Thus, if we imagine two astronomers making
observations of the sun from opposite sides of the
earth at the same time, it is obvious that to these
observers the sun will appear to be at two different
points in the sky. Half the angle measuring this difference
would be known as the sun's parallax. This
would depend, then, upon the distance of the earth
from the sun and the length of the earth's radius.
Since the actual length of this radius has been determined,
the parallax of any heavenly body enables
the astronomer to determine its exact distance.

The parallaxes can be determined equally well, however,
if two observers are separated by exactly known
distances, several hundreds or thousands of miles apart.
In the case of a transit of Venus across the sun's disk,
for example, an observer at New York notes the image
of the planet moving across the sun's disk, and notes
also the exact time of this observation. In the same
manner an observer at London makes similar observations.
Knowing the distance between New York
and London, and the different time of the passage, it is
thus possible to calculate the difference of the parallaxes
of the sun and a planet crossing its disk. The
idea of thus determining the parallax of the planets
originated, or at least was developed, by Halley, and
from this phenomenon he thought it possible to conclude
the dimensions of all the planetary orbits. As
we shall see further on, his views were found to be
correct by later astronomers.