quent corrections
and repeated measurement will change Mr. Pease's result somewhat,
but it is almost certainly within 10 or 15 per cent of the truth.
We may therefore conclude that the angular diameter of Betelgeuse
is very nearly the same as that of a ball one inch in diameter,
seen at a distance of seventy miles.

[Illustration: Fig. 26. Arcturus (within the white circle), known
to the Arabs as the "Lance Bearer," and to the Chinese as the "Great
Horn" or the "Palace of the Emperors" (Hubble).

Its angular diameter, measured at Mount Wilson by Pease with the
20-foot Michelson interferometer on April 15, 1921, is 0.022 of a
second, in close agreement with Russell's predicted value of 0.019
of a second. The mean parallax of Arcturus, based upon several
determinations, is 0.095 of a second, corresponding to a distance of
34 light-years. The linear diameter, computed from Pease's measure
and this value of the distance is about 21 million miles.]

But this represents only the angle subtended by the star's disk.
To learn its linear diameter, we must know its distance. Four
determinations of the parallax, which determines the distance,
have been made. Elkin, with the Yale heliometer, obtained 0.032
of a second of arc. Schlesinger, from photographs taken with the
30-inch Allegheny refractor, derived 0.016. Adams, by his spectroscopic
method applied with the 60-inch Mount Wilson reflector, obtained
0.012. Lee's recent value, secured photographically with the 40-inch
Yerkes refractor, is 0.022. The heliometer parallax is doubtless
less reliable than the photographic ones, and Doctor Adams states
that the spectral type and luminosity of Betelgeuse make his value
less certain than in the case of most other stars. If we take a
(weighted) mean value of 0.020 of a second, we shall probably not
be far from the truth. This parallax represents the angle subtended
by the radius of the earth's orbit (93,000,000 miles) at the distance
of Betelgeuse. By comparing it with 0.047, the angular diameter of
the star, we see that the linear diameter is about two and one-third
times as great as the distance from the earth to the sun, or
approximately 215,000,000 miles. Thus, if this measure of its distance
is not considerably in error, Betelgeuse would nearly fill the
orbit of Mars. All methods of determining the distances of the
stars are subject to uncertainty, however, and subsequent measures
may reduce this figure very appreciably. But the