CHAPTER X
UNAWARES IN THE ENEMY'S COUNTRY
During the evening, while engaged in their usual discussions the surveying implement was brought up, and it was understood that a crude instrument should be made, so that it would be ready within the next week.
While speaking on the subject of laying off the segments into degrees, the former matter of observing the heavens was alluded to, and Ralph inquired why all calculations of the heavens were made by degrees.
"Because degrees mean angles and not measurements. Positions or distances are determined by angles and by measurements in miles."
"But in looking at the stars how can mariners tell where they are, simply by getting the angles?"
"That is a matter which can be calculated with such accuracy on the high seas that the distance can be determined with absolute certainty to within three or four miles. Furthermore, when the moon is shining, a mariner, even without an instrument, will be able to make a pretty accurate determination, if the moon is in sight."
"This is interesting information, and I would like to know how it is done."
"Simply by noting how far the moon is from some fixed star. It is difficult to explain this without making a drawing of the heavens or drawing a map which shows the positions of the fixed stars, and the two sketches (Figs. 13 and 14) that I have drawn out will, I hope, make it clear to you. The first figure (13) is a map of the sky for the northern hemisphere, and the second drawing (Fig. 14) of the southern hemisphere, where we are.
