On Orbs and Stuff
Morin notes (1st half of 17th century) that “...how great this orb is for the individual planets has hitherto remained uncertain among astrologers, on account of the unknown fundamental of that quantity; whence, some attribute a greater orb to a particular planet and others a lesser orb, although they do not seem to differ much in turn.”
Morin then addresses the problem of establishing the semi-diameter of planetary orbs thus: The Sun, as is recognized even today by all astronomers, has a “semi-diameter of illumination” of 18°. Astronomical Twilight begins and ends when the Sun is 18° below the horizon; total darkness occurs only then. Morin then notes the Sun’s distance below the horizon when each of the planets becomes visible (or fades from visibility.) He gives 11°30’for Mars, 11° for Saturn, 10° for Jupiter and Mercury, and 5° for Venus. He then subtracts these values from 18° and so establishes the orb for each of the planets according to the strength of its brightness (at maximum.) Therefore the semi-diameters of the orbs of influence for each of the five planets is:
Venus, 13°; Jupiter and Mercury, 8°; for Saturn 7° and Mars, 6°30’.
The Moon is given 12°. The Sun, of course, receives 18°.
(Note that the sky in the times before the Industrial Revolution was crystalline, not dirty as it is today. Also note, Kepler had only recently published the meticulous naked-eye observations of Tycho Brahe and himself; these were available to Morin.)