Who dare decide MY birth date and birth sign?
Imagine yourself in an island. An island like the one Sindbad visits during his adventures, which is completely without a trace of human life. You are there right from your birth. You
don�t know about anything that you know now. You hunt for food, eat and then sleep. When you are leading a life like this, at some point you would realize the concept of time. Of course, not in terms of sec, min and hour as you know now. You would have observed that the day and the night occur at equal intervals. After some years you would also have observed that the seasons repeat at equal intervals. So, by intuition would you not have these as the basis or units to measure your time?
Thus the units for measuring time were acutally developed by observing nature.
You should always define some unit to measure anything. Metre is a unit to measure length. How do you define a metre? There is a particular scale in France. The length of this scale is taken as a metre. This is just an example of how you standardize and define units for measurements.
So how are we going to define our units of time? We shall use the behaviour and positions of celestial objects.
One day can be defined as the average time taken by the Sun to come back to the same position in the sky, as viewed from the earth. In other words, it is the mean time period between two sunrises. By now you should have known that the 'day' we are referring to is the 'Mean Solar Day'. The duration of this is 24 hours. Now, we've said that it is the mean time between two sunrises. Does it mean that the time between two sunrises vary? Of course! While talking about seasons, we said that at the equinoxes, the days and the nights are equal. During summer solistice, the duration of the day is at its maximum and during the winter solistice the duration of the night is at its maximum, and in the period in-between the duration of the day and the night varies. We've seen that the stars take about 23 hours and 56 minutes to come back to its same position. This duration is referred as 'Sidereal day'.
Thus, from the natural phenomenon of sunrises, we can define the day and hour. An hour is obtained by dividing the mean period between sunrises by 24. We say the time in terms of expressions like 6'o clock in the morning, 3'o clock in the afternoon etc. How do we fix this? Generally the time of the sunrise is around 6'o clock in the morning and that of sunset is 6'o clock in the night. But the time of the sunrise and the sunset differs from place to place. To standardize them, we use the concept of time zones.
Why do you have so many units (mm, cm, m, km etc�) for measurements? To measure something small you need a smaller unit and a larger unit to measure something large. You cannot say the distance from Chennai to Chicago in terms of mm and your height in km.
So we have defined one smaller unit of time (day). Click here to see how we define some larger units of time.
We've seen about the various units of time and how to define them. But how to measure them apart from observing the sky closely? For this purpose several instruments were used. Let's take a quick look at the Sun Dial.
So far, we've looked at units of time starting from a day and going upto Maha Yugas, Kalpas and Manvantharas. But how to subdivide a day into smaller units, which we can perceive?
Back to Astronomy