JUPITER. 115 



LESSON 51. 



Jupiter. 



J jpiTER is the largest of all the planets. His diameter is 

 eighty-nine thousand miles. He revolves round the sun at 

 the mean distance of four hundred and ninety millions of 

 miles, completes a revolution in a little less than twelve years, 

 and turns on his axis in the short interval of nine hours' and 

 fifty-six minutes. With the exception of Venus, Jupiter is 

 the^ most brilliant of the planets, and, when viewed through 

 a telescope, its surface is remarkable for being always co- 

 vered with a number of belts or stripes of various shades. 

 They are not regular or constant in their appearance, and 

 their breadth is also variable, one belt growing narrower 

 while another in its neighbourhood becomes broader, as if 

 one had flowed into the other. Sometimes one or more spots 

 are formed between the belts, which increase until the whole 

 are united in one large dusky band. Bright spots also may 

 be discovered on Jupiter's surface, which are more perma- 

 nent than the belts, and re-appear after unequal intervals of 

 time. For the cause of these appearances, we are referred 

 by eminent philosophers, to his swift diurnal motion, to the 

 changes in the density of his atmosphere, as occasioned by 

 variations of temperature, and to other incidental agencies. 

 The axis of Jupiter is perpendicular to the plane of his orbit ; 

 his inhabitants, therefore, will experience no change of sea- 

 sons, nor difference in the length of their days and nights. 

 At the equator there will be perpetual summer, and at the 

 poles unceasing winter. The degree of light and heat is 

 about twenty-five times less than at the earth. 



The satellites of Jupiter are invisible to the naked eye, 

 but through a telescope they make a beautiful appearance. 

 As our moon turns round the earth, enlightening the nights 

 by reflecting the rays of the sun, so these also enlighten the 

 nights of Jupiter, and move round him in different periods 

 of time, proportioned to their several distances. They often 

 pass behind the body of the planet, and also into its shadow, 

 and are eclipsed. These eclipses are of use for ascertaining 

 the longitude of places. They have led to the discovery, 

 that light is about eight minutes in coming from the sun ta 

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