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Other uses: Combustion is a common noun whose verb form is combust or Heliacal Setting

"Combustion" occurs when the unassisted view of a planet from the earth is obscured by the light of the sun.

The Moon, Mars, Mercury, Jupiter, Venus and Saturn become combust, during their direct motion, when their longitudinal arc is 12, 17, 14, 11, 10 & 16 degrees respectively from the Sun on either side. During retrograde motions Mercury and Venus become combust when the said arc is 12 & 8 degrees respectively. Exact longitudinal arc depends upon declination of the Sun and the concerned planet. Daily ephemeredes do indicate the time and date of start and end the combustion of different planets as seen from a specific place on the earth. The Moon, Mars, mercury Jupiter, Venus and Saturn remain combust for 2, 118, 11 to 20, 28, 53 and 57 days approximately during their direct motion and during retrograde motion Mercury and Venus remain combust for 21 to 33 and 5 days respectively. Mars Jupiter and Saturn do not combust when in their retrograde motion. It is astronomically termed as “heliacal setting”.

In Vedic / Hindu system of astrology, a planet is said to be devoid of its strength when in combustion. This astronomical phenomenon weakens a planet most adversely. However Mercury is said to be extremely weak if it is in the same sign as the Sun in Navamsa divisional chart (the Navamsa is a harmonic chart which divides each sign into 9 parts.) A combust Moon is to be avoided in all good Jyotish election charts. A planet within one degree from the Sun is said to the most powerless. In addition to adverse effects in the natal astrology, a combust planet is too weak to render beneficial effects during its favorable transits to the natal position.

An eclipse of the luminaries is analogous to combustion, in which case either of the luminaries is not visible because of occlusion. The period of the occlusion is a very feared astrological effects, especially in electional astrology. The constellation occupied by the Moon at the time of solar or lunar eclipse must be left in elections being highly malefic. The period for which the said constellation is to be shunned, depends upon the degree of severity of the eclipse. As per Hindu electional astrology, the intervening period between the two eclipses is prohibited in elections.

For technical purposes, in Western astrology, most ancient and medieval authorities considered a planet combust or burnt when its position was within 5-8 degrees on either side of the Sun. However, a planet will continue to be weakened by Sun until it has elongated by 15-17 degrees from it. (Lilly says 17 degrees on p. 113 of Christian Astrology.) This positioning is said to be under the beams of the Sun, and although it may be stronger than being combust, it will still cause a noticeable weakening in a planet's effectiveness.

As per the ancient western astrologers, a planet within arc of 17 minutes the Sun’s longitude is “in heart of the Sun” & is said to be a cazimi planet. William Lilly opines it to be “ wondrous strong”. The traditional astrologers give (+) 5 points to evaluate strength of a cazimi planet. But some modern astrologers disagree with this and opine it as “silly distinction”. Further in evaluation of strength of planets, (-) 5 points are allocated to combust planet (between 17' and 8.5° from the Sun); and (-) 4 points under beams of the Sun (between 8.5° and 17° of the Sun). Thus a planet when even not heliacal set (as per astronomy) is considered week by the accidental debilities mode of calculation.

Since Mercury remains 28 degrees from either side of the Sun and is hence combust most of the time (about 150 days in a year), it is difficult to apply the strictures of combustion to that planet. Furthermore, the Moon is combust two days out of every lunar month, although the period of the New Moon, when the Moon is in this condition, has always been regarded as a perilous one astrologically.

See also: Cazimi, Conjunction (astronomy)