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 Rashi is the sign in which Moon was placed at your birth time. It's also called the Moonsign. The Moonsign is one of the most important points in Vedic Jyotish. Considerations like the planetary dasha, transits etc. are made in reference to the Rashi.