Did you ever wonder what “o’clock” means, as in “it’s 3 o’clock”? It is a short form of the phrase “of the clock”. Before mechanical clocks dominated, the day was divided into 24 hours. There were 12 daylight hours and 12 night hours. Therefore, an “hour” in the summer was longer than an “hour” in the winter. But that was fine because we were basically an agrarian society and didn’t need precision.
When clocks were introduced, they kept track of time uniformly. Each hour was the same length. When mechanical clocks started to appear, that forced people to specific whether they meant a solar hour or a clock hour. If you’re telling someone to meet you at 3 – you needed to specify “meet me at 3 hours of the clock.”
Image by Flickr user Nick Webb used under Creative Commons License