«بالضرورة كل قمر منخسف وقت حيلولة الأرض بينه وبين الشمس لا دائما»
Necessarily the moon is eclipsed when the Earth intervenes between it and the sun, but not permanently.
The time of intervention identifies the appointed condition; necessity is asserted under that condition. “Not permanently” denies that eclipse holds throughout the moon's entire existence.
As an empirical qualification, a real lunar eclipse requires the appropriate celestial alignment, not mere verbal mention of Earth's intervention. The textbook's logical point is the combination specified time + non-permanence.