Measurement of Time: Atomic Clock, Unit Time and Time Period

The measurement of time employs various different units of measurement, based on the time scales and periods under consideration. These encompass a wide range of scales, from the nearly microscopic units used in physics to the more substantial units used in geological and cosmic time scales (such as seconds, minutes, hours, days, months, and years).

There are two major approaches to measuring time. A clock is a mechanical device that counts the passing of time. It is typically used for tracking time more precisely and for periods shorter than a day. A calendar is a mathematical model used to calculate extended periods of time (i.e. longer than a day). 

• An atomic clock is the most accurate clock in the world. It tells the time by measuring the vibration within the atoms. 
• A clock that measures time by keeping track of the resonance frequency of atoms is known as an atomic clock.
• It is based on the theory that atoms have various energies.
• In an atom, different electron states correspond to different energy levels, and as these states change, they interact with electromagnetic radiation at a very particular frequency.

Read More: Speed Time Graphs

KeyTerms: Atomic Clock, Speed, Frequency, Atoms, Gravity, Temperature, Oscillation, Motion, Electrons. 

Measurement of Time

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Measurement of time is called chronometry. Time can be defined as the interval between any two events. Time helps to track changes by assigning dates to moments, defining event durations, and indicating the order of the events. Time can be expressed as the precise second at which a specific event occurs or as a time interval, or as the length of an ongoing event.

• It is also known as Periodic events, which correspond to the events that repeat themselves after an equal interval of time.
• A perfect example of a periodic movement is the simple pendulum or the rotation of a wheel of the car moving at a constant speed.
• The methods of measuring time are different according to the place and profession.

Units and Measurements Video Lecture

Read More: Frequency and Wavelength

Measurement of Time Using the Atomic Clock

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An atomic clock is defined as a device that works on the usage of hyperfine transition frequency in an electron or microwave transition frequency in the ultraviolet or optical region of the electromagnetic spectrum of the atoms to be used as a standard value of frequency to keep the time on track. Atomic Clock also called the primary clock, corresponds to the clock which is based to work with atoms. The crossing radiation of electrons deals with the frequency of the clock.  

• Measurement of time is most accurate using an atomic clock because of the standard frequency.
•  it is also referred to as the primary standard for the system of international time distribution.
• It is used in the global navigation system in satellites and to manage the wave frequency of TV broadcasts.

Read More: Displacement vector

Similar to a regular clock, an atomic clock also depends upon the oscillation process to keep a record of time. Oscillation is a periodic and repetitive variation about a central value and it could even be between two or more various states, which is created by the change in energy.

For example: Consider a pendulum clock whose time depends upon the oscillation of the pendulum hanging which moves back and forth.

• The time of the clock depends totally upon the frequency of the pendulum.
• The frequency of the pendulum can be wrong or right or anything depending on the factors like gravity, temperature, and pressure of different regions.
• An atomic clock’s frequency totally depends upon the oscillation of atoms, which is always considered to be accurate.

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Ways to Measure Time

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Three basic ways to measure time are-

1. A simple pendulum
2. An atomic clock, and
3. A stopwatch

Measurement of Time in History vs Modern World

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Measurement of time in the ancient period and the modern world are as follows:

• Measurement of time in the ancient period corresponds to the occurrence of day and night.
• The rising and setting of the Sun was considered to be periodic motion. 
• The advancement of science and technology helped to measure time more precisely.
• Modern science divided the whole day into 24 hours, every hour into 60 minutes, and every minute into 60 seconds.
• Every second is further divided into 100 microseconds.

Read More: Resolution of vectors

History of the Atomic Clock

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Back in 1945, a physics professor at Columbia University whose name was Isidor Rabi proposed the fact that we can keep track of time using atomic vibrations. This theory came from something that he had developed which was known as Atomic Beam Magnetic Resonance.

After four years, the National Institute of Standards and Technology which was formerly known as the National Bureau of Standards developed the so-called Atomic Clock in which the Ammonia molecules were used. The United States’ current standard- NIST-F1 uses cesium atoms. There is another atomic standard clock present in Paris. Both of them are the two atomic clocks with the most accurate time.

