Any must match electricity production to consumption, both of which vary significantly over time. Energy derived from and varies with the weather on time scales ranging from less than a second to weeks or longer. is less flexible than , meaning it cannot easily match the variations in demand. Thus, without storage presents special challenges to .
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In Ptolemaic Egypt, the Egyptian tradition merged with Greek astronomy and Babylonian astronomy, with the city of Alexandria in Lower Egypt becoming the centre of scientific activity across the Hellenistic world.OverviewEgyptian astronomy started in times, in the . In the 5th millennium BCE, the stone circles at may have made use of astronomical alignments. By the time the historical .
Egyptian astronomy dates back to prehistoric times. The presence of at in from the show the importance of to the religious life of ,. .
Following the , The region came to be . It was ruled by the , and up until the 10th century, when the founded their own.
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Electricity can be stored directly for a short time in capacitors, somewhat longer electrochemically in , and much longer chemically (e.g. hydrogen), mechanically (e.g. pumped hydropower) or as heat. The first pumped hydroelectricity was constructed at the end of the 19th century around in Italy, Austria, and Switzerland. The technique rapidly expanded during the 196.
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