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ont 24 to the heavens, thus dividing the whole concave surface into 24 sectiona each 15° in width. These meridians mark the space which the heavenly bodies appear to describe, every hour, for the 24 hours of the day. They are thence sometimes denominated Hour Circles.

In measuring distances and determining positions on the Earth, the equator, and some fixed meridian, as that of Greenwich, contain the primary starting points; in the heavens, these points are in the ecliptic, the equinoctial, and that great meridian which passes through the first point of Aries, called the equinoc

tial colure.

Latitude on the Earth, is distance north or south of the equator, and is measured on a meridian.

Latitude in the Heavens, is distance north or south of the ecliptic, and at right angles with it.

Longitude on the Earth, is distance either east or west from some fixed meridian, measured on the equator.

Longitude in the Heavens, is distance east from the first point of Aries, measured on the ecliptic.

A

Declination is the distance of a heavenly body either north or south of the equinoctial, measured on a meridian.

Right Ascension is the distance of a heavenly body east from the first point of Aries, measured on the equinoctial.

It is more convenient to describe the situation of the heavenly bodies by their declination and right ascension, than by their latitude and longitude, since the former correspond to terrestrial latitude and longitude.

Latitude and declination may extend 90° and no more. Terrestrial longitude may extend 180° either east or west; but celestial longitude and right ascen sion, being reckoned in only one direction, extend entirely round the circle, or 360°.

In consequence of the Earth's motion eastward in its orbit, the stars seem to have a motion westward, besides their apparent diurnal motion caused by the Earth's revolution on its axis; so that they rise and set sooner every succeeding day by about four minutes, than they did on the preceding. This is called their daily acceleration. It amounts to just two hours a month.

EXAMPLE.-Those stars and constellations which do not rise until 10 o'clock this evening, will, at the same hour, one month hence, be 30° above the horizon; and, for the same reason, those stars which we see directly over head this evening, will at the same hour, three months hence, be seen setting in the west; having in this time, performed one fourth of their apparent annual revolution.

The following table of sidereal revolutions, shows the difference between solar and sidereal time. The first column contains the numbers of complete revolutions of the stars, or of the Earth's rotation on its axis; the second exhibits the

Into how many sections, do these meridians divide the concave surface of the heavens? Of what width are these sections? Why are these meridians sometimes called hour circles? In measuring distances on the Earth, what circles contain the primary starting points? Where are these points in measuring distances in the heavens? What is latitude on the Earth? What is latitude in the heavens? What is longitude on the Earth? What is longitude in the heavens? What is declination? What is right ascension? Why is it more convenient to describe the situation of the heavenly bodies by their de clination and right ascension, than by their latitude and longitude? How many degrees may latitude and declination extend? How many terrestrial longitude? Ho many celestial longitude? What is meant by the daily acceleration of the stars? To now many minutes does it amount? Illustrate this subicct with an example.

times in which these revolutions are made; and the third, shows how much the Stars gain on the Sun every day-that is, how much sooner they rise and come to the meridian every succeeding day, than they did on the preceding

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On this account, we have not always the same constella tions visible to us throughout the year. While some, that were not visible before, are successively rising to view in the east, and ascending to the meridian, others sink beneath the western horizon, and are seen no more, until, having passed through the lower hemisphere, they again reappear in the east.

It is easy to convert right ascension into time, or time into right ascension for if a heavenly body is one hour in passing over 150, it will be one fifteenth of an hour, or 4 minutes, in passing over 10.

If the first point of Aries be on the meridian at 12 o'clock, the next hour line, which is 150 E. of it, will come to the meridian at 1 o'clock; the second houi line at 2 o'clock; the third at 3, &c. Of any two bodies whose right ascensions are given, that one will pass the meridian first which has the least right ascension. The first map of the atlas represents, upon a large scale general view of the solar system.

This will be more fully described in the Second Part of the work.

Do we always see the same constellations? Explain the manner of converting righ scension into time, and time into right ascension.

The next six maps represent different sections of the concave surface of the heavens. The first of these exhibits the principal constellations visible to us in October, November and December; the second, those visible in January, February and March; the third, those visible in April, May and June; and the fourth, those visible in July, August and September; with the exception, however, of the constellations which lie beyond the 50th degree of north and south declination, of which, indeed, those around the North Pole are always, and those around the South Pole, never, visible to us.

These constellations are represented on the sixth and seventh maps, called circumpolar maps, which are an exact continuation of the cthers, and if joined to them at their corresponding degrees of right ascension and declination, they might be considered as constituting one map. The scale on which all the above-mentioned maps are drawn is that of a 16 inch globe. The lines drawn on the maps have been already defined; and their use, being nearly the same with those in Geography, will be readily understood. Those which are drawn from right to left, on each side of the equinoctial and parallel to it, are called Parallels of Declination. Those which are drawn up and down through the maps, at intervals of 150, are called Meridians of Right Ascension, or Hour Circles. The scale at the top and bottom of the first four maps, and in the circumference of the circumpolar maps, indicates the daily progress of the stars in right ascension, and shows on what day of the month any star will be on the me ridian at 9 o'clock in the evening.

