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visible hemisphere, and the lower one, the invisible hemisphere. It is the plane of this circle which determines the rising and setting of the heavenly bodies.
The Sensible or Apparent Horizon, is the circle which termi nates our view, where the Earth and sky appear to meet.
To a person standing on a plain, this circle is but a few miles in diameter. If the eye be elevated five feet, the radius of the sensible horizon will be less than two miles and three quarters; if the eye be elevated six feet, it will be just three miles. The observer being always in the centre of the sensible horizon, it will move as he moves, and enlarge or contract, as his station is elevated or depressed.
8. The Poles of the Horizon are two points, of which the one is directly overhead, and is called the Zenith; the other is directly underfoot, and is called the Nadir.
Vertical Circles are circles drawn through the Zenith and Nadir of any place, cutting the horizon at right angles.
The Prime Vertical is that which passes through the east and west points of the horizon.
9. The Ecliptic is the plane of the Earth's orbit; or the great circle which the Sun appears to describe annually among the stars. It crosses the Equinoctial, a little obliquely, in two opposite points, which are called the Equinoxes. The Sun rises in one of these points on the 21st of March; this point is called the Vernal Equinox. It sets in the opposite point on the 23d of September; this point is called the Autumnal Equinox. One half of the Ecliptic lies on the north side of the Equinoctial, the other half on the south side, making an angle with it of 234°. This angle is called the obliquity of the Ecliptic. The axis of the Ecliptic makes the same angle with the axis of the heavens; so that the poles of each are 234° apart.
This angle is perpetually decreasing. At the commencement of the Christian era, it was about 23° 45'. At the beginning of 1836, it was only 23° 27′ 38′′, showing an annual diminution of about half a second, or 45".70 in a hundred years. A time will arrive, however, when this angle, having reached its minimum, will again increase in the same ratio that it had before diminished, and thus it will continue to oscillate at long periods, between certain limits, which are said to be comprised within the space of 20° 42'.
10. The Ecliptic, like every other circle, contains 360°, and it is divided into 12 equal arcs of 30° each, called signs, which the ancients distinguished by particular names. This division commences at the vernal equinox, and is continued eastwardly round to the same point again in the following order: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capri
8. Poles of the horizon? Vertical circles? Prime Vertical? 9. Ecliptic? Equinoxes? How is the Ecliptic situated with respect to the Equinoctial? Obliquity of Ecliptic? Is this angle permanent? 10. How is the Ecliptic divided? Where cominenced, and how reckoned? Name signs in order? How does the Sun proceed through the signs?
cornus, Aquarius, Pisces. The Sun, commencing at the first degree of Aries, about the 21st of March, passes, at a mean rate, through one sign every month.
11. The Zodiac is a zone or girdle, about 16 degrees in breadth, extending quite round the heavens, and including all the heavenly bodies within 8° on each side of the ecliptic. It includes, also, the orbits of all the planets, except some of the asteroids, since they are never seen beyond 8° either north or south of the ecliptic.
12. Parallels of Latitude are small circles imagined to be drawn on the Earth's surface, north and south of the equator, and parallel to it.
Parallels of Declination are small circles, imagined to be drawn on the concave surface of the heavens, north and south of the equinoctial, and parallel to it; or they may be considered as circles formed by producing the parallels of latitude to the heavens.
13. The Tropic of Cancer is a small circle, which lies 234° north of the Equinoctial, and parallel to it. The Tropic of Capricorn is a small circle, which lies 234° south of the Equinoctial, and parallel to it. On the celestial sphere, these two circles mark the limits of the Sun's farthest declination, north and south. On the terrestrial sphere, they divide the torrid from the two temperate zones. That point in the ecliptic which touches the tropic of Cancer, is called the Summer Solstice; and that point in the ecliptic which touches the tropic of Capricorn, is called the Winter Solstice.
The distance of these two points from the equinoctial, is always equal to the obliquity of the ecliptic, which, in round numbers, is 23c°; but, as we have seen, the obliquity of the ecliptic is continually changing; therefore the position of the tropics must make a correspondent change.
