It is not necessary to confine ourselves to right
angles in these measurements, for the same principles hold true in
any angles. Now, suppose two observers on the equator should look at
the moon at the same instant. One is on the top of Cotopaxi, on the
west coast of South America, and one on the west coast of Africa.
They are 90 deg. apart--half the earth's diameter between them. The one
on Cotopaxi sees it exactly overhead, at an angle of 90 deg. with the
earth's diameter. The one on the coast of Africa sees its angle with
the same line to be 89 deg. 59' 3"--that is, its parallax is 57". Try
the same experiment on the sun farther away, as is seen in Fig. 27,
and its smaller parallax is found to be only 8".85.

It is not necessary for two observers to actually station themselves
at two distant parts of the earth in order to determine a parallax.
If an observer could go from one end of the base-line to the other,
he could determine both angles. Every observer is actually carried
along through space by two motions: one is that of the earth's
revolution of one thousand miles an hour around the axis; and the
other is the movement of the earth around the sun of one thousand
miles in a minute. Hence we can have the diameter not only of [Page
70] the earth (eight thousand miles) for a base-line, but the
diameter of the earth's orbit (184,000,000 miles), or any part of
it, for such a base. Two observers at the ends of the earth's
diameter, looking at a star at the same instant, would find that it
made the same angle at both ends; it has no parallax on so short a
base. We must seek a longer one. Observe a certain star on the 21st
of March; then let us traverse the realms of space for six months,
at one thousand miles a minute. We come round in our orbit to a
point opposite where we were six months ago, with 184,000,000 of
miles between the points. Now, with this for a base-line, measure
the angles of the same stars: it is the same angle. Sitting in my
study here, I glance out of the window and discern separate bricks,
in houses five hundred feet away, with my unaided eye; they subtend
a discernible angle. But one thousand feet away I cannot distinguish
individual bricks; their width, being only two inches, does not
subtend an angle apprehensible to my vision. So at these distant
stars the earth's enormous orbit, if lying like a blazing ring in
space, with the world set on its edge like a pearl, and the sun
blazing like a diam