Time

The rotation of the Earth and moon determines the duration of years, seasons, months, and days. The cycle of rotation of the Earth around the Sun is one year. During this time, the stars are replaced by other in the night sky, and in weather there are seasonal changes.
Once was discovered this sequence of events, people learned to extract from their observations of practical use. For example, the appearance of a certain star meant that the time has come for Seva. The ancient Egyptians knew; the annual flooding of the Nile always began soon after the appearance on the horizon of a star Sirius.