Shula is the ninth nityayoga, and its name means a spear or a sharp pain. It is one of the inauspicious yogas, associated with conflict, injury, and piercing difficulty, and the panchanga counts it among the configurations to be avoided for auspicious beginnings, especially journeys and ceremonies. Like the others it is derived from the combined longitude of Sun and Moon divided into twenty-seven equal parts. In choosing a muhurta an astrologer sets aside the malefic yogas, and Shula, with its imagery of the spear, is one the tradition treats with caution. These are the conventions of the calendar tradition, which fixes the character of each yoga; the astronomical basis is only the arithmetic of the two lights' summed motion through the month.
Shula is also the name of the trident, trishula, the weapon most closely associated with Shiva in Hindu iconography, though the yoga's own name uses the simpler, unprefixed word for a spear or a piercing pain rather than the specifically three-pronged trident; the two words share a root sense of a piercing point rather than naming identical objects. Classical Indian medicine, Ayurveda, also uses shula as a technical term for a broad category of sharp, stabbing bodily pain, so the yoga's inauspicious association with injury and piercing difficulty draws on a term with an established, non-astrological technical use in the medical literature as well as in general vocabulary.