14. The Colures are two great circles which pass through the poles of the heavens, dividing the ecliptic into four equal parts, and mark the seasons of the year. One of them passes through the equinoxes at Aries and Libra, and is thence called the Equinoctial Colure; the other passes through the solstitial points or the points of the Sun's greatest declination north and south, and is thence called the Solstitial Colure.
The Sun is in the equinoctial points the 21st of March and the 23d of September. He is in the solstitial points the 22d of June and the 22d of December.
15. The Polar Circles are two small circles, each about 661°
11. What is the Zodiac? 12. Parallels of latitude? Of declination? 13. The tropics? Cancer? Capricorn? What do these circles mark in the celestial sphere? On the terrestrial? 14. The Colures? Where situated? When is the Sun at the equiLoctial points? The solsticial? 15. What are the Polar Circles?
from the equator, being always at the same distance from the poles that the tropics are from the equator. The northern is called the Arctic circle, and the southern the Antarctic circle.
16. Meridians are imaginary great circles drawn through the poles of the world, cutting the equator and the equinoctial at right angles.
Every place on the Earth, and every corresponding point in the heavens, is considered as having a meridian passing through it; although astronomers apply but 24 to the heavens, thus dividing the whole concave surface into 24 sections, 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 equinoctial colure.
17. Latitude on the Earth, is distance north or south of the equator, and is measured on the 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.
18. 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 corresponds 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 ascension, being reckoned in only one direction, extend entirely round the circle, or 360°.
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 15°, it will be one fifteenth of an hour, or four minutes, in passing over 1°.
If the first point of Aries be on the meridian at 12 o'clock, the next hour line, which is 15° E. of it, will come to the meridian at 1 o'clock; the second hour 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.
19. 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
16. Meridians? How many? What other name? How measure distances on the earth? In the heavens? 17. What is latitude on the earth? In the heavens? Longitude on the earth? In the heavens? 18. Declination? Right ascension? Why describe by D. and R. A. Extent of latitude? Declination? Longitude and R. A? How convert R. A. into time? Which of two bodies given will first pass the meridian? 19. What apparent motion of stars? Cause? Results?
called their daily acceleration. It amounts to just two hours a month. On this account we have not always the same constellations 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.
DESCRIPTION AND USE OF THE MAPS.
20. 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.
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.
21. 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 Ilour 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 meridian at 9 o'clock in the evening.
20. What said of maps? First? Next six? Describe lines? Scale indicates what?
21. Sixth and seventh? Scale?
22. The first four maps of the heavens are so constructed that the pupil in using them must suppose himself to face the south, and to hold them directly overhead 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 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 overhead 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 overhead.
CLASSIFICATION OF STARS, NEBULÆ, &c.
23. FOR purposes of convenience in finding or referring to particular stars, recourse is had to a variety of artificial methods of classification. First, the whole concave of the heavens is divided into sections or groups of stars, of greater or less extent, called Constellations.-(Of the origin of these figures see page 143). Next, they are classified according to their magnitudes, (as already stated art. 4), and designated on the maps accordingly. Thirdly, the stars of each constellation are classified according to their magnitudes in relation to each other, and without reference to other constellations. Thus, for instance, the largest star in Taurus is marked a, Alpha; the next largest ß, Beta; the next, y, Gamma, &c., till the Greek alphabet is exhausted. Then the Roman (or English) is taken up, and finally, if necessary, recourse is had to figures.
This useful method of designating particular stars by the use of the Greek and Roman alphabet, was invented by John Bayer, of Augsburg, in Germany, in 1603. It has been adopted by all succeeding astronomers, and extended by the addition of the Arabic notation 1, 2, 3, &c., wherever the stars in a constellation outnumber both alphabets.
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.
22. How use the first four maps of the heavens? Circumpolar? What exception? What point of projection chosen, and why? 23. Classification or designation of stars? By whom invented, and when?