By means of the sextant he determines the exact moment at which the sun crosses the meridian, or the exact noon hour at his locality and he also learns the angle or declination of the sun above the horizon. By means of tables he is thus enabled to work out his latitude, or his distance north or south of the equator, and then by marking the spot on his chart where the longitude and latitude cross, he indicates the exact position of his ship.
It is an easy matter to learn to “shoot the sun” and to compute latitude and longitude. Every amateur sailor will do well to acquire this knowledge, even if it never becomes necessary to use it.
For all ordinary purposes, however, dead reckoning will serve and many sailors, and not a few captains of large vessels find dead reckoning sufficiently accurate for their needs if near land or only sailing for comparatively short distances out at sea.
Dead reckoning consists of computing a vessel’s position by the distance sailed from one time to another; the drift or leeway made and the directions in which the boat has sailed.
To find the distance sailed it is necessary to multiply the number of hours by the speed per hour. To determine this an instrument called a log is used. In former days the log was in reality a log, and consisted of a drag of wood attached to a marked line on a reel. The log was thrown over the vessel’s stern and as the line ran out it was timed by a sandglass and the number of knots on the line which ran out while the sand fell through the glass gave the speed of the ship. Today instruments known as patent logs are used which are like small propellers attached to a line connected with a clocklike arrangement, and as the log whirls around by being dragged through the water the hands on the dials indicate the speed of the vessel.
But while the log has been changed to a metal whirler and the line and sandglass have given place to an accurate and complicated mechanism of wheels and cogs, yet the name log is still retained and sailors always speak of knots instead of miles.
By marking off the number of knots sailed in the proper direction the sailor might easily tell where he was, provided there were no currents or tides and the vessel moved at a uniform speed and made no leeway. As a matter of fact the ocean is full of currents, streams and tides and moreover a vessel, when sailing, or steaming for that matter, is carried to one side or the other and forwards and backwards by these as well as by the wind.
Besides a sailing vessel moves more slowly or faster according to the strength of the wind and is often obliged to tack or to alter its course to suit the winds.
All of these matters must be considered in working out a position by dead reckoning and the course of a sailing vessel when thus pricked out on a chart often looks like the track made by a drunken man, as it zigzags hither and thither, swings about, and varies widely from one side or the other of its true course.
In order to come anywhere near accuracy by means of dead reckoning a mariner must be thoroughly familiar with the tides and currents through which he is sailing. He must be able to judge the strength of the wind; he must know just what his vessel will do under varying conditions; he must be able to guess very closely the leeway she makes, and he must bear in mind all the changes of course, all the tacks and all the shifting of sails which have been made. Only by such knowledge and by long practice can a sailor determine where he is by dead reckoning and even then he can only locate his position approximately. It seems remarkable that any man can come anywhere near the truth by such means but many sea captains have become so expert that they can figure out their position by dead reckoning and come within a very few miles of the right result every time.