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.
Sidereal year = 365.256366 days (period of Earth's revolution about the Sun with respect to the stars). Keeps star time. Tropical year (period of Earth's revolution about the Sun with respect to the vernal equinox) = 365.242199 days.