visible only to that small portion of the hemisphere where the moon's shadow falls.

An occultation is when a star is hid from our sight by the moon or a planet.

CHAP. VII.

THE CELESTIAL GLOBE.

DEFINITIONS.*

THE artificial celestial globe is a representation of the heavens, on which are placed the fixed stars in their natural position and magnitude.

When the globe is turned round upon its axis, it represents the apparent diurnal motion of the sun, moon, and stars. The pupil, in using this globe, is to imagine it transparent and himself in the centre of it, whence he can view the stars in their natural position, as in one vast concave.

1. Latitude of a star or planet, is its distance from

*As the definitions of most of the lines, &c. on the terrestrial globe accord with those of the celestial, in order to avoid a repetition, we shall here only take notice of those more immediately connected with celestial objects.

the ecliptic, north or south, measured on a great circle towards the pole of the ecliptic. The sun has no latitude, being always in the ecliptic.

2. Longitude of a star or planet, is its distance from Aries, reckoned eastward quite round the globe; hence extends to 360 degrees on the ecliptic. The longitude of the sun on the celestial globe is the same as what is called the sun's place on the terrestrial globe.

3. Right ascension of the sun or stars, is that part of the equinoctial which rises or sets with the sun or star when the globe is in the position of a right globe, or sphere, reckoned from Aries eastward round the globe, being 360°.

4. Oblique ascension of the sun or stars, is that part of the equinoctial which comes to the meridian with the sun or star, in an oblique globe or sphere, and is reckoned on the equinoctial, from Aries, quite round the globe eastward, being likewise to 360°.

5. Ascensional difference of the celestial objects, is the difference between their right ascension and oblique ascension, and, likewise, it is the time that the sun rises or sets before or after six.

6. Equinoctial points,* are where the ecliptic crosses the equator, viz. at Aries and Libra.

*Precessions of the equinoxes is a gradual motion which the equinoctial points have from east to west, contrary to the

7. The solsticial points, are where the ecliptic touches the tropics, viz. at Cancer and Capricorn.

8. The poles of the ecliptic, are where the celestial meridians meet, and are the same to the ecliptic as the poles of the earth are to the equator. The pole of the ecliptic, on the celestial globe, is 2310 from the pole of the equator.

9. Declination of the sun or stars, is their distance from the equator north or south. The sun's greatest declination is 2310, but the greatest declination of the stars extends to the poles, being 90°.

order of the signs, which is from west to east. This motion, from observations, is found to be about 50 seconds in a year, so that in the space of 25,791 years the celestial objects make an entire revolution westward round the globe, found by dividing the seconds in 360° by 504".

In the time of Hipparchus, the equinoctial points were fixed in Aries and Libra, but the signs that were then in conjunction with the sun, when he was in the equinox, are now 30° eastward of it, so that Aries is now in Taurus, Taurus in Gemini, &c. Hence the stars that rose and set at any particular season of the year, in the times of Hesiod, Pliny, &c. do not answer to the description given of them by these writers.

CHAP. VIII.

Contains twenty-two Problems on the Celestial Globe; including the Phenomena of the Harvest Moon ; Armillary Sphere, with Descriptions, &c.

The day of the month being given, to find the sun's right ascension, or the right ascension of a star.

BRING the sun's place for the given day, or the star to the meridian, then the degree cut by the meridian on the equator, reckoning from Aries eastward, is the ascension required.

Given the 20th of March; required the sun's right ascension for that day? Answer, O.

What is the sun's right ascension on the 23d of July? Answer, 122° 10'.

Required the sun's right ascension on the 26th of September? Answer, 185° 30'.

* Problems i, xiv. xv. xx. xxvii. xxviii. and xxxii. on the terrestrial globe, being common to both globes, it is not necessary to repeat them among the Problems of the celestial.

PROBLEM II.

A place and its latitude, day and month being given, to find the sun's oblique ascension and descension.

Rectify the globe for the latitude, bring the sun's place to the eastern verge of the horizon; then the degree of the equinoctial, cut by the east part of the horizon, is the sun's oblique ascension; turn the globe till the sun's place come to the western verge of the horizon, and the degree cut by it on the equinoctial, reckoning from Aries in both cases, will be the sun's oblique descension for the given day.

Given London, the 20th of March; required the sun's oblique ascension and descension? Answer, the sun being on the equator, and in the first degree of Aries, has neither ascension nor descension.

What is the sun's oblique ascension and descension at London on the 23d of July? Answer, his oblique ascension is 95°, and oblique descension 150°.

Required the sun's oblique ascension and descension at London, on the 27th of September? Answer, oblique ascension is 187°, and oblique descension 182° 30'.

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