Our perception of time is based on the earth's revolution around the sun and its rotation around its own axis. One complete cycle around the sun is a year, while a complete rotation on its axis constitutes a day.

Based on these observations, a day has 24 hours while 365 days constitute a year. The day is further divided into hours, minutes and seconds.

Time measured by the earth's rotation relative to the sun is called solar time. For any given point, there could be two values of solar time — apparent and mean. Apparent time is measured by direct observation of the sun by a sundial. Mean solar time, however, is measured by assuming that relative to the earth, the sun is at the same position after every 24 hours. Most clocks and watches are based on mean solar time.

Measurement of time based on the earth's rotation and revolution has its limitations and hence the unit of time defined by the International System of Units is not based on astronomical observation.

The length of a second is defined according to the vibrations of caesium atoms at various atomic clocks. International Atomic Time is based on a system of about 270 atomic clocks. Signals from these clocks are transmitted to the International Bureau of Weights and Measures located in Sevres, France, which uses the signals to form the International Atomic Time. Seconds measured by atomic clocks are the most accurate. These clocks are predicted to be off by less than a second in 50 million years.

Based on these observations, a day has 24 hours while 365 days constitute a year. The day is further divided into hours, minutes and seconds.

Time measured by the earth's rotation relative to the sun is called solar time. For any given point, there could be two values of solar time — apparent and mean. Apparent time is measured by direct observation of the sun by a sundial. Mean solar time, however, is measured by assuming that relative to the earth, the sun is at the same position after every 24 hours. Most clocks and watches are based on mean solar time.

**What is the most accurate measure of time?**Measurement of time based on the earth's rotation and revolution has its limitations and hence the unit of time defined by the International System of Units is not based on astronomical observation.

The length of a second is defined according to the vibrations of caesium atoms at various atomic clocks. International Atomic Time is based on a system of about 270 atomic clocks. Signals from these clocks are transmitted to the International Bureau of Weights and Measures located in Sevres, France, which uses the signals to form the International Atomic Time. Seconds measured by atomic clocks are the most accurate. These clocks are predicted to be off by less than a second in 50 million years.