Physics Topics are often described using mathematical equations, making them precise and quantifiable.
What are Some Ancient Time Measuring Devices
Time is a moment or duration in which things occur. The knowledge of time is essential for carrying on our daily life activities. For example, our school starts at a particular ‘time’. We use our wrist watch to know the time so that we may reach the school on time.
Similarly, we need to know time to catch a bus, a train or an aeroplane for making a journey. We also have to know time to listen to a particular radio programme or to watch a favourite television programme. We measure time with the help of clocks and watches.
Measurement of Time in Ancient Times
The people did not have clocks or watches for measuring time in olden days. In ancient times, people used some natural events which repeated regularly after fixed time intervals, to measure time. For example, it was observed that full moon occurs regularly after fixed time interval and the sunrise also occurs regularly after fixed time interval. So, the two natural ‘periodic events’ which were used in ancient times to measure time were the occurrence of ‘full moon and ‘sunrise.
- The time from one full moon to the next full moon was called a ‘month’.
- The time from one sunrise to the next sunrise was called a ‘day’.
In order to measure time intervals shorter than ‘one day’, a number of time measuring devices were invented in ancient times. Some of the important time measuring devices used in ancient times were : Sundial, Sand clock and Water clock. We will now describe all these devices briefly.
Sundial is one of the earliest devices used for measuring time. A sundial measures time by the position of the shadow cast by the sun. We know that the sun rises in the east, appears to move in the sky during the day, and finally sets in the west.
Due to this apparent movement of the sun in the sky , the position of shadow of a vertical object on the ground changes continuously throughout the day. The sundial has a triangular blade which is fixed vertically on a dial marked with hours of the day (see Figure).
The sun casts a shadow of this vertical blade on the dial. As the position of sun changes in the sky, the position of shadow of blade on the dial also changes. The position of shadow of vertical blade on the dial gives the time of the day. At many places, huge sundials have been constructed on the ground in the form of triangular walls. Some historical sundials still exist in India at Jantar Mantar in Delhi and at Jaipur in Rajasthan.
2. Sand Clock
Another early device for measuring time was the sand clock. Sand clock is also known as sand-glass or hour-glass. The sand clock uses the flow of sand (from one glass bulb to another) to measure time. The length of time taken by the sand to pass from one bulb of the sand clock to the other, gives a constant time interval. A sand clock consists of two glass bulbs joined together through a narrow tube (see Figure).
Some sand is filled in one of the glass bulbs. The two glass bulbs are closed at the outer ends. When the sand clock is kept in such a way that the sand is in the upper bulb, the entire sand flows down to the lower bulb slowly, in a fixed time interval (The whole sand does not fall from the upper bulb to the lower bulb at once because it has to pass through a narrow tube).
When the sand clock is to be used again, it is turned upside down so that the sand containing bulb comes at the top. The sand clock (or sand glass) is usually filled with so much sand that it takes ‘one hour’ to fall from the upper bulb to the lower bulb. This is why sand clock (or sand-glass) is also called an hour-glass.
3. Water Clock
Water clock was a yet another time measuring device used in ancient times. A water clock uses the rate at which water drips from one vessel to another to measure time intervals. Water was allowed to drip from one vessel to another vessel kept at a lower level (see Figure).
The time taken by the entire water to drip from upper vessel to lower vessel was used for measuring time intervals. Every time the upper vessel is filled with water, its water takes exactly the same time to drip into the lower vessel.
The ancient time-measuring devices such as sundial, sand clock and water clock could not be used for the measurement of small time intervals (such as minutes and seconds). The measurement of small time intervals became possible when the pendulum clocks were developed. Before we study the pendulum clocks, we should know the various units of measuring time. These are discussed below.
Units of Time
The standard unit of measuring time is ‘second’. The symbol of second is ‘s’. The standard unit of time is also called basic unit of time. So, we can also say that the basic unit of time is ‘second’. The larger units of time than second are minute and hour. The symbol of ‘minute’ is ‘min’ and the symbol of ‘hour’ is ‘h’. Thus, time is normally measured in the units of hours, minutes and seconds. The units of hour, minute and second are related to one another as follows :
1 hour = 60 minutes
and 1 minute = 60 seconds
The bigger units of time for expressing longer time intervals are: Day, Month and Year.
24 hours = 1 day
30 days = 1 month
and 12 months = 1 year
The still bigger units for expressing very long time intervals are decade, century and millenium.
10 years = 1 decade
100 years (or 10 decades) = 1 century
1000 years (or 10 centuries) = 1 millenium
Different units of time are used depending on the purpose for which they are to be used. For example, the time taken to cover the distance between our home and school should be expressed in minutes (or hours) and not in days or months. Similarly, the age of a person should be expressed in years and not in days or hours. The times of historical events should be stated in terms of centuries or milleniums.
The ages of stars and planets are expressed in billions of years (1 billion = 1,000,000,000). On the other hand, the two extremely small units of time which are used for expressing very, very small time intervals are ‘microsecond’ and ‘nanosecond’. One microsecond is one-millionth of a second. A nanosecond is one-billionth of a second.