MERIDIAN

, in Astronomy, is a great circle of the celestial sphere, passing through the poles of the world, and both the zenith and nadir, crossing the equinoctial at right angles, and dividing the sphere into two equal parts, or hemispheres, the one eastern, and the other western. Or, the Meridian is a vertical circle passing through the poles of the world.

It is called Meridian, from the Latin meridies, midday or noon, because when the sun comes to the south part of this circle, it is noon to all those places situated under it.

, in Geography, is a great circle passing through the poles of the earth, and any given place whose Meridian it is; and it lies exactly under, or in the plane of, the celestial Meridian.

These Meridians are various, and change according to the longitude of places; so that their number may be said to be infinite, for that all places from east to west have their several Meridians. Farther, as the Meridian invests the whole earth, there are many places situated under the same Meridian. Also, as it is noon whenever the centre of the sun is in the celestial Meridian; and as the Meridian of the earth is in the plane of the former; it follows, that it is noon at the same time, in all places situated under the same Meridian.

First Meridian, is that from which the rest are counted, reckoning both east and west; and is the beginning of longitude.

The fixing of the First Meridian is a matter merely arbitrary; and hence different persons, nations, and ages, have fixed it differently: from which circumstance some confusion has arisen in geography. The rule among the Ancients was, to make it pass through the place farthest to the west that was known. But the Moderns knowing that there is no such place on the earth as can be esteemed the most westerly, the way of computing the longitudes of places from one fixed point is much laid aside.

Ptolomy assumed the Meridian that passes through the farthest of the Canary Islands, as his first Meridian; that being the most western place of the world then known. After him, as more countries were discovered in that quarter, the First Meridian was remov farther off. The Arabian geographers chose to the First Meridian upon the utmost shore of the western ocean. Some fixed it to the island of St. Nicholas near the Cape Verd; Hondius to the isle of St. James; others to the island of Del Co<*>vo, one of the Azores; because on that island the magnetic needle at that time pointed directly north, without any variation: and it was not then known that the variation of the needle is itself subject to variation. The latest geographers, particularly the Dutch, have pitched on the Pike of Teneriffe; others on the Isle of Palm, another of the Canaries; and lastly, the French, by order of the king, on the island of Fero, another of the Canaries.

But, without much regard to any of these rules, geographers and map-makers often assume the Meridian of the place where they live, or the capital of their country, or its chief observatory, for a First Meridian; and from thence reckon the longitudes of places, east and west.

Astronomers, in their calculations, usually choose| the Meridian of the place where their observations are made, for their First Meridian; as Ptolomy at Alexandria; Tycho Brahe at Uranibourg; Riccioli at Bologna; Flamsteed at the Royal Observatory at Greenwich; and the French at the Observatory at Paris.

There is a suggestion in the Philos. Trans. that the Meridians vary in time. And it has been said that this is rendered probable, from the old Meridian line in the church of St. Petronio at Bologna, which is said to vary no less than 8 degrees from the true Meridian of the place at this time; and from the Meridian of Tycho at Uranibourg, which M. Picart observes, varies 18 minutes from the modern Meridian. If there be any thing of truth in this hint, Dr. Wallis says, the alteration must arise from a change of the terrestrial poles (here on earth, of the earth's diurnal motion), not of their pointing to this or that of the fixed stars: for if the poles of the diurnal motion remain fixed to the same place on the earth, the Meridians, which pass through these poles, must remain the same.

But the notion of the changes of the Meridian seems overthrown by an observation of M. Chazelles, of the French Academy of Sciences, who, when in Egypt, found that the four sides of a pyramid, built 3000 years ago, still looked very exactly to the four cardinal points. A position which cannot be considered as merely fortuitous.

Meridian of a Globe, or Sphere, is the brazen circle, in which the globe hangs and turns.

It is divided into four 90's, or 360 degrees, beginning at the equinoctial: on it, each way, from the equinoctial, on the celestial globes, is counted the north and south declination of the sun, moon, or stars; and on the terrestrial globe, the latitude of places, north and south. There are two points on this circle called the poles; and a diameter, continued from thence through the centre of either globe, is called the axis of the earth, or heavens, on which it is supposed they turn round.

On the terrestrial globes there are usually drawn 36 Meridians, one through every 10th degree of the equator, or through every 10th degree of longitude.

The uses of this circle are, to set the globes in any particular latitude, to shew the sun's or a star's declination, right ascension, greatest altitude, &c.

Meridian Line, an arch, or part, of the Meridian of the place, terminated each way by the horizon. Or, a Meridian line is the intersection of the plane of the Meridian of the place with the plane of the horizon, often called a north-and-south line, because its direction is from north to south.

The Meridian line is of most essential use in astronomy, geography, dialling, &c; and the greatest pains are taken by astronomers to fix it at their observatories to the utmost precision. M. Cassini has distinguished himself by a Meridian line drawn on the pavement of the church of St. Petronio, at Bologna; being extended to 120 feet in length. In the roof of this church, 1000 inches above the pavement, is a small hole, through which the sun's image, when in the meridian, falling upon the line, marks his progress all the year. When finished, M. Cassini, by a public writing, quaintly informed the mathematicians of Eu- rope, of a new oracle of Apollo, or the sun, established in a temple, which might be consulted, with entire confidence, as to all dissiculties in astronomy. See Gnomon.

To draw a Meridian Line.—There are many ways of doing this; but some of the easiest and simplest are as follow:

1. On an horizontal plane describe several concentric circles AB, ab, &c, and on the common centre C erect a stile, or gnomon, perpendicular to the horizontal plane, of about a foot in length. About the 2 1st of June, between the hours of 9 and 11 in the morning, and between 1 and 3 in the afternoon, observe the points A, a, B, b, &c, in the circles, where the shadow of the stile terminates. Bisect the arches AB, ab, &c, in D, d, &c. If then the same right line DE bisect all these arches, it will be the Meridian line sought.

