Brihat Samhita
तामसकीलकसंज्ञा राहुसुताः केतवस्त्रयस्त्रिंशत् । वर्णस्थानऽकारैस्तान्दृष्ट्वा अर्के फलं ब्रूयात् ॥ ३-०७ ॥
The dark spots, also known as Ketus, the sons of Rāhu are Tāmasa, Kīlaka and the like, and are 33 in number. How they affect the earth depends upon their color, position and shape.
english translation
tAmasakIlakasaMjJA rAhusutAH ketavastrayastriMzat । varNasthAna'kAraistAndRSTvA arke phalaM brUyAt ॥ 3-07 ॥
hk transliteration by Sanscript