Imágenes de página

wards some determinate region. It becomes an interesting questionDoes our sun move in an immense orbit of its own, carrying the whole solar system with it?

4. A constellation is a group of stars.

5. Planets are opaque bodies which only shine by reflecting the light of the sun.

The name is derived from the Greek grannens (a wanderer). A planet does not twinkle as the fixed stars do.

6. Secondary planets, or satellites, are the moons which revolve round the primary planets, as those bodies revolve round the sun.

7. Comets consist for the most part of a large and splendid, but ill defined, nebulous mass of light, called the head; from which there usually proceeds, in a direction opposite to the sun, a stream of light called the tail.

8. The celestial poles are the extremities of the earth's axis produced to the heavens: they are those two points round which the stars appear to revolve.

9. The equinoctial is the equator supposed to be continued to the heavens.

10. Parallels of declination are small circles drawn parallel to the equinoctial; or, they are the parallels of latitude supposed to be continued to the heavens.

11. Celestial meridians are lines drawn from pole to pole, directly across the equinoctial; they are also called circles of declination; or, they are the terrestrial meridians supposed to be continued to the heavens.

12. The declination of the sun, moon, or stars, is their distance north or south from the equinoctial.

13. Right ascension is that degree of the equinoctial which comes to the meridian with the sun, moon, or stars, reckoning eastward from the first point of Aries; or it is that degree which comes to the horizon with the sun, moon, or stars, in a right sphere.

14. Oblique ascension is that degree which comes to the horizon with the sun, or a star, in an oblique sphere.

15. Ascensional difference is the difference between the right and oblique ascension. Expressed in time, it gives the sun's rising before or after 6 o'clock.

16. Right descension, oblique descension, and descensional difference, have the same reference to the setting of the sun, or of a star, as the above terms have to rising.

17. The latitude of any celestial body is its distance north or south from the ecliptic.

18. Parallels of celestial latitude are small circles parallel to the ecliptic.

19. The longitude of any celestial body is its distance from the first point of Aries, reckoned eastward in signs, degrees, and minutes, upon the ecliptic.

The latitude and longitude of celestial bodies have the same reference to the ecliptic, as the lat. and long. of places upon the earth have to the equator; the longitude is reckoned all round to 360°.

20. The rising of any celestial object is when its centre appears in the eastern part of the horizon; culminating when it comes to the meridian, and its setting when its centre disappears in the western part of the horizon.

21. Azimuth, or vertical circles, pass through the zenith and nadir, and are perpendicular to the horizon.

22. The prime vertical is that vertical circle which passes through the east and west points of the horizon.

If the quadrant of altitude be fixed upon the zenith, and brought to any part of the horizon, it will represent the quadrant of a vertical circle; if brought to the east or west point of the horizon, it will represent the quadrant of the prime vertical.

23. Azimuth is the distance of a vertical circle passing through any celestial object, from the north or south point of the horizon, and is either easterly or westerly.

24. Amplitude is the distance of any celestial object from the east or west point of the horizon, at the time of rising or setting, -and is either north or south.

The azimuth and amplitude are both found upon the wooden horizon ; the amplitude being numbered from the east and west points towards the north and south, and the azimuth from the north and south

points towards the east and west. If the azimuth be not marked upon the horizon, it may be found from the amplitude, being its complement, or what it wants of 90°.

25. The zodiac is a zone which surrounds the heavens, extending go on each side of the ecliptic: it contains 12 constellations, each bearing the name of some object,from which the signs of the ecliptic have their names.

26. The altitude of any celestial object is the arc of a vertical circle, intercepted between the centre of the object and the horizon.

27. The zenith distance is an arc of a vertical circle contained between the centre of a celestial object and the zenith; or it is what the altitude wants of 90'.

28. The meridian altitude, or zenith distance, is the alt. or zenith dist. when the object is in the meridian.

29. Orbit is the path which a body describes in its revolution round the sun.

30. A body is in conjunction with the sun when it has the same longitude, and in opposition when the difference of longitude is 180°. The occultation of a star or planet is its eclipse, occasioned by the interposition of the moon or other planet between the earth and it.

