Original Text
On that day the day decreases by +two+ parts, and amounts to ten parts and the night to eight parts.
Simple English
On that day, the day becomes two parts shorter, making it ten parts long, while the night is eight parts long.
On that day the day decreases by +two+ parts, and amounts to ten parts and the night to eight parts.
On that day, the day becomes two parts shorter, making it ten parts long, while the night is eight parts long.