Rivers were used to turn watermills and the Romans even used geothermal water for heating. Until the middle of the 18th century renewable sources of energy were the only forms of energy available, but then, with the advent of the Industrial Revolution, fossil fuels began to be exploited.
The Sidereal Zodiac is the position of the sun referenced against the star background, as a measure of the flow of time. The framework for the star background is the constellations the sun passes in front of. This is the one astronomers use.
The synodic period differs from the sidereal period due to the Earth orbit around the Sun, with one less synodic period than lunar orbit per solar orbit. The sidereal period is the temporal cycle that it takes an object to make one full orbit around the Sun, relative to the stars.