31. The geocentric place of a planet is its place in the heavens as seen from the earth; the heliocentric place is its place as seen from the sun.

32. Disc of the sun or moon is its round face, which, from the great distance of the object, appears flat.

33. A digit is the twelfth part of the diameter of the sun and moon.

Geometrical Definitions. 1. A great circle divides a globe into two equal, a small circle into two unequal parts.

2. A right or direct sphere is that which has the poles in the horizon, and the equinoctial and parallels of declination perpendicular to it.

3. An oblique sphere is that which has one of the poles elevated above the horizon less than 90°, and the other depressed below it; the

equinoctial and parallels of declination form with the horizon oblique angles.

4. A parallel sphere is that which has the equator in the horizon and the poles in the zenith and nadir.


OF THE STARS. A clear winter evening affords one of the most brilliant prospects in nature. The canopy of the heavens is covered with an innumerable multitude of stars, some shining with greater, and others with less, splendour.

To the eye they appear to be all placed at the same distance from the earth; and their different apparent magnitudes and brightness we are apt to attribute to the size of the bodies themselves, rather than to the different distances at which they are placed. From the irregular manner in which they seem scattered about, as well as from their apparently infinite numbers, any attempt to arrange them in classes, or to count their numbers, would at first view appear impossible. Yet we find that this was done in the very infancy of astronomy. The shepherds of Chaldea are supposed to have been the first who directed their attention to this subject: the nature of their employment invited them to the work, and the continued serenity of their sky enabled them to pursue it without interruption. In the time of Job some of the constellations were well known; hence the following apostrophe, “ Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion ? Canst thou bring forth Mazzaroth in his season? Or canst thou guide Arcturus with his sons ?” Stars continue visible through telescopes during the day as well as the night; from the bottoms of deep narrow pits bright stars may be discerned by the naked eye.

Astronomers, to assist the imagination and the memory in conceiving and retaining the number and position of the stars, divided them into certain groups, called constellations, which, by a stretch of fancy, they supposed to resemble the figure of a man, or other object.

The number of the ancient constellations was 48, but the present number upon the globe is 70, though, by some, it amounts to 91 ; of which 34 belong to the northern hemisphere, 12 to the zodiac, and the remaining 45 to the southern hemisphere: those stars which do not come into any of the constellations are called unformed stars.

Stars are further divided into classes according to their apparent size. The largest are said to be of the first magnitude, the next in size of the second magnitude, and so on. The individual stars in each constellation are marked with the letters of the Greek alphabet; the first letter, a, (alpha,) being put for the largest star in the constellation —the second letter, B, (beta,) for the next largest—, (gamma,) for the next-d, (delta,) <, (epsilon,) Ś, (zeta,) ng (ēta,) for the next-and so on; and when there are more stars in a constellation than letters in the Greek alphabet, the rest are marked by italic letters. This serves to point out the stars, as well as if particular names were given to each. But, besides this method of distinguishing them, some of the most remarkable have proper names assigned them.

The lowest magnitude visible to the naked eye is the sixth, but astronomers carry the classification of the stars as far as the sixteenth magnitude. All the stars of the same magnitude do not possess equal brilliancy. Thus some stars, said to be of the first magnitude, are scarcely superior to the brightest of the second magnitude, or the feeblest of the second to the brightest of the third, and so on.

The following is a catalogue of the stars in each constellation, with the names of the most remarkable stars.

The figures placed against each constellation denote the number of stars composing it, and the figure attached to each principal star marks its magnitude.

1. CONSTELLATIONS NORTH OF THE ZODIAC. ']. Ursa Minor The Little Bear 24 Pole Star 2. Ursa Major The Great Bear87 Dubhe Perseus Perseus

Al'genib 8. Caput Meduse Medusa's Head 59

Algol 4. Auriga

The Waggoner 66 Capella 5. Boötes

The Herdsman 54 Arcturus 6. Draco

The Dragon 80 Ras'taben 7. Cepheus

35 Aldera'min & Canes Venatici

The Hounds 9. Cor Caroli Charles's Heart 10. Triangulum

The Triangle

« AnteriorContinuar »