Democratic Sentinel, Volume 15, Number 49, Rensselaer, Jasper County, 25 December 1891 — A Knot or Nautical Mile. [ARTICLE]

A Knot or Nautical Mile.

How much is a knot? This question is asked, we believe, in every sea passage by some passenger or other, and never meets with a clear reply. Sailors themselves do not describe it distinctly, and books of reference differ as to its dimensions. We purpose to answer the question here. A knot is one-sixtieth of a mean degree of the earth’s meridian. This definition requires explanation and also numerical computation. The earth’s meridian is commonly described as any circle whose center is the center of the earth, and whose circumference passes through the poles. This is not exact, because the meridian is not a true circle. Evidently, it would be a true circle if the earth were a true sphere, but the earth is not a true sphere; it is a spheroid, its diameter measured on the axis being less than its diameter at, the equator Hence the circumference of a section of the earth by a plane passing through its center and the poles, which circumference is a meridian, is not a true circle but an oval. Bearing this in mind, it will be easy to understand the meaning of a mean degree of the earth’s merid ian. If three hundred and sixty separate degrees be set off from the center of a perfect circle, it is evident that the circular measure of each degree measured on the circumference of the circle will be the same. But if they be set off from the center of an oval, the measurements on the circumference of the oval will not all be the same. That this is the case any one may demonstrate for himself by drawing an oval and its minor axis, and then, from the center of the oval, with radius equal to its semi-minor axis, inscribing a circle in the oval. If, now, degrees, or rather, for convenience, equimultiples of a degree, be set off from the common center, the geometry of the figure will show at once the variations in the circular measurements on the circumference of the oval. Now, a mean degree of the earth’s

meridian is the average length oi these three hundred and sixty unequal measurements, and it is obtained by dividing the length of the meridian by three hundred and sixty. Astronomers have measured the earth’s meridian and found it to be 131,259,287 English feet. Dividing this by three hundred and sixty, we get 364,609.13 feet as the length of a mean degree of the meridian. Onesixtieth of this, then, is a knot; and thus, by division, a knot is found to be 6076.818 feet, or 2025.6 yards, or 1 mile 265.6 yards. It will now be convenient to notice that a knot being 6076.818 feet, and a mile being 5,280 feet, the proportion of a knot to a mile is very- nearly as 6,076 is to 5,280; or, dividing by four, as 1,519 is to 1,320, which is very nearly asils to 13. So that, for ordinary purposes, knots may be converted into miles by taking thirteen knots as equal to fifteen miles, and vice versa.