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29. Are there any stars which never appear above the horizon at the equator?
30. Where must I be never to see Menkar?
OF THE SUN. The sun is the centre of the solar system, all the planets moving round it at different distances, and in different periods.
The ancients conceived that the earth was at rest, and that the sun, moon, planets, and stars all moved round it. Such is the equable motion of the earth that we have great difficulty in supposing that it is not only revolving on its axis, but is rolling rapidly in its path round the sun. But how much more probable is it that this little globe revolves once in the 24 hours upon its axis, than that the sun and fixed stars, all vastly greater than the earth, and at enormous and very different distances from it, complete a revolution round it in that period. The ancients noticed that the sun was continually shifting his place among the fixed stars, and that whilst he made only 365 revolutions round the earth, the stars had made 366. To account for these facts, they supposed that whilst the sun is carried round the earth by the general motion of the starry sphere, he has a sphere of his own which travels in the contrary direction, and makes one revolution round the earth in a year. They are as satisfactorily and much more simply accounted for upon the supposition that the earth has an annual course round the sun. In order to account for the apparently capricious motions of the planets, the ancients devised a complex system of independent spheres, the mechanism of which it was difficult to comprehend. Upon the Copernican theory, which supposes the sun to be the centre of the solar sys. tem, and the earth to have a double motion, all is plain and easy. Tycho Brahe, an able and very useful astronomer, unable to resist the truth of the theory of Copernicus, and yet unable to throw off entirely the prejudices of centuries, invented another system. He admitted that the sun was the centre round which all the planets, except the earth, revolved, but that the sun, with all his followers, revolved round the earth as the cetnre of the whole. This theory obtained few followers. The sun, though very nearly, is not exactly the centre of the solar system, the real centre being a point which is the common centre of gravity of the sun and the other bodies which compose the system.
Owing, however, to the immense quantity of matter contained in the sun, this point is almost identical with the sun's centre.
The figure of the sun is nearly globular, and its diameter is equal to 111 times that of the earth, being about 883,000 miles ; hence its surface is 12,300 times, and its bulk, or solid content, 1,380,000 times that of the earth. The sun is composed of lighter materials than the earth, an equal bulk of the sun's substance possessing less than
of the density of that of the earth ; notwithstanding this, its gigantic dimensions give it a force of gravity 27 times greater than the earth. It is the attraction of the sun that retains the planets in their orbits, and to it they are indebted for light, heat, and motion. The sun is not absolutely at rest; it probably has a motion of its own through space; and it is found, by the spots on its surface, to turn round on its axis from west to east in about 25 days. The mean distance of the sun from the earth is 95,000,000 miles.
The sun agrees with the fixed stars in the property of emitting light continually ; and it is not improbable that they have many other properties in common. The sun is therefore considered as a fixed star comparatively near us, and the stars as suns at immense distances.
From being the source of light and heat, the sun was long supposed to be a body of fire; but Sir W. Herschel supposed that the body of the sun is an opaque habitable planet, surrounded by a double atmosphere, the outer being luminous, diffusing light and heat through the whole system; the inner, a cloudy stratum protecting the body of the sun from the heat of the luminous one. The luminous atmosphere, being at times intercepted and broken, gives us a view of the dark one beneath, (the penumbra,) and of the body of the sun.
The spots on the sun are interesting telescopic objects, and large ones may be seen by instruments of moderate power, the eye of the observer being protected by coloured glasses. The spots consist of a perfectly dark central part surrounded with a kind of border, less completely dark, called a penumbra. Their general appearance is represented in the cut illustrating solar eclipses. They are irregular in shape and very various in size. Sometimes they exceed 45,000 m. in diameter.
When watched from day to day they appear to be subject to violent agitation; they enlarge and contract, break up into two or more, change their forms, disappear altogether, or new ones appear. They hardly ever last longer than six weeks. The neighbourhood of great spots, and the places where spots frequently afterwards break out, are generally observed to be covered with strongly marked curved or branching streaks more luminous than the rest, called faculæ. These are, perhaps, the ridges of immense waves in the luminous regions of the sun's atmosphere, indicative of violent agitation in their neighbourhood.
The annual revolution of the earth produces the apparent motion of the sun among the stars in the ecliptic, by which he describes his annual path. This produces a daily change in right ascension and declination. The sun's amplitude and azimuth vary, both with the day of the month and the latitude of the place.
