Examples for practice.

On what day is the sun vertical at Lima? at Cape Comorin, in the East Indies? and at the mouth of the river Amazon?

PROBLEM XIX.-To find the sun's meridian altitude at any given place.

Rule.-Elevate the globe for the latitude of the place. Find the sun's place in the ecliptic, and bring it to the brazen meridian: count the number of degrees contained on the meridian, between the horizon and the sun's place, which is the altitude required.

Ex. Thus, on the 21st of May, the sun's meridian altitude at London will be nearly 59°.

Examples for practice.

What is the meridian altitude of the sun at London, October 26?-at St. Petersburg on the longest day?-on Christmas Day, at Bastia in the Island of Corsica ?—at Samarcand on Michaelmas Day?

PROBLEM XX.-To find the altitude of the sun at any given place and hour.

Rule.-Rectify for the latitude, zenith, and sun's place. Turn the globe till the index points to the given hour, and bring the quadrant of altitude to the sun's place at that hour, and the degrees counted on that from the horizon are the altitude sought.

Ex. Thus the altitude of the sun at London, on the 21st of May, at nine in the morning, will be a little more than 43°.

Examples for practice.

What is the altitude of the sun at London on the 24th of January, at eleven o'clock in the forenoon?-at Moscow, at eight in the morning on the 1st of May ?-at Constantinople, at ten in the forenoon on the 24th of June?

PROBLEM XXI.—Any place being given in the north frigid zone, to find the number of days the sun shines constantly without setting at that place, and the number of days he is totally absent.

Rule.-Rectify for the latitude of the place; and bring the ascending part of the ecliptic (that is, the part reckoned from Capricorn, through Aries, Gemini, &c. to Cancer) to the north part of the horizon observe the degree of the ecliptic which cuts that point, and the day in the calendar which answers to that degree shows the time when continual day begins: then bring the descending part of the ecliptic (that is, the part that passes from Cancer, through Leo, &c. to Capricorn) to the said north point of the horizon, and observe the degree as before; and the day on the calendar which answers to it is that in which continual day ends.

Ex. Thus, at latitude 76° north, continual day begins about the 27th of April, and lasts till August 15, that is, during 110 or 111 days, the sun never goes below the horizon. And as the longest night is, in all latitudes, equal to the longest day, the people that live at South Cape, in Spitzbergen, or 76° north latitude, never see the sun for 110 days successively, except by refraction.*

At North Cape, Lapland, or lat. 71°, continual day begins about the middle of May, and lasts to the end of July; and continual night begins about the middle of November and lasts till about the end of January.

* To find when the longest night begins and ends, proceed as before, bringing the ascending and descending points of the ecliptic to the south instead of the north part of the

horizon.

Examples for practice.

What is the length of continual day at the north part of Nova Zembla?

When does continual day begin in Davis's Straits?

When does the sun begin to shine without intermission in latitude 80° north, and what is the length of continual day there?

PROBLEM XXII.-To explain the phenomenon of the harvest-moon.

The harvest-moon is the full moon which happens at or near the time of the autumnal equinox; when a few nights before and after the full, the moon rises nearly at the same time, on account of the horizon being nearly parallel to that part of her orbit at which she then is.*

Rule. Rectify the globe for the latitude; find the moon's place in White's or any other Ephemeris, for four or five days before and after the full moon, and put a patch on each of these places. Bring the sun's place for each day to the brazen meridian, and set the index to twelve at noon: turn the globe westward till the moon's place, corresponding to that day, comes above the horizon, and the index will show the time of rising.

Ex. Thus the difference of the time of the rising of the moon, two or three days before and after full in September, 1803, was about sixteen minutes only.

*When there is the smallest difference between the times of the moon's rising, there will be the greatest difference between the times of her setting; and the contrary.

U

OF THE CELESTIAL GLOBE.

The celestial globe is an artificial representation of the heavens, having the fixed stars drawn upon it in their natural order and situation. The eye is supposed to be placed in the centre.

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 motion of the heavens.

The zodiac is an imaginary belt round the heavens, of about sixteen degrees broad, through the middle of which runs the ecliptic, or the apparent path of the sun.

The twelve signs of the zodiac, which belong to the celestial globe, have been already enumerated on page 205.

The first points of Aries and Libra are called the equinoctial points, because when the sun appears to be in either of them the day and night are equal.

The first points of Cancer and Capricorn are called solstitial points, because when the sun is near either of them he seems to stand still, or to be at the same height in the heavens at twelve o'clock at noon for several days together.

Definition I.—The latitude of the heavenly bodies is measured from the ecliptic north and south. The sun, being always in the ecliptic, has no latitude.

Definition II.-The longitude of the heavenly bodies is reckoned on the ecliptic, from the first point of Aries, eastward round the globe. The longitude of the sun is what is called, on the terrestrial globe, the sun's place.

PROBLEM I.-To find the latitude and longitude of any star.*

Rule.-Put the centre of the quadrant of altitude on the pole of the ecliptic, and its graduated edge on the star; then the arch of the quadrant, intercepted between the star and the ecliptic shows its latitude: and the degree which the edge of the quadrant cuts on the ecliptic, is the degree of its longitude.

Ex. Thus, the latitude of Regulus is 0° 28' N., and its longitude nearly 147°. The latitude of Arcturus is 31° N. nearly; its longitude is about 201°.

Examples for practice.

What are the latitudes and longitudes of Cor Caroli ?-of Aldebaran?-of ẞ in Perseus ?-of a in Canis Minor? of Sirius?-of Capella ?-and of the bright star in the Northern Crown?

PROBLEM II.-To find any place in the heavens by having its latitude and longitude given.

Rule.-Fix the quadrant of altitude, as in the last problem, letting it cut the longitude given on the ecliptic; then seek the latitude on the quadrant, and the place under it is the place sought.

Ex. Thus, if I am asked what part of the heavens that is, whose longitude is 60° 30', and latitude 5° 30' south, I find it is the place which Aldebaran occupies.

Examples for practice.

What star is that whose longitude is 85°, and whose latitude is 16° south?

The latitude and longitude of the planets and moon are given in White's Ephemeris, the Nautical Almanac, &c.