Cross, F: New Navigation

Excerpt from The New Navigation: Presented in a Familiar Way for Captains and Officers of the Merchant Service

The run between sights needs very little consideration, thus obviating carrying the first position on to the second, so making some of the same logarithms available for the second observation.

From personal experience I can recommend this problem on the score of accuracy, as the daily position by this method on the Mersey is always on the dot (as compared with the latitude obtained by meridian altitude, when the double altitude is frequently 2' or 3' out both in latitude and longitude owing to a late second sight.

Another advantage of this problem was pointed out to me by a navigator in the Navy (where this method is used almost exclusively), viz., that as the altitude is the last thing to be worked with, an hour angle may be assumed for the second observation and all the calculation finished before taking the altitude. In which case find the g.m.t. Corresponding to the hour angle used, and then the time-taker sings out stop to the observer, who takes the altitude at that precise second, from which is obtained the true zenith distance, which is compared with the one already calculated, and so the position is obtained within 2 or 3 minutes of the observation. Should the sun be clouded at that precise second, by altering the D. R. Longitude, the time may be made to give the same hour angle as at the time when the altitude was really taken.

In the Western Ocean, also from the Cape to Australia, this problem should be especially valuable, as the weather conditions, more often than not, preclude the observer from choosing his own times for sights, and the following examples will clearly show the extreme ¿exibility of this problem.

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