LATITUDE

, in Geography, or Navigation, the distance of a place from the equator; or an arch of the meridian, intercepted between its zenith and the equator. Hence the Latitude is either north or south, according as the place is on the north or south side of the equator: thus London is said to be in 51° 31′ of north latitude.

Circles parallel to the equator are called parallels of latitude, because they shew the latitudes of places by their intersections with the meridian.

The Latitude of a place is equal to the elevation of the pole above the horizon of the place: and hence these two terms are used indifferently for each other.

This will be evident from the figure, where the circle ZHQP is the meridian, Z the zenith of the place, HO the horizon, EQ the equator, and P the pole; then is ZE the latitude, and PO the elevation of the pole above the horizon. And because PE is = ZO, being each a quadrant, if the common part PZ be taken from both, there will remain the latitude ZE = PO the elevation of the pole.—Hence we have a method of measuring the circumference of the earth, or of determining the quantity of a degree on its surface; for by measuring directly northward or southward, till the pole be one degree higher or lower, we shall have the number of miles in a degree of a great circle on the surface of the earth; and consequently multiplying that by 360, will give the number of miles round the whole circumference of the earth.

The knowledge of the Latitude of the place, is of the utmost consequence, in geography, navigation, and astronomy; it may be proper therefore to lay down fome of the best ways of determining it, both by sea and land.

1st. One method is, to find the Latitude of the pole, to which it is equal, by means of the pole star, or any other circumpolar star, thus: Either draw a true meridian line, or find the times when the star is on the meridian, both above and below the pole; then at these times, with a quadrant, or other fit instrument, take the altitudes of the star; or take the same when the star comes upon your meridian line; which will be the greatest and least altitude of the star: then shall half the sum of the two be the elevation of the pole, or the latitude sought.—For, if abc be the path of the star about the pole P, Z the zenith, and HO the horizon: then is aO the altitude of the star upon the meridian when above the pole, and cO the same when below the pole; hence, because aP = cP, therefore , hence the height of the pole OP, or latitude of Z, is equal to half the sum of aO and cO.

2d. A second method is by means of the declination of the sun, or a star, and one meridian altitude of the same, thus: Having, with a quadrant, or other instrument, observed the zenith distance Zd of the luminary; or else its altitude Hd, and taken its complement Zd; then to this zenith distance, add the declination dE when the luminary and place are on the same side of the equator, or subtract it when on different sides, and the sum or difference will be the latitude EZ sought. But note, that all altitudes observed, must be corrected for refraction and the dip of the horizon, and for the semidiameter of the sun, when that is the luminary observed.

Many other methods of observing and computing the Latitude may be seen in Robertson's Navigation; see book 5 and book 9. See also the Nautical Almanac for 1771.

Mr. Richard Graham contrived an ingenious instrument for taking the latitude of a place at any time of the day. See Philos. Trans, N°. 435, or Abr. vol. 8. pa. 371.|

Latitude

, in Astronomy, as of a star or planet, is its distance from the ecliptic, being an arch of a circle of latitude, reckoned from the ecliptic towards its poles, either north or south. Hence, the astronomical latitude is quite different from the geographical, the former measuring from the ecliptic, and the latter from the equator, so that this latter answers to the declination in astronomy, which measures from the equinoctial.

The sun has no latitude, being always in the ecliptic; but all the stars have their several latitudes, and the planets are continually changing their latitudes, sometimes north, and sometimes south, crossing the ecliptic from the one side to the other; the points in which they cross the ecliptic being called the nodes of the planet, and in these points it is that they can pass over the face of the sun, or behind his body, viz, when they come both to this point of the ecliptic at the same time.

Circle of Latitude, is a great circle passing through the poles of the ecliptic, and consequently perpendicular to it, like as the meridians are perpendicular to the equator, and pass through its poles.

Latitude

, of the Moon, North ascending, is when she proceeds from the ascending node towards her northern limit, or greatest elongation.

Latitude

, North descending, is when the moon returns from her northern limit towards the descending node.

Latitude

, South descending, is when she proceeds from the descending node towards her southern limit.

Latitude

, South ascending, is when she returns from her southern limit towards her ascending node.

And the same is to be understood of the other planets.

Heliocentric Latitude, of a planet, is its latitude, or distance from the ecliptic, such as it would appear from the sun.—This, when the planet comes to the same point of its orbit, is always the same, or unchangeable.

Geocentric Latitude, of a planet, is its latitude as seen from the earth.—This, though the planet be in the same point of its orbit, is not always the same, but alters according to the position of the earth, in respect to the planet.

The latitude of a star is altered only by the aberration of light, and the secular variation of latitude.

Difference of Latitude, is an are of the meridian, or the nearest distance between the parallels of latitude of two places. When the two latitudes are of the same name, either both north or both south, subtract the less latitude from the greater, to give the difference of latitude; but when they are of different names, add them together for the difference of latitude.

Middle Latitude, is the middle point between two latitudes or places; and is found by taking half the sum of the two.

Parallax of Latitude. See Parallax.

Refraction of Latitude. See Refraction.

previous entry · index · next entry

ABCDEFGHKLMNOPQRSTWXYZABCEGLMN

Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

This text has been generated using commercial OCR software, and there are still many problems; it is slowly getting better over time. Please don't reuse the content (e.g. do not post to wikipedia) without asking liam at holoweb dot net first (mention the colour of your socks in the mail), because I am still working on fixing errors. Thanks!

previous entry · index · next entry

LANDEN (John)
LARBOARD
LARMIER
LATERAL Equation
LATION
* LATITUDE
LATUS Rectum
LEAGUE
LEAVER
LEE
LEGS