Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 5 · Portents, Comets
ग्रहचारः केतवश्च
तस्मादन्वेषणीयं तत्कालज्ञानाय धीमता ।उत्पातरूपाः केतूनामुदयास्तमया नृणाम् ॥ ९२ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 92.
So the wise should look for that, to know the time. The rising and setting of comets are portents for people.
Simple meaning: A wise person watches the rising and setting of comets to know the time and its results.
दिव्यांतरिक्षा भौमास्ते शुभाशुभफलप्रदाः ।
Comets are of the heavens, the mid-sky or the earth, and give good or bad results.
स्तम्भशूलांकुशाकारा आंतरिक्षाः प्रकीर्तिताः ।
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.