Measurement of Time

Things to remember

• The duration between any two events is called time.
• The SI unit of time is second. 
• One standard second is defined as the time taken by cesium -133 atom to make 9192631770 vibrations. 
• One second is defined as the 1/86,400 th part of the mean solar day. A particular point in the space is called zenith.
• A solar day is defined as the time interval between the consecutive passages of the sun through the zenith.
• The time taken by the earth to revolve once around the sun is called one year.