THE RADIATION OF THE SUN.! 
By ©. G. ABBort, D. Se., 
Director Astrophysical Observatory, Smithsonian Institution. 
[With 4 plates. | 
It is really extraordmary how much has been found out about 
the sun, when it is considered that the sun lies at the immense dis- 
tance of 93,000,000 miles. There are various methods of ascer- 
taining the distance of the sun, resting upon extremely diverse 
foundations, so that the close accord of their results to within about 
one-tenth of 1 per cent gives us great confidence in the accuracy of 
the mean value. The angular diameter of the sun is also known to 
a great accuracy, and from this and the distance one determines at 
once that the diameter of the sun is 865,000 miles. How great this 
is as compared with the diameter of the earth—7,918 miles! From 
a consideration of the motions of the earth and the moon it is found 
that the mass of the sun is 332,800 times the mass of the earth. In 
accordance with this, the gravitation on the sun is enormous com- 
pared with that upon the earth, so that a body which weighs 100 
pounds at the earth’s surface would be pulled toward the center of 
the sun from the sun’s surface with a force of nearly 14 tons. 
In accordance with the measurements of the diameter and the 
mass of the sun, it follows that the average density of the material 
composing the sun is very much less than that composing the earth. 
In fact, it comes out that the sun’s material has only 1.41 times the 
density of water, whereas the mean density of the material composing 
the earth is 5.5 times the density of water. Notwithstanding this 
remarkable fact, it has been shown by spectroscopic work that the 
heavy metallic elements, such as iron, nickel, zinc, tin, copper, and 
others, occur in the sun as well as in the earth. The explanation for 
the discrepancy of density between the two bodies lies probably in 
the very high temperature of the sun, so that the elements found 
there are in the form of gases, whereas upon the earth they are in the 
form of solids. We shall return to this fact later. 
1 Presented at the meeting of the Section of Physics and Chemistry held Thursday, Jan. 8, 1914. Re- 
printed by permission from the Journal of The Franklin Institute, June, 1914. 
Fig. 1 on pl. 1 is from The Astrophysical Journal, by permission of The University of Chicago Press. 
Text fig. 1; fig. 2, pl. 1; and pls. 2 and 3 are from Abbot’s The Sun, by permission of D. Appleton & Co. 
Qn 
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