When astronomy had reached a stage of development so that such things as eclipses could be predicted with anything like accuracy, the occult knowledge of the astronomer would be unquestioned.

Turning this apparently occult knowledge to account in a mercenary way would then be the inevitable result, although it cannot be doubted that many of the astrologers, in all ages, were sincere in their beliefs.
Later, as the business of astrology became a profitable one, sincere astronomers would find it expedient to practise astrology as a means of gaining a livelihood.

Such a philosopher as Kepler freely admitted that he practised astrology "to keep from starving," although he confessed no faith in such predictions.

"Ye otherwise philosophers," he said, "ye censure this daughter of astronomy beyond her deserts; know ye not that she must support her mother by her charms." Once astrology had become an established practice, any considerable knowledge of astronomy was unnecessary, for as it was at best but a system of good guessing as to future events, clever impostors could thrive equally well without troubling to study astronomy.

The celebrated astrologers, however, were usually astronomers as well, and undoubtedly based many of their predictions on the position and movements of the heavenly bodies.