«كل قمر منخسف بالضرورة وقت حيلولة الأرض بينه وبين الشمس»
Every moon is necessarily eclipsed when Earth intervenes between it and the Sun.
The textbook specifies the occasion when Earth lies between Sun and Moon, treating the eclipse relation as necessary in that arrangement.
The logical lesson is the time qualification: the claim does not assert that the Moon is eclipsed throughout its existence. It applies to the stated occasion. Confusing the time-specific mode with unrestricted permanence would erase that essential limitation.