Historical Time Measurers
The Sundial
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The sundial is probably one of the earliest ways in which people were able to keep track of time. It utilized the angle of the sun in the sky to cast a shadow from the gnomon (the part of the sundial that sticks up) onto the dial. The time was then determined by where the shadow was laying when the gnomon was facing north. Because the sundial is dependent upon the sun's location in the heavens it has several flaws. The most obvious one is the fact that it requires direct sunlight to even give the time at all so on an overcast day the time would have been relatively unknown.
The other main flaw a sundial has is that the length of the hour changes with the seasons of the earth. This has to do with the fact that daylight lasts longer in the summer than in the winter and that the orbit of the sun is not circular, or in other words "uniform". So if one wanted to have everyday be the same length then the mean time would have to used rather than the apparent time which sundials naturally give. (see this link for more info on apparent and mean time).
Another flaw with sundials is that they have to be made specifically for a certain location because the angle the sun has in the sky is different at all latitudes on the earth. For instance, "a sundial brought to Rome (41°54? N) from Catania Sicily (37°30? N), in 263 B.C. told Romans the wrong time for 100 years" (encyclopedia Britannica).
Other Less Prominent Devices
Other devices that were used to keep time were the Clepsydra, the Candle Clock, and the Astrolabe. Each device has its own particular methedology, uses, and disadvantages. The Clepsydra is a device that utilizes a container of water that fills and empties periodically to measure time. The Candle Clock uses the burning of a candle to tell how much time has passes: lines on the edge of the candle mark an hour of the day and as the candle burns down it reaches each of these marks in turn. The Astrolabe is a device that utilizes the movements of the starts in the heavens to track what time it is. If you would like to learn more about how these devices work check out this website.