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 sidereal zodiac is set using the position of the fixed stars as the starting point of 0 degrees Aries. And tropical astrology always starts 0 degrees Aries at the Spring equinox. This is when the plane of the ecliptic intersects with the plane of the equator (hence the name Tropical -- Tropic of Cancer/Capricorn).