Page images

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 others, 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 15°, 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 be 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 it?

pupil in using them must suppose himself to face the south, and to 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 certainty 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 distances 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, however, 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.

[merged small][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][merged small][merged small][merged small][ocr errors][subsumed][merged small][subsumed][merged small][merged small][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][merged small][ocr errors][subsumed][subsumed][merged small][merged small][merged small]

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-dian as a starting point?

constellation by the letters of the Greek and Roman Alphabets; assigning the Greek letter a to the principal stars in each constellation, to the second in magnitude, to the third, and so on; and when the Greek alphabet was exhausted, the notation was carried on with the Roman letters, a, b, c, &c. That the memory might not be perplexed with a multitude of names, this convenient method of designating the stars has been adopted by all succeeding astronomers, who have farther enlarged it by the Arabic notation, 1, 2, 3, &c. whenever the stars in the constellations outnumbered both alphabets.




[merged small][ocr errors][subsumed][merged small][subsumed][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][subsumed][merged small][merged small][merged small][merged small][merged small][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small][merged small][merged small][merged small][ocr errors][subsumed][subsumed][merged small]


[merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small]






























162 164






If we look directly over head at 10 o'clock, on the 10th a November, we shall see the constellation celebrated in fable, by the name of ANDROMEDA It is represented on the map by the figure of a woman having her arms extended, and chained by her wrists to a rock. It is bounded (N. by Cassiopeia, E. by Perseus and the head of Medusa, and S. by the Triangles and the Northern Fish.) It is situated between 20° and 50° of N. declination. Its mean right ascension is nearly 15°; or one hour E. of the equinoctial colure.)

It consists of 66 visible stars, of which three are of the 2d magnitude, and two of the 3d; most of the rest are small.

(The stars directly in the zenith, are too small to be seen in the presence of the moon, but the bright star Almaack, of the 2d magnitude, in the left foot, may be seen 13° due É., and Merach, of the same magnitude, in the girdle, 7° south of the zenith. This star is then nearly on the meridian, and with two others N. W. of it forms the girdle.)

The three stars forming the girdle are of the 2d, 3d, and 4th magnitude, situated in a row, 3° and 4° apart, and are called Merach, Mu and Nu.

About 20 from Nu at the northwestern extremity of the girdle, is a remarkable nebula of very minute stars, and the only one of the kind which is ever visible to the naked eye. It resembles two cones of light, joined at their base, about length, and 40 in breadth.


If we look directly over head at 10 o'clock on the 10th of November, what constellation shall we see? How is it represented on the map? How is it bounded? What are its right ascension and declination? How many visible stars has it? Describe the girdle of Andromeda. Describe the appearance of a remarkable nebula which lies at its northwestern extremity.

« PreviousContinue »