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meridian, pole, equator, declination and sun

LATITUDE, the distance of a place from the equator, or an arc of the meri dian intercepted between the zenith of the place and the equator. Hence lati tude is either northern or southern, ac cording as the place, whose latitude is spoken of, is on this or that side of the equator. Thus London is said to be in fifty-one degrees thirty-two minutes north latitude. Circles parallel to the equator, are called parallels of latitude, because they shew the latitudes of places by their intersection with the meridian. If through the poles of the world we conceive innu merable great circles drawn, these are called secondaries of the equator, and by their help, the position of every point, either on earth or in the heavens, with regard to the equinoctial ; that is, the latitude of any point is determined. One of the secondaries, passing through any place on the earth's surface, is called the meridian of that place, and on it the lati tude of that place is measured. The lati tude of a place, and the elevation of the pole of that place above the horizon, are terms used indifferently for each other, because the latitude and elevation of the pole are always equal. The knowledge of the latitude of a place is of the utmost consequence in navigation ; and the me thods of determining it, both at sea and land, are generally the same. As the altitude of the pole is always equal to the latitude, the latitude is consequently best found by observing the pole's height ; but as the pole is only a mathe matical point, and no ways to be observ ed by our senses, its height cannot be de termined in the same manlier as that of the sun and stars, &c.; for which reason

another manner has been contrived. A meridian line is first drawn, on which is placed a quadrant, so that its plane may be in the plane of the meridian ; then some star near the pole is taken ; for ex ample, the pole star, (which never sets) and observation is made of both its great est and least altitude. The latitude may also be found by having the sun or a star s declination and meridian altitude, taken with a quadrant or astrolabe. The me thod is this : observe the meridian and distance of the sun from the vertex or zenith, which is always the complement of his meridian altitude ; correct for the clip of the horizon, refraction, and add to this the sun's declination, when the sun and the place are on the same side of the equator ; and subtract the declination when they are of different sides ; the sum, in the former case, and the differ ence, in the latter, will be the latitude re quired. But when the declination of the sun is greater than the latitude of the place, which is known from the sun's be ing nearer to the elevated pole than the vertex of the place is, as it frequently happens in the torrid zone, then the dif ference between the sun's declination and his zenith distance, is the latitude of the place. If the sun or star have no declination, but move in the equinoctial that day, then the elevation of the equa tor will be equal to his meridian alti tude, and consequently his meridian alti tude is the complement of the latitude to ninety.