Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/853

]  Elements of Jupiter's Satellites.

Elements of Saturn's Satellites.

No. Sidereal Revolution. Distance in Radii of Sarurn. Diameter in miles (?). Eccen tricity. Discoverer. d. li. m. 1 22 37 3-360 1000 0-06SS9 Sir W. Ilerschel. 2 1 8 53 4-312 Do. 3 1 21 18 5-339 500 0-0051 J. D. Cassini. 4 2 17 41 6-839 500 0-02 Do. 5 4 12 25 9-552 1200 0-02269 Do. 6 15 22 41 22-145 3300 0-029223 C. Huvghens. 7 21 7 7 28- W.Bond. 8 79 7 53 64-359 1800 0-115 J. D. Cassini.

Elements of Uranus's Satellites.

No. Sidereal Revolu tion. Distance in Radii of Uranus. Maximum Klongation. Discoverer. d. h. m. 1 2 12 28 7-44 12 W. Lassell. 2 4 3 27 10-37 15 Do. 3 8 16 55 17-01 35 Sir W. ITerschel. 4 13 11 6 22 75 49 Do.

Elements of Neptune's Satellite.

Sidereal Revolution. Distance in Radii of Neptune. Maximum Klongation. Discoverer. d. h. m. 5 21 8 12-00 18 W. Lassell.

The following are the elements of Saturn's ring system according to the best authorities:—

Longitude of ascending node on ecliptic 167 43 29 Inclination 28 10 22&quot; Exterior diameter of outer ring, in miles .... 166,920 Interior do. do. do 147,670 Exterior do. inner ring do 144,310 Interior do. do. do 109,100 Interior do. dark ring do 91,780 Breadth of outer bright ring do 9,625 Breadth of division between the rings do. ... 1,680 Breadth of inner bright ring do.... 17,605 Breadth of dark ring do. ... 8,660 Breadth of system of bright rings do. ... 28,910 Breadth of entire system of rings do. ... 37,570 Space between planet and dark rings do. ... 9,760

—The Sun—Sun Spots—Rotation of the Sun—His Physical Constitution and Surroundings—Prominences—Corona—Zodiacal Light.

The sun, the central and ruling body of the planetary system, and the source of light and heat to our earth and all the members of that system, is a globe about 852,900 miles in diameter. So far as observation extends, his figure is perfectly spherical, no difference having been observed between his polar and spherical diameters. It has been well remarked, indeed, by Sir G. Airy, that if any observer could by ordinary modes of measurement satisfy himself that a real difference existed between the diameters, that observer would have proved the inexactness of his own work ; for the absence of any measurable com pression comes out as the result of comparisons between thousands of observations of the sun s limbs made at Greenwich and other leading observatories. The volume of the sun exceeds the earth s 1,252,700 times. His mean density is almost exactly one- fourth of the earth s, and his mass exceeds hers about 316,000 times. Gravity at the surface of the sun exceeds terrestrial gravity about 27 1 times, so that a body dropped from rest near the sun s surface would fall through 436 feet in the first second, and have acquired a velocity of 872 feet per second.

Viewed with the naked eye, the sun appears only as a luminous mass of intense and uniform brightness ; but when examined with the telescope, his surface is frequently observed to be mottled over with a number of dark spots, of irregular and ill-defined forms, constantly varying iu appearance, situation, and magnitude. These spots are occasionally of immense size, so as to be visible even with out the aid of the telescope ; and their number is frequently so great that they occupy a considerable portion of the sun s surface. Sir W. Herschel observed one in 1779, the diameter of which exceeded 50,000 miles, more than six times the diameter of the earth ; and Scheiner affirms that he has seen no less than fifty on the sun s disk at once. Most of them have a deep black nucleus, surrounded by a fainter shade, or umbra, of which the inner part, nearest to the nucleus, is brighter than the exterior portion. The boundary between the nucleus and umbra is in general tolerably well defined ; and beyond the umbra a stripe of light appears more vivid than the rest of the sun. The discovery of the sun s spots has been attributed to Fabricius, Galileo, and Scheiner, and has been claimed for the English astronomer Harriot. Amidst these conflicting pretensions it is perhaps impossible to arrive at the truth ; but the matter is of little importance ; the discovery is one which followed inevitably that of the telescope, and an accidental priority of observation can hardly be considered as establishing any claim to merit.

Fig. 23 The solar spots furnish an extensive subject of curious speculation. They are, first, interesting on account of their establishing the fact of the rotation of the sun, and afford- the means of determining its period. Let M, M, M&quot;, fig. 23, be the successive positions of a spot on the surface of the sun on different days. Then, correction being made for the earth s advance dur- Fl S- 23 - ing these days, M, M, M&quot; will represent the apparent path of the spot on the sun s disk This path, as thus corrected, is in general an oval slightly differing from an eclipse ; and it is found that all the spots observed at the same time describe similar and parallel curves. They also return to* the same relative positions in the same time, and their period is about 27 J days. Fig. 24. Fig. 25. Fig. 26. Fig. 27. Diagrams showing motion of sun spots at different seasons. The paths described by the spots undergo very consider able changes, according to the season of the year at which they are observed. About the end of November and beginning of December they appear simply as straight lines, Mm, M ? , M&quot;m&quot; (fig. 24), along which the spots move in the direction Mm; that is, they enter on the eastern and disappear on the western edge of the sun s disk, and the points at which they disappear are more elevated, or nearer the north pole of the ecliptic than those at which they enter. After a certain time the lines M?n, &c., begin to assume a curved appearance. During the winter and spring the 