Parallel of latitude

“ An altitude may be taken of any body and after a suitable change in bearing has taken place (not less than 30°) a second altitude may be taken and the first longitude advanced for the run during the interval to the parallel of the latitude by dead reckoning at the time of second sight. ”

“ The latitude at which the arc length of the parallel separating the meridians passing through two specific points is exactly equal to the departure in proceeding from one point to the other by mid-latitude sailing. The mean latitude is used when there is no practicable means of determining the middle latitude. ”

“ Any definite line is one along which remains constant while varies continuously, and inversely any definite line is one along which remains constant while varies continuously. In much the same way, on the earth’s surface a latitude line or parallel is one along which the latitude remains constant and the longitude varies, whereas a longitude line or meridian is one along which the reverse is true. ”

“ As they move north they are continually coming to parts of the globe having smaller circles of latitude than those they have left, and therefore not moved forward as rapidly by the earth's daily rotation as the latitudes nearer the equator. ”

“ As at sea there is, within certain degrees of latitude and longitude, an uphill and a downhill, made by the convexity of the globe, we, perhaps, may have reached the meridian of the great voyage, and may have begun to feel the inclination which will set us forward more swiftly to the end. ”

“ In order that a body will cross the prime vertical, the latitude must be of the same name and greater than the declination. In conditions cited above the declination of the sun is south and the latitude is north, hence the body will never be on the prime vertical. ”

“ The latitude of a spot may always be found by measuring the distance from the zenith of a star whose altitude, i.e. distance from the equator, is known. The words 'At their brightest' are only used to point the antithesis with the 'dark eclipses' used to measure longitude. ”

“ This, therefore, at any instant defines the position where the sun is vertical; its latitude must equal the sun’s declination and its longitude in time equal the time since noon at Greenwich. Now at distance of 60 m. in every direction on the surface of the earth from the point thus defined the sun will have an altitude of 89° and in all directions at a distance of 1200 m. ”

“ Since (if not certain) it is at least probable, that the latitude itself, or rather the effect of the latitude, can be found (even in very dark weather) much more accurately from that declination alone, than can either the longitude or the effect of the longitude from the variation, though the sun itself is shining brightly or all the stars are visible, with the most skilful employment likewise of all the most exact instruments. ”

“ It cannot be doubted that the change of situation with respect to longitude has a much greater influence upon the variation of the needle, than change of latitude. The variation increased sensibly in proportion as we advanced farther eastward. ”

“ From these two data the length of an arc of a meridian corresponding to a difference of latitude of 1° at once follows. The latitude of a place is the angle which the vertical at the place makes with the equator, or, expressed in a slightly different form, is the angular distance of the zenith from the celestial equator. The vertical at any place may be defined as a direction perpendicular to the surface of still water at the place in question, and may be regarded as perpendicular to the true surface of the earth, accidental irregularities in its form such as hills and valleys being ignored. ”

“ The latitudes and longitudes of all, or most of the principal head-lands and bays, may be confided in, for we seldom failed of getting an observation once at least every day, by which to correct the latitude of our reckoning, and observations for settling the longitude were equally numerous, no opportunity that was offered by the sun and moon being suffered to escape. ”

“ Respecting the latter, he says, in an inscription on the chart: “On account of which considerations, we have increased gradually the length of the degrees of latitude toward each pole proportionate to the increase of the parallels beyond the length which they have on the globe, relatively to the equator:—“Quibus consideratis, gradus latitudinum versus utrumque polum paulatim auximus pro incremento parallelorum supra rationem quam habint ad acquinoctialem.” ”

“ Another very important point to bear in mind when observing a body with the view of computing its hour angle, is its azimuth. When the bearing is nearly east or west, on the prime vertical, the body is rising or falling faster than at any other time, and an error in altitude or latitude will produce the least error in the resulting longitude. ”

“ The navigator has merely to measure the exact elevation of the sun as it crosses the meridian,—that is to say when it is at its highest point,—and, having made certain corrections for so-called dip and refraction, to which we shall refer more at length in a moment, a very simple calculation reveals the latitude—that is to say, the distance from the terrestrial equator. ”

“ But here the degrees of latitude towards the middle will fall short of their just quantity, and at the extremities exceed it: which hurts the eye. Artists may use either rule: or, in most cases, they need only make Cx to CM as the arc ML is to its tangent, and finish the map; either by a projection, or, as in the first method, by dividing that part of xt which is intercepted by the secants thro’ L and l, into equal degrees of latitude.
Mr. Mountaine justly observes, “that my rule does not admit of a zone containing N. and S. latitudes.” ”

“ The observations thus far made showed that the changes in latitude were periodic in character, that is, the latitude of any place would increase for a certain length of time and then decrease to a minimum value and so on, continuing to oscillate between certain limits. It is easily seen that such changes in the latitude of any place may be ”

“ This, with other reasons equally plausible, led him to suppose that the earth was flattened near the poles, in the form of an oblate spheroid, and that a degree of latitude would, of consequence, be greater near the pole than at the equator. ”

“ The vertical circle passing through the east and west points is called the prime vertical, and has a distinction above other vertical circles by virtue of its being the most favorable position for a body in observations for longitude. The heavens are further swept by an infinite number of parallels of altitude which are, as their name implies, parallel to the horizon. ”

“ In calculating a meridian altitude, an error in the dip directly affects the result by a corresponding amount, so extra care should be exercised in this respect. In this instance, we endeavor to locate the body relative to our zenith and anything that affects its altitude directly affects the latitude. In a time sight, a different principle is involved. Here the position of the body as defined by the latitude locates the apex of one angle of the astronomical triangle and hence a small error in the altitude will very likely cause a greater effect on the longitude. ”

“ Now, were the earth a perfect sphere, the meridian of the earth would be a perfect circle, and the distance between any two places, differing one degree in latitude, would be exactly equal to the distance between any other two places, differing in latitude to the same amount. ”

“ And since the moon, whilst it lengthens its distance from the ecliptic, has more degrees towards the west than towards the east, it must necessarily have more latitude on one side (of the globe) than on the other: and when the latitude is known, by measuring the degrees and minutes with the astrolabe, it will be known whether it is found, and how far it is found towards the east or the west. But in order to ascertain the longitude, you must know in what latitude the moon ought to be at that same moment in the place from which you sailed, for instance, in Seville. ”

“ A space of nearly forty degrees of latitude remains as little known to us, as if it were a part of the surface of Saturn, or an orb revolving round a star of the eighth magnitude. We know nothing of the inhabitants of those regions, or what kind of animate beings exist in them.
It was a prevailing opinion among the ancients, the correctness of which they for ages never called in question, that the temperate zones of our globe were alone habitable.—The torrid zone they imagined was composed of nothing but sandy deserts, scorched up by the vertical and insupportable beams of a burning sun. ”

“ In reality, in consequence of the flattening of the earth's globe, the degrees of latitude are not equal, being larger the farther we go from the equator. Moreover, strictly speaking, it is incorrect to speak of distances being divided into degrees, or to say that a degree of latitude or longitude contains so many miles ”

“ On this projection there is a constant distance between the meridians, whereas on the earth they actually converge toward the poles. The distance between the parallels increases in passing toward the poles, approximately in the proportion of the secant of the latitude. For each small portion of the map the relative proportions are maintained as on the earth. ”

“ If you are living not far from latitude 40° north, which is the median latitude of the United States, you must, after determining as closely as you can the situation of the north point, look upward along the meridian in the north until your eyes are directed to a point about 40° above the horizon. ”