sun dips, which is a signal that the sun is now beginning to lose altitude. The sextant then shows the maximum altitude attained.
INSTANT OF TRANSIT
The method of determining the exact instant of transit is explained by Dutton in connection with Todd's method of finding the interval to LAN. An observation taken at the exact instant of transit gives the maximum altitude. Under ordinary conditions, an error of 1 minute (more or less) does not affect the result.
NUMEROUS SIGHTS
The method of taking numerous sights is a modification of the maximum altitude method. It is useful under conditions where heavy sea, clouds, and the like may make steady observation impossible. Well before watch time of LAN, the observer begins taking a series of altitudes. Their number depends on the difficulties of the situation and the possible error in computed time of transit. The observer reads off the altitudes to a recording assistant, turning the tangent screw slightly after each observation to ensure that the next altitude is an independent sight. Observations are discontinued when the altitude definitely shows signs of decreasing.
