While off the Yucatan peninsula recently, an opportunity to obtain an unusual celestial fix was taken advantage of during meridian transit of the sun. The fix was obtained entirely through a submarine periscope and is believed to be one of the first ever obtained from a submarine while submerged.
The declination of the sun was within about 2° of the D.R. latitude and hence the subsolar point (the point on the earth’s surface directly beneath the sun) at the time of meridian transit was only 150 miles from the position of the submarine. A subsolar fix was possible at this time except for the fact that the ship was submerged, and the whole fix was, therefore, taken by means of bearings rather than altitudes. The procedure followed was quite simple.
The subsolar track was laid out on a plotting sheet using G.H.A. as longitude and declination as latitude. Timed azimuths were taken at half-minute intervals during a 10- minute period on each side of L.A.N. The reciprocals of these bearings were corrected to mercator bearings by using Table 1, Bowditch, and these azimuths were laid off from the corresponding subsolar points. The point of intersection of these lines then indicated the position of the ship.
Since the submarine was proceeding at slowest submerged speed, own ship’s movement was disregarded. For absolute accuracy, of course, this factor can be taken into account in the usual manner, but a course was chosen to give an error which could safely be omitted.
During the 20-minute period of observations the azimuth of the sun changed about 90°, and the bearing lines intersected in a point which gave a very beautiful and accurate fix.
The only difficulty encountered occurred when the ship took a small angle during one stage of the observations. Since the altitude of the sun was so near 90°, a very slight angle caused a considerable and immediately apparent error in the bearings. Fortunately the sea was flat and depth control well-nigh perfect, so that only a very few of the azimuths were in error. These errors were eliminated by fairing a curve through a plot of time versus azimuth and picking off the correct bearings. The reciprocals of these bearings were then converted to mercator bearings and the final plot was made.
Because of the difficulty in eliminating the effect of roll and pitch on bearings of bodies at high altitudes, this procedure for taking a fix is not believed to be of more than academic interest on a surface ship, but it is believed to be of definite value to a submarine conducting a submerged patrol near the ecliptic when it is not desirable to surface for a sun line. An accurate fix can be obtained by bearings of the sun or moon during transit at any time when within 15° of the subsolar or sublunar track. In fact, to carry the idea to the extreme, a noon fix of some value could be obtained at any time regardless of position with relation to the sun. If own ship were in 50° N. latitude, sun 22° S. declination (which covers practically all Pacific submarine patrol areas) and observations were taken over a one-hour period around L.A.N., a 10° convergence would be obtained which would still give a fix of reasonable accuracy. A total of only five or six observations over a 20-minute period when the declination is as much as 15° away from the D.R. latitude will give a minimum of about 20° change of bearing. These bearings, corrected to mercator bearings as in the above example, will give a fix accurate to about 2 miles. Since morning and afternoon sun lines are also impossible during an all day submergence, this accurate fix at noon should be of definite assistance to a submarine navigator.
Due to security reasons, we are unable to give the usual biographical information on the writer of this article.