Narada Purana (Jyotisha Selections) · by Vyasa
Chapter 6 · Calendar, Muhurta Elements
कालमानं मुहूर्ताङ्गानि च
विद्यते सा त्वखंडा स्यान्यूना चेत्खंडसंज्ञिता ।तिथेः पंचदशो भागः क्रमात्प्रतिपदादयः ॥ १५५ ॥
In the full Narada Purana (Purva Bhaga) this is chapter 56, verse 155.
... is a whole tithi (moon day). If it is shorter, it is called a part tithi. One fifteenth of a tithi, in order, is named Pratipad and the rest.
Simple meaning: A moon day lasting to sunset is whole, a shorter one partial. Each moon day splits into fifteen named parts.
क्षणसंज्ञास्तदर्द्धानि तासामर्द्धप्रमाणतः ।
Those fifteen parts are called kshanas. The Sun is fixed, the Moon movable, Mars cruel and Mercury mixed.
लघुरीज्यो मृदुः शुक्रस्त्रीक्ष्णो दिनकरात्मजः ।
Jupiter is light, Venus soft and Saturn sharp. An oil massage on a Sunday can bring trouble.
ऋक्षेशे कांतिभाग्भौमे व्याधि सौभाग्यमिंदुजे ।
An oil massage may give beauty on Monday, illness on Tuesday, loss on Friday, prosperity on Saturday.