Kepler cast horoscopes for a living, most famously for Wallenstein, and thought most of what other astrologers did indefensible. His position is set out in De Fundamentis Astrologiae Certioribus of 1602 and in the Tertius Interveniens of 1610, which warns theologians and physicians that in throwing out astrology they may be throwing out something worth keeping along with it.
What he wanted kept was narrow. He rejected the twelve houses, distrusted the signs of the zodiac as arbitrary divisions of a continuous circle, and would not accept that planets carry characters or govern parts of the body. What he thought defensible was the aspects, the angular relations between planets, because an angle is a real geometrical fact and because the ratios that make a musical interval consonant are the same ratios. On that reasoning he added aspects the tradition did not have, dividing the circle by five and by eight as well as by three, four and six: the quintile at seventy two degrees, the biquintile, the semi-square and the sesquiquadrate. Western astrology took them up and uses them still, usually without knowing where they came from.
He also held that the soul responds to these geometrical relations at the moment of birth, which is how he could hold both halves of his position at once. His mother Katharina was accused of witchcraft and stood trial between 1615 and 1621. He conducted much of her defence himself, and she was released.