178 
sun’s distance is exceedingly simple; but 
in detail, it is. exceedingly complex and 
technical. The nature of the problem may 
be explained from Fig. 2. 
Dp 
Ss Els A 
Fig. 2. 
If the earth is at H and the asteroid at 
A, an observer on the earth’s surface at P 
will see the asteroid projected on the sky 
at P’, whereas an observer at ZL will see it 
at LZ’. If the distance PL is known, as it 
is, and the angle PAL is obtained from the 
observations, the data for solving the tri- 
angle PLA are at hand, and the distance 
EA may be obtained by simple computa- 
tion. Asa second step in the solution, the 
orbit of Eros will be determined very ac- 
curately, basing the determination upon 
Newton’s law of gravitation. It will then 
be a simple matter to compute the ratio of 
the earth’s distances from the sun (HS) and 
the asteroid (HA). The distance HA hav- 
ing already been determined in miles, the 
desired distance ES will follow. 
If two observatories on opposite sides of 
the earth secure observations simultane- 
ously, say at P (Pukowa in Russia) and 
at L (Mt. Hamilton), the desired data are 
theoretically complete ; but practically they 
are insufficient. There are unavoidable 
errors in the measurement of the very 
small angle PAZ, and in the assumed posi- 
tions of the two observatories, which will 
be materially reduced by having observa- 
tions made at a large number of stations. 
The direct distance between the |Pulkowa 
and Lick Observatories is about 5,000 miles. 
This is a base line from which to measure 
a distance of 27,000,000 miles, and thence 
the three-and-one-half-fold greater distance 
to the sun. The form and the dimensions 
of the earth thus enter directly into the 
problem. An error of a quarter of a mile 
SCIENCE. 
[N. S. Vou. XIII. No. 318. 
in the assumed length of the base line leads 
to a large error in the final result. 
Instead of simultaneous observations 
made in the morning in Russia and in the 
evening in America, the angle PAL 
can be determined by evening and 
P’ morning measures secured at one 
station. 
The July conference of astronomers at the 
Paris Exposition afforded an opportunity for 
a hurried formation of cooperative plans to 
secure the observations needed. It may’ 
be said that every one of the contributing 
observatories is devoting to this problem 
all its resources which in any way prom- 
ise to improve the results. The measures 
are most advantageously made in Novem- 
ber and December; and perhaps a hundred 
observers are giving their time to this 
work. ‘The reductions will require one or 
two years’ labor, and the value of the sun’s 
distance, resulting from a combination of 
all the work of all the observatories, will 
probably not be available for two or three 
years. 
The Lick Observatory is contributing as 
follows: 
Astronomer R. H. Tucker, assisted by 
Mr. Crawford, is securing two meridian- 
circle observations of each of 700 stars, to 
determine their positions with the utmost 
accuracy. These positions, furnished by 
perhaps a dozen or more observers, will 
form the triangulation system, or ground- 
work, upon which the whole structure of 
the determination will be based. 
Astronomer W. J. Hussey and assistant- 
astronomer R. G. Aitken, with the assist- 
ance of Mr. Wright and Dr. Reese, are us- 
ing the great 36-inch telescope five or six 
nights per week, weather permitting, to 
measure the evening and morning positions 
of Eros with reference to the fixed stars 
in the asteroid’s vicinity. The positions 
of these reference stars will be secured 
by means}of photographs of the regions 
, 
