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I can explain what Horrocks did from my perspective, as a mathematician.
He was active in the late 1630s. By that time Kepler laws were more than 100 years old
and were universally accepted by astronomers.

However, the motion of planets does not satisfy Kepler's laws exactly.
The largest deviation from Kepler's laws happens for the Moon, and at the same 
time the Moon is most important
(in the ancient times for eclipse predictions, in 17 century for longitude.)

So the main challenge was to explain those deviations from Kepler laws.
Kepler's laws are obeyed exactly by a system of two bodies which attract each 
other by the inverse square law.
So all deviations must be explained  by gravitational effect of other bodies.
In the case of the Moon the three relevant bodies are Earth, Moon and Sun.

And the first success here is due to Horrocks, who built the best theory 
theory of Moon's motion at that time.
(He also explained perturbations of Saturn's orbit by Jupiter by gravitation. And tides as well).

This was 40 years before Newton. 
The first main applications of Newton's theory of gravitation were: 
explanation of Moon motion, explanation of
tides and explanation of the shape of the Earth. Of these Moon motion is the 
most challenging mathematical
problem, and also it has an important practical application (to the 
longitude). Newton spent a lot of efforts on it,
but did not succeed much in improving on Horrocks's theory. Only much later, by combined efforts
of Clairaut, Euler and Mayer, Lunar theory was developed sufficiently for 
applications to determination of longitude. 

So from my point of view, Horrocks's contribution to Lunar theory, and the general idea to apply
gravitation to perturbation of orbits is more important than observation of
a transit of Venus. 

Alex.


 

   

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