Quinlan-McGrath, M., Influences: Art, Optics, and Astrology in the Italian Renaissance, 36-7.
The intersections of the horizon circle and the meridian circle divide the heavens into four quadrants relative to the location on the Earth that is to be studied. A third circle, the ecliptic, the belt of the zodiacal constellations against which the planets appear to move, intersects the meridian and horizon circles relative to that earthly point as well. Over the course of the year, this ecliptic circle gradually seems to ascend and descend relative to the merid-ian at that place on the Earth. Owing to this continual shining of the heavens during the course of a year, we might visualize the quadrants as seasonally expanding and contracting.
Within the quadrants, the cusps or divisions between these twelve houses are measured and marked along the 360 degrees of that ecliptic circle. There were slightly different ways to apportion the 360 degrees that were divided among the twelve houses, depending on the house system chosen by the astrologer. But, whatever the house system chosen, the four cardines or cusps of the quadrants - marked by the Ascendant (the point at which the ecliptic crosses the eastern horizon), the Midheaven (the point at which the ecliptic intersects the meridian circle above the horizon), the Descendant (the point at which the ecliptic intersects the western horizon), and the Lower Midheaven (the point at which the ecliptic intersects the meridian circle below the horizon) - are usually the same in all house systems.
The degree of the ecliptic circle that intersects the eastern horizon at the time in question becomes the Ascendant degree. The rest of the ecliptic circle follows counterclockwise from this point, and all the subsequent cusps in the horoscope diagram are determined from it. The “moving” sky is continually passing through this Ascendant point - ascending in the east with the passage of the twenty-four-hour day. This is means that each of the 360 degrees takes roughly four minutes of clock time to rise or set. Moment by moment, the stars and planets, traveling at diff ering speeds, some of the planets occasion-ally retrograde, move through this framework of the horoscope chart. Moment by moment, the angles of the celestial rays, hitting a unique point on the Earth, are changing. This is ever-changing complexity of radiation was, we will see, understood to influence the great variety of earthly entities as these were all in the process of coming into existence or fading out of it.
Quinlan-McGrath, M., 2013, Influences: Art, Optics, and Astrology in the Italian Renaissance. Chicago/ Londres: The University of Chicago Press.
