- A planet is combust when it is in conjunction with the Sun and therefore hidden from sight by the light of the Sun. Traditionally this is a serious debility and implies that the planet is weakened or restricted in power. However, if the planet is within 17 minutes of the Sun, it is termed Cazimi - in the heart of the Sun - and considered strengthened by the union.
In his Introduction, William Lilly stated that the combust planet should be in the same sign as the Sun and within 8° 30' - beyond this distance, but within 17 ° of the Sun, the planet is said to be under the Sun's beams. This condition is debilitating, but not as severe as combustion.
Lilly wrote of this: "The significator of the Querent combust, shows him or her in great fear, and overpowered by some great person". (CA., p.113)
However, later, in his horary volume, (p.300): he also writes:
A Planet within 12 degrees of the Sun, is said to be under his Beames, and then hath not fortitude, let it be in what Signe it will; when a Planet is within 16 minutes of the Sun, he is said to be in Cazimi, or heat of the Sun, and then it's an addition of fortune, and he is wonderous strong.
That aphorism was no doubt translated from the work of an older author, but it appears to acknowledge the controversy regarding whether a planet can be combust when in a different sign to the Sun. For a modern discussion of this controversial issue, see the forum thread: 'Combustion'.