The amplitude is always of the same name with the declination : the greatest amplitude north is, when the sun is in the north tropic; and south, when he is in the south tropic. Places that have the greatest latitude (not greater than 6619) have the greatest variation of amplitude; places at the equator have the least variation.
Bring the sun's place to the brass meridian ; the degree over it shows the declination, and the degree of the equinoctial under the meridian shows the right ascension.
EXAMPLES.-Required the sun's right ascension and declination for the following days. RIGHT ASCENSION.
DECLINATION. In degrees. In time. 1. Jan. 1. 282° 22' 18h. 17 m. 21° 59' S. ); 2. Feb. 10. 324 22 21 37 14 10 S.) 3. March 22. 4. May 12. 5. June 22. 6. August 10. 7. September 22. 8. December 21.
PROBLEM XV. To find the Sun's Oblique Ascension, Ascensional Dif
ference, Eastern Amplitude, and Time of Rising, on any given day, at any given place.
1. Elevate the globe for the latitude, bring the sun's place to the meridian, and set the index to 12.
2. Bring the sun's place to the eastern side of the horizon, and the degree of the equinoctial now at the horizon is the sun's oblique ascension.
3. The right ascension being found by the last problem, the difference between it and the oblique ascension will be the ascensional difference.
4. The number of degrees on the horizon, intercepted between the east point and the sun's place, is the eastern or rising amplitude.
5. The hour shown by the index, when the sun is at the horizon, is the time of its rising.
From the ascensional difference, the time of the sun's rising may be found without the globe thus :-If the sun's declination and the latitude of the place are of the same name, the ascensional difference, reduced to time, and subtracted from six o'clock, will give the time of the sun's rising. If the declination and latitude are of different names, the ascensional difference must be added to six.
EXAMPLES 1. Required the sun's oblique ascension, ascensional difference, eastern amplitude, and time of rising, at London, May 1st. Ans. Ob. as., 19°; As. diff., 19° 48'; E. amp., 25° N.; Rising, 4h. 40 m.
2. The same for Gibraltar, Nov. 25th. Ans. Ob. as., 257° 7' ; As. diff., 15° 41'; E. amp., 26° 9' S.; Rising, 7 h. 4 m.
3. For Halifax, (America,) Dec. 25th. Ans. Ob. as., 300"; As. diff., 25° 38'; E. amp., 34o S.; Rising, 7 h. 45 m.
4. Required the same for Hanover, June 4th. 5. Required the same for Newcastle, July 29th. 6. Required the same for St. Petersburg, June 21st.
PROBLEM XVI. To find the Sun's Oblique Descension, Descensional Dif
ference, Western Amplitude, and Time of Setting, on any given day, at any given place.
Proceed as in the last problem, only bring the sun's place to the western horizon.
The sun's setting may be found from the descensional difference.
If the declination and latitude be of the same name, the descensional difference, added to six o'clock, will give the time of the sun's setting ; if they be of different names, the descensional difference, subtracted from six o'clock, will give the time of the sun's setting.
The sun's ascensional and descensional difference, as found by the globe, being equal to each other, either of them may be used in finding the rising and setting of the sun.
EXAMPLES.—Required the sun's oblique descension, descensional difference, western amplitude, and time of setting at the following times and places :
1. C. Good Hope, July 19th.—Ans. Ob. des., 103° 44'. Des. diff., 15° 8'. W. amp., 25° 32'. Setting 5 h.
2. Quebec, May 15th. Ans. Ob. des., 74o. Des. diff., 21° 39'. W. amp., 28° N. Setting, 7 hrs. 23 m.
3. Alexandria, Jan. 21st. 4. Konigsberg, Aug. 12th. 5. Liverpool, May 14th. 6. Washington, Dec. 21st. 7. Archangel, June 21st. 8. Edinburgh, Jan. 1st. 9. Malta, June 9th.
PROBLEM XVII. The latitude, hour of the day, and day of the month being
given, to find the Sun's Altitude and Azimuth. This problem is the same as Problem VI., page 277, only the quadrant of altitude must be brought over the sun's place, instead of being brought over the star.
EXAMPLES.--Required the sun's altitude and azimuth at the following places and times :
h. m. Altitude. Azimuth. 1. Lisbon, May 18, 7 30 a.m. 30° N. 88° E. 2. Madrid, April 15, 10 0a.m. 50. S. 47 E.