Recreations in Astronomy eBook

This eBook from the Gutenberg Project consists of approximately 199 pages of information about Recreations in Astronomy.

In one respect Jupiter seems like a minor sun—­he is royally attended by a group of planets:  we call them moons.  This system is a favorite object of study to everyone possessing a telescope.  Indeed, I have known a man who could see these moons with the naked eye, and give their various positions without mistake.  Galileo first revealed them to ordinary men.  We see their orbits so nearly on the edge that the moons seem to be sliding back and forth across and behind the disk, and to varying distances on either side.  Fig. 64 is the representation of their appearance at successive observations in November, 1878.  Their motion is so swift, and the means of comparison by one another and the planet so excellent, that they can be seen to change their places, [Page 166] be occulted, emerge from shadow, and eclipse the planet, in an hour’s watching.

[Illustration:  Fig. 64.—­a. Various Positions of Jupiter’s Moons; b. Greatest Elongation of each Satellite.]

ELEMENTS OF JUPITER’S SATELLITES.
+-----------------------------------------------------------
--+ | | Mean Distance | | | | | from Jupiter. | Sidereal Period. | Diameter.| | |---------------+------------------+----------| | | Miles. | Days Hrs.  Min. | Miles. | | I. Io | 260,000 | 1 18 28 | 2,352 | | II.  Europa | 414,000 | 3 13 43 | 2,099 | | III.  Ganymede | 661,000 | 7 3 59 | 3,436 | | IV.  Callisto | 1,162,000 | 16 18 5 | 2,929 | +-----------------------------------------------------------
--+

It is seen by the above table that all these moons are larger than ours, one larger than Mercury, and the asteroids are hardly large enough to make respectable moons for them.  They differ in color:  I. and II. have a bluish tinge; III. a yellow; and IV. is red.  The amount of light given by these satellites varies in the most sudden and inexplicable manner.  Perhaps it may be owing to the different distributions of land and water on them.  The mass of all of them is .000171 of Jupiter.

[Page 167] If the Jovian system were the only one in existence, it would be a surprising object of wonder and study.  A monster planet, 85,000 miles in diameter, hung on nothing, revolving its equatorial surface forty-five miles a minute, holding four other worlds in steady orbits, some of them at a speed of seven hundred miles a minute, and the whole system carried through space at five hundred miles a minute.  Yet the discovery of all this display of power, skill, and stability is only reading the easiest syllables of the vast literature of wisdom and power.

SATURN.

The god or time; sign [Symbol], his scythe.

MEAN DISTANCE FROM THE SUN, 881,000,000 MILES.  DIAMETER, POLAR, 66,500 MILES; EQUATORIAL, 73,300 MILES.  AXIAL REVOLUTION, 10H. 14M.  PERIODIC TIME, 29T YEARS.  MOONS, EIGHT.

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Recreations in Astronomy from Project Gutenberg. Public domain.
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