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grees of latitude should be reckoned from this great circle. But from east to west there is no natural division of the earth, each meridian line being a great circle, dividing the earth into two hemispheres, and hence there is no natural reason why longitude should be reckoned from one meridian any more than another. It has, therefore, been customary for writers and mariners to reckon longitude from the capital of their own country, as the English from London, the French from Paris, and the Americans from Washington. But this mode, it is apparent, must occasion much confusion, since each writer of a different nation would be obliged to correct the longitude of all other countries, to make it agree with his own. More recently, therefore, the writers of Europe and America have selected the royal observatory, at Greenwich, near London, as the first meridian, and on most maps and charts lately published, longitude is reckoned from that place.

The latitude of any place is determined by taking the altitude of the sun at mid-day, and then subtracting this from 90 degrees, making proper allowance for the sun's place in the heavens. The reason of this will be obvious, when it is considered that the whole number of degrees from the Zenith to the horizon is 90, and therefore, if we ascertain the sun's distance from the horizon, that is, his altitude, by allowing for the sun's declination north or south of the equator, and subtracting this from the whole number, the latitude of the place will be found. Thus suppose that on the 20th of March, when the sun is at the equator, his altitude from any place north of the equator should be found to be 48 degrees above the hori zon; this, subtracted from 90, the whole number of the degrees of latitude, leaves 42, which will be the latitude of the place where the observation was made.

If the sun, at the time of observation, has a declination, north, or south of the equator, this declination must be added to, or subtracted from the meridian altitude, as the case may be. For instance, another observation being taken at the place where the latitude was found to be 42, when the sun had a declination of 8 degrees north, then his altitude would be 8 degrees greater than before, and therefore 56, instead of

Why are the degrees of latitude reckoned from the equator? What is said concerning the places from which the degrees of longitude have been reckoned? What is the inconvenience of estimating longitude from a place in each country? From what place is longitude reckoned in Europe and America?

48 Now subtracting this 8, the sun's declination, from 56, and the remainder from 90, and the latitude of the place will be found 42, as before. If the sun s declination be south of the equator, and the latitude of the place, north, his declination must be added to the meridian altitude, instead of being subtracted from it. The same result may be obtained by taking the meridian altitude of any of the fixed stars, whose declinations are known, instead of the sun's, and proceeding as above directed.

There is more difficulty in ascertaining the degrees of longitude, than those of latitude, because as above stated, there is no fixed point, like that of the equator, from which its degrees are reckoned. The degrees of longitude are therefore, estimated from Greenwich, and are ascertained by the following methods.

When the sun comes to the meridian of any place, it is noon, or 12 o'clock, at that place, and therefore, since the equator is divided into 360 equal parts, or degrees, and since the earth turns on its axis once in 24 hours, 15 degrees of the equator will correspond with one hour of time, for 24 hours being divided by 360 degrees, will give 15. The earth, therefore, moves in her daily revolution, at the rate of 15 degrees for every hour of time. Now as the apparent course of the sun is from east to west, it is obvious that he will come to any meridian lying east of a given place, sooner than to one lying west of that place, and therefore it will be 12 o'clock to the east of any place. sooner than at that place, or to the west of it. When, therefore, it is noon at any one place, it will be 1 o'clock at all places 15 degrees to the east of it, because the sun was at the meridian of such places an hour before; and so on the contrary, it will be 11 o'clock, 15 degrees west of the same place, because the sun has still an hour to travel, before he reaches the meridian of that place. It makes no difference, then, where the observer is placed, since if it is 12 o'clock where he is, it will be 1 o'clock 15 degrees to the east of him, and 11 o'clock 15 degrees to the west of him, and so

How is the latitude of a place determined? Give an example of the method of finding the latitude of the same place at different seasons of the year. When must the sun's declination from the equator be added to, and when subtracted from, his meridian altitude? Why is there more difficulty in ascertaining the degrees of longitude than of latitude? How many degrees of longitude does the surface of the earth pass through in an hour? Suppose it is noon at any given place, what o'clock will it be 15 degrees to the east of that place? Explain the reason. How may longitude be determined by an eclipse?

in this proportion, let the time be more or less. Now if any celestial phenomenon should happen, such as an eclipse of the moon, or of Jupiter's satellites, the difference of longitude between two places where it is observed, may be determined by the difference of the times at which it appeared to take place. Thus if the moon enters the earth's shadow at 6 o'clock, in the evening, as seen at Philadelphia, and at half past 6 o'clock at another place, then this place is half an hour, or 7 degrees to the east of Philadelphia, because 71⁄2 degrees of longitude are equal to half an hour of time. To apply these observations practically, it is only necessary that it should be known exactly at what time the eclipse takes place at a given point on the earth.

Longitude is also ascertained by means of a chronometer, or true time piece, adjusted to any given meridian; for if the difference between two clocks, situated east and west of each other, and going exactly at the same rate, can be known, at the same time, then the distance between the two meridians where the clocks are placed will be known, and the difference of longitude may be found.

