_Astr. and Astrophysics_,
vol. xi., p. 720.]

[Footnote 732: _Proc. Roy. Irish Acad._, vol. ii., No. 2, 1892.]

[Footnote 733: _Am. Jour. of Sc._, vol. xxi., p. 187.]

[Footnote 734: _Amer. Jour. of Science_, vol. v., p. 245, 1898.]

[Footnote 735: For J. W. Draper's partial anticipation of this result,
see _Ibid_. vol. iv., 1872, p. 174.]

[Footnote 736: _Phil. Mag._, vol. xiv., p. 179, 1883.]

[Footnote 737: "The Solar and the Lunar Spectrum," _Memoirs National
Acad. of Science_, vol. xxxii.; "On hitherto Unrecognised Wave-lengths,"
_Amer. Jour. of Science_, vol. xxxii., August, 1886.]

[Footnote 738: _Astroph. Jour._, vol. i., p. 162.]

[Footnote 739: _Annals of the Smithsonian Astroph. Observatory_, vol.
i.; _Comptes Rendus_, t. cxxxi., p. 734; _Astroph. Jour._, vol. iii., p.
63.]

[Footnote 740: _Phil. Mag._, July, 1901.]

[Footnote 741: _Comptes Rendus_, t. xcii., p. 701.]

[Footnote 742: _Nature_, vol. xxvi., p. 589.]

[Footnote 743: _Phil. Trans._, vol. cxxxii., p. 273.]

[Footnote 744: _Ann. de Chim._, t. x., p. 321.]

[Footnote 745: _Ibid._, t. xi., p. 505.]

[Footnote 746: _Comptes Rendus_, t. cxii., p. 1200.]

[Footnote 747: _Wied. Ann._, Bd. xxxix., p. 294; Scheiner, _Temperatur
der Sonne_, pp. 36, 38.]

CHAPTER VI

_THE SUN'S DISTANCE_

The question of the sun's distance arises naturally from the
consideration of his temperature, since the intensity of the radiations
emitted as compared with those received and measured, depends upon it.
But the knowledge of that distance has a value quite apart from its
connection with solar physics. The semi-diameter of the earth's orbit is
our standard measure for the universe. It is the great fundamental datum
of astronomy--the unit of space, any error in the estimation of which is
multiplied and repeated in a thousand different ways, both in the
planetary and sidereal systems. Hence its determination was called by
Airy "the noblest problem in astronomy." It is also one of the most
difficult. The quantities dealt with are so minute that their sure grasp
tasks all the resources of modern science. An observational inaccuracy
which would set the moon nearer to, or farther from us than she really
is by one hundred miles, would vitiate an estimate of the sun's distance
to the extent of sixteen million![748] What is needed in order to attain
knowledge of the desired exactness is no less