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degrees to the quadrant of altitude above the horizon on the west, so will the index point the hour twilight begins.*
This short specimen of problems by the globes, as commonly mounted, it is presumed, will be sufficient to enable the pupil to solve any other.
* The quadrant of altitude is nov generally divided to 18° below o, so that observing the sun's place 18° below the horizon, will more readily give the duration of twilight. Epit.
AS MOUNTED IN THE COMMON MANNER.
THE celestial globe is an artificial representation of the heavens, having the fixed stars drawn upon it, jn their natural order and situation ; whilst its rotation on its axis represents the apparent diurnal motion of the sun, moon, and stars.
It is not known how early the ancients had any tiling of this kind: we are not certain what the sphere of Atlas or Musceus was; perhaps Palamcdes, who lived about the time of the Trojan war, had. something of this kind ; for of him it ,is said,
To mark the signs that cloudless sties bestow,
From Pliny it would seem that Hipparc Jtus had a celestial globe with the stars delineated upon it.
It is not to be supposed, that the celestial globe is so just a representation of the heavens as the terrestrial globe is of the earth; because here the stars are drawn upon a convex surface, whereas they naturally appear in a concave one. But suppose the, globe was made of glass, then, to an eye placed in the centre, the stars which are drawn upon it would appear in a concave surface, just as they do in the heavens.
Or if the reader was to suppose that holes were made in each star, and an eye placed in the centre of the globe, it would view, through those holes, the same stars in the heavens that they represent.
As the terrestrial globe, by turning on its axis, represents the real diurnal motion of the earth, so the celestial globe, by turning on its axis, represents the apparent diurnal motion of the heavens.
For the sake of perspicuity, and to avoid continual references, it will be necessary to repeat here some articles which have been already mentioned.
The ecliptic is that graduated circle which crosses the equator in an angle of about 23-i- deg. and the angle is called the obliquity of the ecliptic:7 t •
This circle is divided into twelve equal parts, consisting of 30 degrees each; the beginning of them are marked with characters, representing the twelre signs.
Aries "f, Taurus ft , Gemini n, Cancer 95, Leo ft, Virgo 1T£, Libra £h, Scorpio TT[, Sagittarius f, Capricornus V?, Aquarius zz, Pisces ^.
Upon my father's globes, just under the ecliptic, the month, and days of each month, are graduated, for the readier fixing the artificial sun upon it» place in the ecliptic,
The two points where the ecliptic crosses th$ ^quinoptial (the circle that answers to the equator on the terrestrial globe) are called the equinoctial points; they are at the beginning of Aries and Libra, and are so called, because when the sun is ia either of them, the day and night is every where equal.
The first points of Cancer and Capricorn are called solstitial points; because, when the sun arrives at either of them, he seems to stand, in a manner still for several days, in respect to his distance from the equinoctial; when he is in one solstitial point, ho makes to us, the longest day; when in the other, the longest night.
The latitude and longitude of stars are determined from the ecliptic.
The longitude of the stars and planets is reckoned upon the ecliptic; the numbers beginning at th§ first points of Aries, where the ecliptic crosses the equator, and increases according to the order of the signs.
-Thus, suppose the sun to be in the tenth degree of Leo, we say, his longitude, or place, is four signs, ten degrees; because he has already passed the four signs, Aries, Taurus, Gemini, Cancer, and is ten degrees in the fifth.
• The latitude of the stars and planets is determined by their distance from the ecliptic upon a secondary fcr great circle passing through its poles, and crossing it at right angles
Twenty-four of these circular lines, which cross the ecliptic at right angles, being fifteen degrees from each other, are drawn upon the surface of our celestial globe; which being produced both ways^ those on one side meet in a point on the northern polar circle, and those on the other meet in a point on the southern polar circle.
The points determined by the meeting of these circles are called the poles of the ecliptic, one north, the other south.
From these definitions it follows, that longitude
and latitude, on the celestial globe, bear just the same relation to the ecliptic, as they do on the terrestrial globe to the equator.
Thus, as the longitude of places on the earth is measured by degrees upon the equator, counting from the first meridian; so the longitude of the heavenly bodies is measured by degrees upon the ecliptic, counting from the first point of Aries.
And as latitude on the earth is measured by degrees upon the meridian, counting from the equator; so the latitude of the heavenly bodies is measured by degrees upon a circle of longitude, counting either north or south from the ecliptic.
The sun, therefore, has no latitude, being always in the ecliptic ; nor do we usually speak of hislongi^ tude, but rather of his place in the ecliptic, expressing it by such a deg. and min. of such a sign, as !> deg. of Taurus, instead «f 35 deg. of longitude.