Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 5 · Portents, Comets
ग्रहचारः केतवश्च
स्तम्भशूलांकुशाकारा आंतरिक्षाः प्रकीर्तिताः ।नक्षत्रसंस्थिता दिव्या भौमा ये भूमिसंस्थिताः ॥ ९४ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 94.
Those shaped like a pillar, a spear or a goad, with the above, are called comets of the mid-sky. Those standing in the nakshatras (stars) are heavenly. Those on the ground are of the earth.
Simple meaning: Comets in the stars are heavenly, those shaped like objects are of the mid-sky, those on the ground earthly.
एकोऽपि भिन्नरूपः स्याज्जंतुर्नाम शुभाय वै ।
A comet can take different forms. How long it is seen sets how long its result lasts.
तावन्मासैः फलं यच्छत्यष्टौ सारव्यवत्सरैः ।
A comet seen for some days gives results for that many months.
ह्रस्वः स्निग्धः सुप्रसन्नः श्वेतकेतुः सुवृष्टिकृत् ।
A short, bright white comet that soon fades brings good rain. A long one brings drought.