Page:Popular Science Monthly Volume 3.djvu/737

Rh, the Nautical Almanac. The astronomer, in his fixed observatory, finds this almanac essential to the prosecution of his observations; the student of theoretical astronomy has continual occasion to refer to it; but, to the sea-captain, the Nautical Almanac has a far more important use. The lives of sailors and passengers are dependent upon its accuracy. It is, again, chiefly for the sailor that our great nautical observatories have been erected, and that our astronomer-royal and his officers are engaged. What other work they may do is subsidiary, and, as it were, incidental. Their chief work is to time this great clock, our earth, and so to trace the motions of those celestial indices, which afford our fundamental time-measures, as to insure as far as possible the safety of our navy, royal and mercantile.

Let us see how this is brought about, not, indeed, by inquiring into the processes by which, at the Greenwich Observatory, the elements of safety are obtained, but by considering the method by which a seaman makes use of these elements.

In the measures heretofore considered, the captain of a ship in reality relies on terrestrial measurements. He reasons that, being on such and such a day in a given place, and having in the interval sailed so many miles in such and such directions, he must at the time being be in such and such a place. This is called "navigation." In the processes next to be considered, which constitute a part of the science of nautical astronomy, the seaman trusts to celestial observations independently of all terrestrial measurements.

The points to be determined by the voyager are his latitude and longitude. The latitude is the distance north or south of the equator, and is measured always from the equator in degrees, the distance from equator to pole being divided into ninety equal parts, each of which is a degree. The longitude is the distance east or west of Greenwich (in English usage, but other nations employ a different starting-point for measuring longitudes from). Longitude is not measured in miles, but in degrees. The way of measuring is not very