MAGNITUDE OF NEPTUNE. 



body have any proportion to such prodigious distances and 

 motions ? or whether, indeed, its magnitude at such a distance 

 as 3000 millions of miles, can be at all ascertained ? Difficult, 

 nevertheless, as such a problem may seem, it is, on the con- 

 trary, among the most easy and simple which are presented to the 

 astronomer. 



When a powerful astronomical telescope is directed to a planet, 

 the object which to the naked eye appears as a mere stellar point of 

 light, is seen with a circular disc like that of the moon, but in 

 general much smaller. The visual angle of the moon's disc is 

 1800"; now it is found that the visual angle of the disc of Neptune is 

 only 2 '8", and, therefore, is very nearly 643 times less than the 

 disc of the moon ; but the distance of Neptune is 30 times greater 

 than that of the sun, and the distance of the sun is 400 times 

 greater than that of the moon. Therefore the distance of Neptune 

 is 12000 times greater than that of the moon, consequently it will 

 follow, that if the moon were removed to Neptune's distance, its 

 visual angle would be 12000 times less than it is ; but from what 

 has been just stated it appears that the visual angle of Neptune 

 is only 643 times less than that of the moon. It follows, there- 

 fore, that the actual diameter of Neptune must be greater than 

 the actual diameter of the moon, in the proportion of 12000 to 

 643, or 19 to 1 very nearly. But since we know the diameter of 

 the moon to be a little more than 2000 miles, it follows that the 

 actual diameter of Neptune will be about 38000 miles. 



This is a rough method of calculation which we have adopted 

 to render the point familiar to those who are not accustomed to 

 the more exact methods of astronomical calculations. 



How little the result nevertheless varies from the truth, will 

 be perceived when we state, that, according to the most exact 

 observations and calculations of astronomers, the actual diameter 

 of Neptune is 37500 miles. 



21. A satellite of this planet was discovered by Mr. Lassell in 

 October, 1846, and was afterwards observed by other astronomers 

 both in Europe and the United States. The first observations 

 then made raised some suspicions as to the presence of another 

 satellite as well as of a ring analogous to that of Saturn. 

 Notwithstanding the numerous observers, and the powerful instru- 

 ments which have been directed to the planet since the date of 

 these observations, nothing has been detected which has had any 

 tendency to confirm these suspicions. 



The existence of the satellite first seen by Mr. Lassell has, how- 

 ever, not only been fully established, but its motion, and the 

 elements of its orbit, have been ascertained, first by the observa- 

 tions of M. 0. Struve in September and December, 1847, and later, 



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