The constellation called the Great Bear is an exception to this rule; in this constellation the principal stars are marked in the order of their right ascension. That point of projection for the maps which would exhibit each successive portion of the heavens directly over head at 9 o'clock in the evening, was chosen, because in summer at an earlier hour the twilight would bedim our observation of the stars, and at other seasons of the year it is easier to look up to stars that want an hour of their meridian altitude than to those which are directly over head.

It will be readily seen that the stars are so represented on the maps as to show their relative magnitudes. The method invented by Bayer, of designating them by the letters of the Greek and Roman alphabets, is adopted. Thus in each constellation the stars are marked alpha, beta, &c., and should the letters of the Greek alphabet be exhausted, those of the Roman are employed. Some of the stars have also proper names.

The first four maps of the heavens are so constructed that the

For what months does the first map represent the heavens? For what months does the second map represent the heavens? "The third? The fourth? What constellations are represented on the sixth and seventh maps? In what manner must these six maps De arranged to form one complete map of the heavens? On what scale are these maps drawn? What is the use of the scale at the top and bottom of the first four maps, and in the circumference of the circumpolar maps? Why was that point of projection for the maps, which would represent each successive portion of the heavens directly over head at 9 o'clock in the evening, chosen? What is the method by which the stars are designated on the maps? How must the pupil, in using either of the first four maps Imagine himself to stand and to hold its

pupil in using them must suppose himself to face the south, and o hold them directly over head in such manner that the top of the map shall be towards the north, and the bottom towards the south; the right hand side of the map will then be west, and the left hand east. In using the circumpolar maps he must suppose himself to face the pole, and to hold them in such a manner that the day of the given month shall be uppermost. The Celestial Planisphere represents the whole heavens lying between 70 degrees of north and south declination, not as the surface of a concave sphere, but of a concave cylinder, and spread out so as to form a plain surface. A great variety of interesting problems, including almost all those that are peculiar to the celestial globe, may be solved upon it with facility and readiness.

We may now imagine the pupil ready to begin the study of the visible Heavens. The first thing of importance is to fix upon the proper starting point. This, on many accounts, would seem to be the North Polar Star. Its position is apparently the same every hour of the night throughout the year, while the other stars are continually moving. Many of the stars also in that region of the skies never set, so that when the sky is clear, they may be seen at any hour of the night. They revolve about the Pole in small circles, and never disappear below the horizon. On this account they are said to be within the circle of perpetual apparition. On the other hand, the identity of the North Polar Star, strange as it may appear, is not so easily determined, by those who are just entering upon this study, as that of some others. For this reason, the point directly over head, called the zenith, is preferable, since upon this point every one can fix with cer tainty in whatever latitude he may be. It will be alike to all the central point of the visible heavens, and to it the pupil will learn imperceptibly to refer the bearing, motion, and dis ances of the heavenly bodies.

That meridional point in each map, whose declination corresponds with the latitude of the place of observation, represents the zenith of the heavens at that place; and those constellations of stars which occupy this position on the maps, will be seen directly over head at 9 o'clock in the evening of the day through which the meridian passes.-Thus in Georgia, for instance, the starting point should be those stars which are situated in this meridian near the 33d degree of north declination, while in New England it should be those which are situated in it near the 42d degree.

How, in using the circumpolar maps? Describe the construction and use of the Ce lestial Planisphere. When the pupil is ready to begin the study of the visible heav ens, what is the first step to be taken? What advantages has the North Polar Star, as a proper starting point? What disadvantages? What point is preferable to the Polar Star? Why is it preferable? How may the point corresponding to this be found upon the maps? At what time in the evening, will the stars which are near this point on the mups, be seen directly over head? Is it indispensably necessary to begin with the stars near this central meridian

We might, nowever, begin with the stars near either of the meridians represented on the maps, the only rule of selection being to commence at that which approaches nearest to being over head at the time required.

We have chosen for our starting point in this work, that meridian which passes through the vernal equinox at the first point of Aries, not only because it is the meridian from which the distances of all the heavenly bodies are measured; but especially because the student will thus be enabled to observe and compare the progressive motion of the constellations according to the order in which they are always arranged in catalogues, and also to mark the constellations of the Zodiac passing over head as they rise one after another in their order, and to trace among them the orbits of the Earth and of the other planets.

As Greek letters so frequently occur in catalogues and maps of the stars and on the celestial globes, the Greek alphabet is here introduced for the use of those who are unacquainted with it. The capitals are seldom used for designating the stars, but are here given for the sake of regularity.

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In 1603, John Bayer, of Augsburg, in Germany, published a complete Atlas of all the constellations, with the useful invention of denoting the stars in every

What is the only rule of selection? What is the starting point chosen for this work What advantages has this meri lian as a starting point?

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