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the periods into which the time between sunset and sunrise was divided
They are so called because watchmen relieved each other at each of these periods. There are frequent references in Scripture to the duties of watchmen who were appointed to give notice of the approach of an enemy (2 Samuel 18:24-27; 2 Kings 9:17-20; Isaiah 21:5-9). They were sometimes placed for this purpose on watch-towers (2 Kings 17:9; 18:8). Ministers or teachers are also spoken of under this title (Jeremiah 6:17; Ezek. 33:2-9; Hebrews 13:17).
The watches of the night were originally three in number.