Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
धातृ इन्द्रादितीज्याख्या विष्णवर्कत्वष्टृवायवः ।अह्नः पंचदशो भागस्तथा रात्रिप्रमाणतः ॥ २२६ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 226.
Dhatri, Indra, Aditi, Jupiter, Vishnu, Sun, Tvashtri and Vayu. A muhurta (48 minutes) is a fifteenth part of the day, and likewise by the measure of the night.
Simple meaning: Each muhurta is a fifteenth part of the day, and a fifteenth part of the night.
मुहूर्तमानं द्वरावक्षणर्क्षाणि समेश्वरम् ।
A muhurta is one fifteenth of day or night. The bad muhurtas for each weekday begin here.
राक्षसाख्यौ ब्राह्मपित्र्यौ भर्गाजांशाविनादिषु ।
Each weekday has one or two bad muhurtas. Skip them for good acts.
येषु ऋक्षेषु यत्कर्म कथितं निखिलं च तत् ।
An act told for a star can also be done in the hour of that star's god.