How did we first determine the Earth's departure from sphericity? I'm quite interested to know the history of the study of the Earth's departure from spherical symmetry. What were the first methods used to model the non-spherical geometry of the Earth? For example, there are geodesy satellite and missions used now to determine the gravity field of the Earth but that is "recent" technology. They obviously had other methods prior to this.
Isaac Newton was the first to put it together and realized the Earth had an oblate shape in 1687.    See 1672 - Wikipedia.

In 1666 Robert Hooke reasoned that pendulums could measure variation in gravity, but this remained unobserved until 1671, when Jean Richter observed that pendulums kept different time near the equator (French Guiana compared to Paris).  

Around that time, 1673, Christian Huygens observed that the wide swing of a pendulum would move in a very slight oval, so to make pendulum clocks more accurate, the swings were kept to a low angle and the swing kept perpenducular   That, however, has to do with the Earth's rotation, but not your question specifically.

Newton was the first to put it all together that the observed gravitational fluctuation was due to the equatorial bulge and faster speed of rotation near the equator.    Article here also credits him with being the first.

Also, related:   https://en.wikipedia.org/wiki/History_of_geodesy