FREE BOOKS

Author's List




PREV.   NEXT  
|<   22   23   24   25   26   27   28   29   30   31   32   33   34   35   36   37   38   39   40   41   42   43   44   45   46  
47   48   49   50   51   52   53   54   55   56   57   58   59   60   61   62   63   64   65   66   67   68   69   70   71   >>   >|  
ow much was learned from the transits of Venus, and how much remained to be settled. They left the result in doubt over a range of about two million miles. SUN'S DISTANCE, IN MILLIONS OF MILES, AS FOUND BY DIFFERENT OBSERVERS =Before the Transits of Venus= estimates varied between =96= million miles (Gilliss and Gould, 1856) and =91= million (Winneche, 1863), a range of 5 million miles. =The Transits of 1874 and 1882= gave results lying between =93-1/4= million (Airy, from British observations of 1874), =92-1/2= million (Stone, from British observations of 1882), and =91-1/2= million (Puiseux, from French observations), a range of 1-3/4 millions. =Gill's Heliometer results= all lie very near =93= millions. The observations of Mars in 1877 give about 100,000 miles over this figure: but the observations of Victoria, Iris, and Sappho, which are more trustworthy, all agree in giving about 100,000 miles _less_ than the 93 millions. It became necessary, therefore, to look to other methods; and before the second transit of 1882 was observed, an energetic astronomer, Dr. David Gill, had already put into operation the method which may be now regarded as the standard one. [Sidenote: Modern method for sun's distance.] [Sidenote: Photography.] [Sidenote: Dr. Gill's expedition to Ascension.] We have said that the _relative_ distance of Venus from the sun is accurately known from observations of the exact time of revolution. It is easy to see that these times of revolution can be measured accurately by mere accumulation. We may make an error of a few seconds in noting the time of return; but if the whole interval comprises 10 revolutions, this error is divided by 10, if 100 revolutions by 100, and so on; and by this time a great number of revolutions of all the planets (except those just discovered) have been recorded. Hence we know their relative distances with great precision; and if we can find the distance in miles of any one of them, we can find that of the sun itself, or of any other planet, by a simple rule-of-three sum. By making use of this principle many of the difficulties attending the direct determination of the sun's distance can be avoided; for instance, since the sun's light overpowers that of the stars, it is not easy to directly observe the place of the sun among the stars; but this is not so for the planets. We can photograph a plane
PREV.   NEXT  
|<   22   23   24   25   26   27   28   29   30   31   32   33   34   35   36   37   38   39   40   41   42   43   44   45   46  
47   48   49   50   51   52   53   54   55   56   57   58   59   60   61   62   63   64   65   66   67   68   69   70   71   >>   >|  



Top keywords:

million

 

observations

 
distance
 

millions

 

revolutions

 

Sidenote

 

results

 

planets

 

British

 

accurately


relative
 

method

 

revolution

 

Transits

 

accumulation

 

measured

 

comprises

 

noting

 

return

 

seconds


interval

 

attending

 

direct

 

determination

 

avoided

 

difficulties

 

making

 

principle

 

instance

 
photograph

observe

 
directly
 

overpowers

 

discovered

 

recorded

 

number

 

distances

 

planet

 

simple

 

precision


divided

 

observed

 

Gilliss

 

varied

 

estimates

 

DIFFERENT

 

OBSERVERS

 
Before
 

Winneche

 

Puiseux