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But this method of "dead reckoning” while well enough for coactevise voyaging, where known marks are frequently seen, is prolific of error. Mistakes are continuous, and cumulative. The mistake of one day is carried to the next, and unless there is a frequent method of "checking" the position, the best captain is likely to be uncertain of his exact location after a day or two.

By little and little, as astronomers figured the movements of the heavenly bodies more accurately, and, as instrumental makers improved the reliability of their appliances, the Mariners used more, and more, the observation method, that is, the determination of his position by figuring it from the known positions of the sun, moon, or a star. He very early learned to find his latitude by this method, but it was a long time before he could find his longitude as well.

First, as to getting the latitude. Latitude, you remember, is simply one's distance from the equator, measured in [degrees], [feet] and [inches].

Suppose the [sun] were always immediately over the equator, and that you were given some method of measuring your distance N or S of the [sun]. There would be your lat. fixed, at once, and the actual practice is quite as simple in theory, although there are many allowances, and corrections to be made.

You know, of course, that the [sun] is actually “over the equator” on only two days in each year; the equinoctial days. Mch 20 and Sept 21. It moves (it is easier explained if we go back to the belief of Hipparchus, and consider the sun as moving and the earth immovable) from a point about 23 1/2° N. of the equator. On the 21st and 22nd of Dec. it is at its S'ern point and starts N'd. March 20th it crosses the equator, June 21st it reaches its most N’ern point, and starts on its S’ern trip. September 21st it crosses the equator..