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.
This would be the period of apparent revolution of the sun in the celestial sphere, or, as we now know it to be, the period of one revolution of the earth in its orbit. This period is called the sidereal year, because it is fixed by the stars. Hipparchus, who flourished about 150 B.C., was the first to take exact observations of the length of the year. Ptolemy, who flourished about 300 years later, made similar ones.