The measurement of Time is based upon the apparent motion of the heavenly bodies caused by the Earth's rotation on its axis. Since the Earth rotates on its axis from West to East, all heavenly bodies (the fixed stars and the sun) appear to revolve from East to West (in a clockwise direction) around the Earth.

There are four kinds of Time:

  1. Sidereal Time
  2. Apparent Solar Time
  3. Mean Solar Time
  4. Standard Time

The first two kinds of time are convenient to the astronomer, while the latter two are convenient for every-day affairs.

Mean Solar Time

The mean solar day is the average of all the apparent solar days of the year. The time which is in common use by the people is the Mean Solar Time or Civil Time. It is the time kept by our clocks and watches.

Mean Solar Day is divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds.

There are two systems of reckoning means solar time:

  1. Civil Time
  2. Astronomical Time

The Civil Day is divided into two periods, one from midnight to noon, and the other from noon to midnight so that the time of an event occurring before mean noon is denoted by the letters A.M. (Ante Meridian), and the time of event occurring after mean noon by the letters P.M. (Post Meridian). The Astronomical Day is divided continuously from zero hour to 24 hours.

Standard Time

In order to avoid confusion arising from the use of different local mean times by the people, it is necessary to adopt the mean time on a particular meridian as the Standard Time for the whole country. This meridian is known as the Standard Meridian and usually lies an exact number of hours from Greenwich. When the Prime Meridian of Greenwich has the sun at the highest point in the sky, all the places along this meridian will have mid-day or noon.

As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich time and those to the west will be behind it. The earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes. Thus, when it is 12 noon at Greenwich, the time at 15° east of Greenwich will be 1 hour ahead of Greenwich time, which means 1 p.m. But at 15° west of Greenwich, the time will be behind Greenwich time by one hour, it will be 11.00 a.m.

At any place a watch can be adjusted to read 12 o’clock when the sun is at the highest point in the sky, i.e., when it is mid-day. The time shown by such a watch will give the local time for that place.

The longitude of the standard meridian adopted in India (Indian Standard Time) is 82°30' E or 5 hours 30 minutes East. Greenwich meridian is the standard meridian for Great Britain.