Suppose two chronometers, which are known to go at exactly the same rate, are made to indicate 12 o'clock by the meridian line of Greenwich, and the one be taken to sea, while the other remains at Greenwich. Then suppose the captain, who takes his chronometer to sea, has occasion to know his longitude. In the first place, he ascertains, by an observation of the sun, when it is 12 o'clock at the place where he is, and then by his time piece, when it is 12 o'clock at Greenwich, and by allowing 15 degrees for every hour of the difference in time, he will know his precise longitude in any part of the world. For example, suppose the captain sails with his chronometer for America, and after being several weeks at sea, finds by observation, that it is 12 o'clock by the sun, and at the same time finds by his chronometer, that it is 4 o'clock at Greenwich. Then because it is noon at his place of observation after it is noon at Greenwich, he knows that his longitude is west from Greenwich, and by allowing 15 degrees for every hour of the difference, his longitude is ascertained.

Explain the principles on which longitude is determined by the chronometer. Suppose the captain finds by his chronometer that it is 12 o'clock where he is, 6 hours later than at Greenwich, what then would be his longitude? Suppose he finds it to be 12 o'clock 4 hours earlier where he is, than at Greenwich, what then would be his longitude?

*Thus, 15 degrees, multiplied by 4 hours, give 60 degrees of west longitude from Greenwich. If it is noon at the place of observation, before it is noon at Greenwich, then the captain knows that his longitude is east, and his true place is found in the same manner.

Fixed Stars.

The stars are called fixed, because they have been observed not to change their places with respect to each other. They may be distinguished by the naked eye from the planets of our system by their scintillations, or twinkling. The stars are divided into classes, according to their magnitudes, and are called stars of the first, second, and so on to the sixth magnitude. About 2000 stars may be seen with the naked eye in the whole vault of the heavens, though only about 1000 are above the horizon at the same time. Of these, about 17 are of the 1st magnitude, 50 of the 2d magnitude, and 150 of the 3d magnitude. The others are of the 4th, 5th, and 6th magnitudes, the last of which are the smallest that can be distinguished with the naked eye.

It might seem incredible, that on a clear night only about 1000 stars are visible, when on a single glance at the different parts of the firmament, their numbers appear innumerable. But this deception arises from the confused and hasty manner in which they are viewed, for if we look steadily on a particu lar portion of sky, and count the stars contained within certain limits, we shall be surprised to find their number so few.

As we have incomparably more light from the moon, than from all the stars together, it is absurd to suppose that they were made for no other purpose than to cast so faint a glimmering on our earth, and especially as a great proportion of them are invisible to our naked eyes. The nearest fixed stars to our system, from the most acurate astronomical calculations, cannot be nearer than 20,000,000,000,000, or 20 trillions of miles from the earth, a distance so immense, that light cannot pass through it in less than three years. Hence were these stars annihilated at the present time, their light would

Why are the stars called fixed? How may the stars be distinguished from the planets? The stars are divided into classes, according to their magnitudes; how many classes are there? How many stars may be seen with the naked eye, in the whole firmament? Why does there appear to be more stars than there really are? What is the computed

distance of the nearest fixed stars from the earth?

to us.

continue to flow towards us, and they would appear to be in the same situations to us, three years hence that they do now. Our sun, seen from the distance of the nearest fixed stars, would appear no larger than a star of the first magnitude does These stars appear no larger to us, when the earth is in that part of her orbit nearest to them, than they do, when she is in the opposite part of her orbit; and as our distance from the sun is 95,000,000 of miles, we must be twice this distance, or the whole diameter of the earth's orbit, nearer a given fixed star at one period of the year, than at another. The difference, therefore, of 190,000,000 of miles, bears so small a proportion to the whole distance between us and the fixed stars, as to make no appreciable difference in their sizes, even when assisted by the most powerful telescopes.

The amazing distances of the fixed stars may also be infer. red from the return of comets to our system, after an absence of several hundred years.

The velocity with which some of these bodies move, whẹn nearest the sun, has been computed at nearly a million of miles in an hour, and although their velocities must be per petually retarded, as they recede from the sun, still in 250 years of time, they must move through a space, which to us would be infinite. The periodical return of one comet is known to be upwards of 500 years, making more than 250 years in performing its journey to the most remote part of its orbit, and as many in returning back to our system; and that it must still always be nearer our system than the fixed stars, is proved by its return; for by the laws of gravi tation, did it approach nearer another system, it would never again return to ours.

From such proofs of the vast distances of the fixed stars, there can be no doubt that they shine with their own light, like our sun, and hence the conclusion that they are suns to other worlds, which move around them, as the planets do around our sun. Their distances will, however, prevent our ever knowing, except by conjecture, whether this is the case or not, since, were they millions of times nearer us than they

How long would it take light to reach us from the fixed stars? How large would our sun appear at the distance of the fixed stars? What is said concerning the difference of the distance between the earth and the fixed stars at different seasons of the year, and of their different appearances in consequence? How may the distances of the fixed stars be inferred by the long absence and return of comets? On what grounds is it supposed that the fixed stars are suns to other worlds?