FEBRUARY 1, 1901. ] 
knowledge of the distance between the earth 
and sun. Fully half the resources of the 
Lick Observatory, the Lick Astronomical 
Department of the University of California, 
have been devoted to this work for three 
months past, and the observations will con- 
tinue another month. 
The determination of the value of this as- 
tronomical unit of distance is one of the 
most famous problems in the science, and 
a great variety of methods has been used. 
Perhaps the best determination is that made 
by Sir David Gill, of the Cape of Good Hope, 
from heliometer observations of the nearer 
asteroids. The four hundred small planets 
—asteroids—discovered up to 1898 all move 
in orbits situated entirely outside the orbit 
of Mars. The observations of Victoria, Iris 
and Sappho, which approach nearer to the 
earth than any of the other members of the 
asteroid group, led to the conclusion that the 
average distance from the earth to the sun 
is about 92,900,000 miles. The unavoidable 
errors to which all such observations are 
subject leave an uncertainty of some 150,000 
miles in this value: it may be too large or 
too small by this amount. 
The accuracy of results obtained in this 
manner depends upon the distance of the 
observed body from the earth and upon its 
definiteness as a point of observation. Mars 
possesses the advantage of being nearer the 
earth than the asteroids, but this advantage 
is greatly outweighed by the fact that an 
asteroid-point can be observed much more 
accurately than a large planet-disk. Gill’s 
splendid work left much to be desired, but 
there was no prospect that his value of the 
unit could be improved with instruments 
now available. 
An asteroid discovered by Dr. Witt in 
Berlin in 1898, to which he gave the name 
Eros, is very remarkable in that its orbit 
lies partly within and partly without the 
orbit of Mars. It approaches the earth 
more closely than any other member of 
SCIENCE. 
led 
our system,except the moon. The forms and 
relative positions of the orbits of the earth, 
Eros and Mars are shown approximately 
by the accompanying diagram, Fig.1. The 
Fig. 1. 
orbits of Mars and Eros appear to intersect, 
but this is merely apparent, from the pro- 
jections of the two on the plane of the paper. 
The plane of the orbit of Eros makes an 
angle of about eleven degrees with the plane 
of the orbit of Mars (and the earth), and 
the two orbits are situated like two adjacent 
links of a chain, without approaching each 
other very closely. 
As soon as the orbit of this wonderfully 
located asteroid was computed and pub- 
lished, astronomers realized its possibilities 
for improving our knowledge of the sun’s 
distance. As stated above, an asteroid’s 
value for this purpose depends upon its 
closeness to the earth. The distance of 
Eros at this opposition will have a mini- 
mum value of twenty-seven million miles. 
Unfortunately, the more favorable opposi- 
tion of 1896, when the distance dimin- 
ished to fifteen million miles, was not 
available, as the planet remained undis- 
covered ; and an equally good opposition 
will not occur again for a quarter of a 
century. 
In outline, this method of measuring the 
