Johannes Kepler had not a conventional attitude toward Astrology. He did question all astrological “rules”.

One should not forget that during the 16th and 17th centuries, many changes were taking place about the way the man was viewing the heavenly bodies:
Copernicus displaced the Earth from the centre of the universe;
Tycho proved that the classic serenity of the heavens was indeed subject to change;
Galileo's telescope was giving literally a new vision of the skies.

Kepler transmitted the idea that astrology would have to adjust to the new astronomy. He was not very respectful of astrological tradition. He criticised the use of the 12 houses as “Arabic sorcery”. He could not see the justification for the conventional 12 houses division of the Zodiac, even if he accepted that angles were important. He also questioned the validity of the Signs of the Zodiac. For him, they were derived from human reasoning and arithmetical convenience and not from any natural division of the heavens. He also didn’t paid any attention to planetary sign rulership and saw no reason for the traditional classification of benefit and malefic planets.

Kepler’s originality was his approach to aspects. Traditional astrology recognises five significant relationships between the planets. Ptolemy taught that their significance was derived by analogy with the ratios of the musical scale:
The conjunction is equivalent to the same two notes played in unison;
The opposition divides the circle in the ratio 1:2, which corresponds to the octave; The sextile (5:6) corresponds to a minor third;
The square (3:4) to a perfect fourth;
The trine (2:3) to a perfect fifth.

As he was placing less emphasis upon the zodiac signs, Kepler explored other possibilities of relationships. His theory was based in empirical observation of astrological effects. From his observations, he concluded that when planets formed angles equivalent to particular harmonic ratios a resonance was set up, both in the archetypal “Earth-soul” and in the souls of individuals born under those configurations.

He considered this 'celestial imprint' more important than the traditional emphasis on signs and houses. This geometric-harmonic imprint constitutes the music that impels the listener to dance as the movements of the planets, that will act as a echo of the natal theme (or vice-versa!)

In addition to the Ptolemaic aspects, Kepler proposed the quintile (72°), bi-quintile (144°) and sesqui-quadrate (135°). Continuing the analogy of the musical scale, the quintile is equivalent to an interval of a major third (4:5), the sesqui-quadrate to a minor sixth (5:8) and the bi-quintile to a major sixth (3:5).

Kepler’s concept of universal harmony will find an echo in the philosophy of astrology. Kepler was indeed the first “ modern” astrologer and his exigency about astrology theory and his disapproval of popular astrology had its roots in his conviction on the divine character of astrology: ... "a testimony of God's works and... by no means a frivolous thing".

After all, he was the author of the famous quote "don't throw the baby out with the bathwater", when he was discussing astrology.