As it is not easy to determine precisely the extremity of the shadow, it will be best to make the stile flat at top, and to drill a small hole through it, noting the lucid point projected by it on the arches AB and ab, instead of marking the extremity of the shadow itself.

2. Another method is thus: Knowing the south quarter pretty nearly, observe the altitude FE of some star on the east side ofit, and not far from the Meridian HZRN: then, keeping the quadrant firm on its axis, so as the plummet may still cut the same degree, direct it to the western side of the Meridian, and wait till you find the star has the same altitude as before, as fe. Lastly, bisect the angle ECe, formed by the intersection of the two planes in which the quadrant has been placed at the time of the two observations, by the right line HR, which will be the Meridian sought.

Many other methods are given by authors, of describing a Meridian line; as by the pole star, or by equal altitudes of the sun, &c; by Schooten in his Exercitationes Geometriæ; Grey, Derham, &c, in the Philos. Trans. and by Ferguson in his Lectures on Select Subjects.

From what has been said it is evident that whenever the shadow of the stile covers the Meridian line, the centre of the sun is in the Meridian, and therefore it is then noon. And hence the use of a Meridian line in adjusting the motion of clocks to the sun.

If another stile be erected perpendicularly on any other horizontal plane, and a signal be given when the shadow of the former stile covers the Meridian line drawn on another plane, noting the apex or extremity of the shadow projected by the second stile, a line drawn through that point and the foot of the stile will be a Meridian line at the 2d place.

Or, instead of the 2d stile, a plumb line may be hung up, and its shadow noted on a plane, upon a signal given that the shadow of another plummet, or| of a stile, falls exactly in another Meridian line, at a little distance; which shadow will give the other Meridian line parallel to the former.

Meridian Line, on a Dial, is a right line arising from the intersection of the Meridian of the place with the plane of the dial. This is the line of noon, or 12 o'clock, and from hence the division of the hourline begins.

Meridian Line, on Gunter's scale, is divided unequally towards 87 degrees, in such manner as the Meridian in Mercator's chart is divided and numbered.

This line is very useful in navigation. For, 1st, It serves to graduate a sea-chart according to the true projection. 2d, Being joined with a line of chords, it serves for the protraction and resolution of such rectilineal triangles as are concerned in latitude, longitude, course, and distance, in the practice of sailing; as also in pricking the chart truly at sea.

Magnetical Meridian, is a great circle passing through or by the magnetical poles; to which Meridians the magnetical needle conforms itself.

Meridian Altitude, of the sun or stars, is their altitude when in the meridian of the place where they are observed.

Meridional Distance, in Navigation, is the same with the Departure, or easting and westing, or distance between two meridians.

Meridional Parts, Miles, or Minutes, in Navigation, are the parts of the increased or enlarged meridian, in the Mercator's chart. Tables of these parts are in most books of navigation; and they serve both for constructing that sort of charts, and for working that kind of navigation.

Under the article Mercator's Chart, it is shewn that the parts of the enlarged Meridian increase in proportion as the cosine of the latitude to radius, or, which is the same thing, as radius to the secant of the latitude; and therefore it follows, that the whole length of the enlarged nautical Meridian, from the equator to any point, or latitude, will be proportional to the sum of all the secants of the several latitudes up to that point of the Meridian. And on this principle was the sirst Table of Meridional Parts constructed, by the inventor of it, Mr. Edward Wright, and published in 1599; viz, he took the Meridional parts of 1′ = the sec. of 1′; of 2′ = sec. of 1′ + sec. of 2′; of 3′ = secants of 1, 2, and 3 min. of 4′ = secants of 1, 2, 3, and 4 min. and so on by a constant addition of the secants.

The Tables of Meridional Parts, so constructed, are perhaps exact enough for ordinary practice in navigation; but they would be more accurate if the Meridian were divided into more or smaller parts than single minutes; and the smaller the parts, so much the greater the accuracy. But, as a continual subdivision would greatly augment the labour of calculation, other ways of computing such a table have been devised, and treated of, by Bond, Gregory, Oughtred, Sir Jonas Moor, Dr. Wallis, Dr. Halley, and others. See MERCATOR's Chart, and Robertson's Navigation, vol. 2, book 8. The best of these methods was derived from this property, viz, that the Meridian line, in a Mercator's chart, is analogous to a scale of logarithmic tan- gents of half the complements of the latitudes; from which property also a method of computing the cases of Mercator's Sailing has been deduced, by Dr. Halley. Vide ut supra, also the Philos. Trans. vol. 46, pa. 559.

Let the semidiameter of the equator be to the distance of the centre from the focus of the generating ellipse, as m to 1. Let A represent the latitude for which the meridional parts are required, s the sine of the latitude, to the radius 1: Find the arc B, whose sine is s/m; take the logarithmic tangent of half the complement of B, from the common tables; subtract the log. tangent from 10.0000000, or the log. tangent of 45°; multiply the remainder by the number 7915.7044679, and divide the product by m; then the quotient subtracted from the Meridienal parts in the sphere, computed in the usual manner for the latitude A, will give the Meridional parts, expressed in minutes, for the same latitude in the spheroid, when it is the oblate one.

Example. If , then the greatest difference of the Meridional parts in the sphere and spheroid is 76.0929 minutes. In other cases it is found by multiplying the remainder above mentioned by the number 1174.078.

When the spheroid is oblong, the difference in the Meridional parts between the sphere and spheroid, for the same latitude, is then determined by a circular arc. See Philos. Trans. no. 461, sect. 14. Also Maclaurin's Fluxions, art. 895, 899. And Murdoch's Mercator's Sailing &c.