1.     Astronomer Edmund Halley was
attempting to calculate the distance from the earth to the sun in 1716. The
method he proposed used the transit of
Venus. His procedure is based on the idea of “Parallax”. This is the effect
when two observers are observing the same object from different orientations,
and therefore will observe a different distance to that object. In the case of
Halley that  meant one at the north pole
and one at the  south pole. Those
distances are measured, and then they form a triangle with the tip at Venus and
the other two vertices at the north and south pole. Based on trigonometry the distance
from the earth to Venus can then be determined. Kepler had already worked out
the relative distances of planets from the sun, so based on that ratio the astronomical
unit could be calculated.