A Massachusetts-based firm, which is a National Company made the first commercial cesium-based atomic clock. It is not easy to use an atomic clock in a wide range because of its large size and high power consumption.

But science, scientists, and engineers make everything possible. The problems faced while using the atomic clock are high power and large size, and NIST recently made an atomic clockwork that has no drawbacks like these. The new technology of manufacturing these within computer chips as it has a size close to the size of a grain of rice. The accuracy of time is 1 second in 126 years. It can easily be used in GPS systems, radios, and cellular telephones.

Read More: Light Sources 

Unit of Time

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A specific time interval that can be used to represent a standard duration of time for measuring purposes may correspond to the unit of time. The SI unit of time is second. According to the International System of Units and the extension of the western world, the second of time is defined by around 9 billion oscillations of Cesium Atoms.

According to The National Institute of Standard Technology, the precise and actual definition says, “The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atoms, to be 9192631770 when expressed in the unit Hz, which is equal to s⁻¹." 

Back then the unit of time was considered through the movements of astronomical objects. A few ancient units of time are mentioned below:

Sun-Based Unit

A year is the time taken by the Earth to complete one revolution around the sun. Some of the year-based units are-

• Olympiad- 4 years
• Lustrum- 5 years
• Indiction- 15 years
• Decade
• Century
• Millennium 

Moon-Based Unit

A month was the time taken by the moon to complete its own orbital period around the Earth.

Earth-Based Unit

A complete day is the time taken by the Earth to rotate on its axis. One week was subdivided into 7 days, and one fortnight had 14 days. One day is referred to as 24 hours, one hour is subdivided into 60 minutes, and then one minute is subdivided into 60 seconds. And now according to science, the second is the International Standard Unit.

Celestial Sphere-Based Unit

The movement of stars was decided to be used as the unit of time. Constellations helped in predicting the length of the year.

Read More: Valence Electrons

Pendulum and Its Time Period

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A pendulum or simple pendulum is made of a metallic bob and a string, be it metallic or non-metallic. The bob is mainly considered as the pendulum and is certainly the important part.

• One side of the string is attached to the bob and the other end needs to be fixed on a support in a way that the pendulum is free to do any movement (back and forth).
• Once the pendulum is set up by letting bob free to move, it is moved in periodic motion.
• The bob of the pendulum oscillates along the string and the wall making a periodic motion.

• The time period of a simple pendulum depends upon the length of the string, the gravity, and the amplitude that corresponds to the maximum angle the pendulum would swing away from the vertical axis.
• It would be surprising to know that the time period is independent of the mass of the bob. 

The formula for the time period of a simple pendulum is-

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Things to Remember

• Measurement of time employs multiple units of measured based on the time scales and durations.
• The two major major approaches used to measure time are by clock and calendar. 
• An atomic clock is a device that calculates the passage of time by monitoring the resonance frequency of atoms.
• Time can be quantified as the precise second at which a certain event takes place, as a time interval, or as the duration of a continuing event.
• The SI unit of time is second.
• The time period of a simple pendulum is T=2π√lg

Read More: Barometer

Previous Years’ Questions

1. A screw gauge has the least count of 0.01 mm… [NEET – 2020]
2. Dimensions of stress are… [NEET – 2020]
3. Lumen is the unit of… [UPSEE – 2019]
4. A quantity z, to be estimated has a dependency… [TS EAMCET – 2019]
5. The nearest star to our solar system is… [AMUEEE – 2018]
6. A physical quantity of the dimensions of length that can… [NEET – 2017]
7. Planck’s constant (ℎ), speed of light in vacuum…  [NEET – 2016]
8. A body weighs 22.42 g and has a measured volume…  [WBJEE – 2010]
9. A student measures the distance traversed in free fall … [NEET – 2010]
10. Which of the following is the smallest unit… [UPSEE – 2010]