ASTRONOMICAL MAGNITUDES AND DISTANCES. 291 

 ASTEONOMICAL MAGNITUDES AND DISTANCES. 



By Professob H. S. CAEHART. 



THE magnitudes and distances considered in physical astronomy 

 are so immense that we cannot hope to reach even a faint con- 

 ception of them except by illustration and comparison. If even then, 

 with our best effort, we fail to measure up to the magnificent dimen- 

 sions of the universe, the attempt will at least enlarge our intellectual 

 conceptions, and lead us out mentally into a broader place. 



The results reached by modern astronomy, respecting the dimen- 

 sions and distances of the heavenly bodies, are based on two lines, the 

 radius (or semi-diameter) of the earth and the radius of its orbit ; the 

 former is accurately known, the latter approximately. In modern times 

 the highest refinements of engineering skill have been applied to the 

 measurement of base-lines, which furnish through triangulation arcs 

 of a meridian. So thoroughly has this work been done that, in the 

 opinion of Prof. Young, the error in the ascertained length of the 

 earth's equatorial radius cannot exceed "HOO feet. This radius forms 

 our base-line for broader operations. The equatorial, horizontal parallax 

 of the moon, or the angle subtended at the moon by the earth's equa- 

 torial radius, is found to have an average value of 57' 2". Hence by 

 plane trigonometry the moon's mean distance is 238,885 miles, or nearly 

 ten times the circumference of the earth. Light, with a velocity of 

 186,500 miles a second, travels from the earth to the moon and back 

 again in two and a half seconds, thus producing that faint illumination 

 of the dark portion of the new moon turned toward us. Knowing the 

 moon's distance, the measurement of its apparent diameter in minutes 

 of arc furnishes immediately its absolute diameter in miles. 



So, then, this queen of the night, once supposed to be a kind of 

 lantern, fed by exhalations from the ocean, is a body -^ as large as 

 the earth. It is our nearest celestial neisrhbor — in fact, a little out- 

 lying, condensed nebulosity ; and if we had a weather-station on the 

 lunar mountain Tycho, connected by telegraph with Washington, Gen- 

 eral Myer would receive the lunar weather-reports in fifteen seconds by 

 electricity. 



Aristarchus, in the third century before the Christian era, attempted 

 to use the moon's distance to compute the greater distance of the sun ; 

 but the method failed, and astronomers were compelled to fall back on 

 the radius of the earth as a base-line for a still grander triangulation. 

 The parallax of Mars, at opposition, gave us the first approximation to 

 the sun's distance ; then the transit of Venus furnished a nearer esti- 

 mate ; latterl}', Le Verrier, who found Neptune by figures, has also 

 determined the distance of the sun by means of planetary perturba- 

 tions ; still a fourth method combines the retardation of the eclipses 



