THE ASTEROIDS OR MINOR PLANETS.

Hecuba was discovered in April, 1869, and others have been discovered sinco -raising the number to 130. Flora, the nearest to the Sun, is 201 millions of miles distant; Cybele nearly 313 millions.

LESSON 21.-THE SYSTEM OF JUPITER 2:
VELOCITY OF LIGHT.

1. Q. What are Jupiter's Dimensions and Distance? A. Jupiter's Distance from the Sun (476 millions of miles) is 5 times; his Diameter (86,000 miles) 11 times; his Year (4,3321 days) about 12 times; and his Day (9h 55m) less than half the Earth's. To equal the Volume of this vast planet 1,280 worlds as large as the earth would be required, but his Density is less than 1-4th, and therefore his Mass, or the quantity of matter of which the planet consists, is only 300 times that of the earth (Lesson 22).

2. Q. What is Jupiter's appearance in the heavens?

A. Jupiter is larger than all the other planets together, and though so distant, he appears, next to Venus, the largest and brightest body in the heavens. Through the telescope the polar flattening, due to the rapid rotation of this huge globe, is distinctly perceptible-the equatorial diameter being to the polar as 17 is to 16-and the disc is seen crossed by dark bands called Belts, which generally run parallel to the equator (Les. 3, 27).

3. Q. How are Jupiter's Belts produced?

A. These Belts, which vary considerably in number, breadth, and situation, are probably the dark body of the planet seen through tracts of clear sky; and they are supposed to be due to atmospheric currents, resembling our TradeWinds. On the Belts are sometimes seen Dark Spots, at other times Bright Spots-probably masses of cloud which reflect more light than the

planet's body-and from the motions of these Spots the Period of Jupiter's Rotation has been determined (Lesson 7).

4. Q. By what number of Moons is Jupiter attended?

A. Jupiter has Four Satellites or Moons, the discovery of which, by Galileo, furnished conclusive evidence of the truth of the Copernican System by showing that the same laws prevail in Jupiter's Little System and in the Great Solar System. Besides, the study of these satellites, on account of their use in determining terrestrial longitudes, led to the discovery of the progessive motion, and the determination of the actual velocity of light (Lesson 30).

5. Q. How are Terrestrial Longitudes determined by means of Jupiter's Satellites?

A. A spectator on Jupiter might witness 4,500 eclipses of the satellites-and as many of the sun -during a Jovian year, for the eclipse of the three inner moons, which may be visible over half our globe, occurs at every revolution. The exact time at which each of these eclipses will happen at Greenwich is given in the "Nautical Almanack," and the difference between this time and that at which the eclipse is observed at any place gives the Longitude of that place in Time.

6. Q. What are the Dimensions of Jupiter's Moons?

A. Europa, the least, is about the size of our moon; Ganymede, the largest, is bigger than Mercury; and Io, the first satellite, whose period is only 14 days, would appear from Jupiter larger than the moon in our sky. From certain variations observed in the brightness of the satellites, as their positions vary with respect to the sun, it

is concluded that they all resemble our moon in turning on their axes in the same time that they revolve round their primary (Lessons 9, 22).

7. Q. How did the discovery of the Progressive Motion of Light result from observations of Jupiter's Satellites?

A. Römer, a Danish astronomer, on comparing a long series of observations on the eclipses of these satellites, found (1675) that the eclipses happened earlier than the calculated time when Jupiter was in Opposition (Earth between Jupiter and the Sun), and later when he was in Conjunction (Sun between Jupiter and the Earth). The eclipses happening too soon when Jupiter's distance from the earth was less, Römer justly concluded that light takes less time to travel a less distance, and must therefore have a progressive motion (Lesson 20).

A.

E

M

ORBITS OF PLANETS.-m, Mercury; V, Venus; E, the Earth; M, Mars; A, the Asteroids; J, Jupiter with his Moons.

8. Q. What is the Velocity of Light?

A. Light was thus proved not to be transmitted instantaneously as many had supposed, but to take 16m 26 to cross the earth's orbit,

which is the difference of Jupiter's greatest and least distances from the earth (e J-E Je E. Fig., p. 109). Now, half the breadth of the earth's orbit is the sun's distance (SE) from the earth -about 91 millions of miles-and this divided by half the above time, or 8m 13, gives for the Velocity of Light about 186,000 miles per second.

LESSON 22-SATURN URANUS H:

:

NEPTUNE.

1. Q. What are Saturn's Dimensions and Attendants? A. Saturn, perhaps the most interesting object in the heavens, has a Diameter of 72,000 miles, a Volume nearly 750 times that of the Earth, and a Period of about 29 years. He is attended by no less than 8 Moons, besides an immense Ring, or rather a System of Rings, which turns on an axis corresponding to that of Saturn, while the whole family-Planets, Moons, and Ringstravel together round the sun, at a Mean Distance from him of 872 millions of miles.

2. Q. In what Periods do Saturn and his Rings complete their Rotation?

A. Saturn Rotates on his Axis in 10h 29m; the Rings take 10 321m, which is about the Periodic Time that the theory of gravitation assigns for a satellite, supposing such to revolve at a Mean Distance from the primary equal to the planet's distance from the middle of the breadth of the Rings. The innermost of the three Rings is much fainter than the others, and appears to be semi-transparent, or at least so constituted that the planet may be seen through it (Lesson 30).