Concept of Time and Ways of Measuring Time

Concept of Time

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Time is the duration between two events, or the interval between the beginning and the end of an event. It is a measure of how long something lasts. We use time to put events in order (what happened first, what happened next) and to say how long they took. Without a way to measure time, we could not talk about speed, acceleration, frequency, power or any quantity that involves “per second”.

 

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Key Facts about Time

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• Fundamental quantity: Time is one of the seven base (fundamental) quantities in the SI system. It cannot be built from other quantities.

• Scalar quantity: It has magnitude only and no direction.

• International System of Units (S.I.): The S.I unit of time is the second (s).

• Symbol: t (for an interval, we often write Δt). The dimension of time is T.

• Always moves forward: Time cannot be stopped, repeated or reversed. It always moves forward.

 

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Instant and Interval

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An instant (or time of day) is a particular moment, such as 8:00 a.m. A time interval is the length of time between two instants. If a lesson starts at 8:00 a.m. and ends at 8:40 a.m., the time of day at the start is 8:00 a.m., but the time interval is 40 minutes.

In physics, formulas such as speed = distance / time use the time interval.

Time Instant	Time Interval
A particular moment, such as 8:00 a.m.	The duration between two moments, such as 8:00 a.m. to 8:40 a.m.
Read directly from a clock	Found by subtracting the earlier reading from the later one
Example: the bell rang at 10:15	Example: the lesson lasted 40 minutes

 

Please read on fundamental, derived and supplementary units with their dimensions here.

 

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Importance of Measuring Time

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• It helps us plan daily activities such as school, work, worship and travel.

• It is needed to calculate speed, velocity, acceleration, power and frequency.

• It is used to time races and sports events.

• It is used in navigation, satellite systems (GPS), communication and computing.

• It is used in science experiments, medicine (pulse rate) and history (dating events).

 

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Units and Conversions of Time

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The second (S.I definition)

One second is defined as the time taken for 9,192,631,770 periods of the radiation emitted by a caesium-133 atom when it changes between two particular energy levels of its ground state. This is the standard used in atomic clocks.

The table below shows the various units of time and their equivalent in seconds.

 

Smaller units are used for very short times:

1 millisecond (ms) = 10⁻³ s

1 microsecond (µs) = 10⁻⁶ s

1 nanosecond (ns) = 10⁻⁹ s

Please read on the measurement of basic quantities and the importance of measurement in physics here.

 

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Ways of Measuring Time

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All timekeeping depends on one idea. The idea is: "use something that repeats at a steady rate, then count the repeats". Such a repeating event is called a periodic event. The time for one complete repeat is called the period.

 

What makes a good timing device?

• It must repeat at a regular, constant rate.

• The repeats must be easy to observe and count.

• It should not be easily affected by temperature, wind, position or other conditions.

• It should be able to measure the interval needed (a few seconds, or many hours) with enough accuracy.

 

The table below show examples of periodic events used to measure time.

 

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Timing Devices at School Level

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The device listed below are used for timing at school level.

• Stop watch / Stop clock: It is used to time events in the laboratory and in sports, and can be started and stopped at will.

• Simple pendulum: This is the time for a number of oscillations, measured with a stop watch.

• Ticker-tape timer: It makes dots on a paper tape at equal time intervals. It is used to study motion.

• Digital timers and light gates: It measure very short times electronically.

Please read the introduction to physics, definition, branches and importance of physics here.

 

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Old (Traditional) Methods of Measuring Time

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Early people used nature and simple devices to tell time. These methods are less accurate, and many depend on weather, daylight or human judgement.

The old methods of measuring time include:

1. Natural Observation

• Position of the Sun, and length of shadows: People estimated the time of day from where the sun was in the sky.

• Moon and Stars: The phases of the Moon were used to count months, and the position of stars to estimate night hours.

• Seasons: The rainy and dry seasons guided farming and were used to count years.

• Animals and Events: This was used in many African communities. They include the cock’s crow, birdsong and market days which were used to mark time.

• Traditional market-day: The traditional markek day formed the local "weeks". For example, the four-day week of many Nigerian communities (such as Eke, Orie, Afo and Nkwo among the Igbo).

You can attempt questions and answers in physics here.

 

2. Sundial (Shadow Clock)

A vertical rod (the gnomon) casts a shadow on a marked surface. As the Sun moves across the sky, the shadow moves and points to different marks.

Ancient Egyptians used shadow clocks and sundials thousands of years ago.

Limitations: It works only in daytime and when the sun is shining, so it fails at night and on cloudy days, and the marks must be adjusted for each location.

 

3. Water clock (Clepsydra)

Water flows out of (or into) a container through a small hole at a steady rate. The water level against marks on the container shows how much time has passed. It was used in ancient Egypt, Greece and other places.

Limitations: The flow rate changes as the water level falls and with temperature. Also, water can evaporate or leak.

 

4. Hour Glass (Sand Clock)

Two glass bulbs are joined by a narrow neck. Sand runs from the top bulb to the bottom bulb in a fixed time, such as one hour or a few minutes. It can be turned over to measure again.

Limitations: It measures only one fixed interval, and the interval is not very exact.

 

5. Burning methods

It includes the candle clock and the incense (or oil lamp clocks)

• Candle clock: A candle marked in equal sections burns down at roughly a steady rate.

• Incense or oil lamp clocks: The amount of incense burnt shows the time.

Limitations: The burning rate changes with wind and the material used.

 

6.Early mechanical and pendulum clocks

Mechanical clocks driven by falling weights appeared in Europe in the Middle Ages. In 1656 the Dutch scientist Christiaan Huygens built the first practical pendulum clock, which was far more accurate than earlier clocks because a pendulum swings at a very regular rate.

Many textbooks treat the pendulum clock as a bridge between old and modern methods.

Please read on position, distance and displacement here.

 

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Modern Methods of Measuring Time

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1. Mechanical Watches and Clocks

A coiled spring stores energy and drives a set of gears. A balance wheel swings back and forth at a steady rate and controls the release of energy, so the hands move at an even rate. These need to be wound regularly (or are wound by wrist movement in automatic watches).

 

2. Electric (Electronic) Clocks / Watches

These use a battery or the mains supply to run a motor or electronic circuit. They are easier to use and often more accurate than purely mechanical ones.

 

3. Quartz Clocks and Watches

A small crystal of quartz is made to vibrate by an electric voltage (the piezoelectric effect). Common watch crystals vibrate 32,768 times every second. An electronic circuit counts the vibrations and uses them to drive the display or the hands. Quartz watches are cheap, small and accurate to a few seconds (±15 to ±20 seconds) per month (about half a second per day). This is far more precise than standard mechanical watches, which usually drift by several seconds per day.

 

4. Digital Stop Watches and Timers

These count the vibrations of a quartz crystal or electronic pulses and show time in seconds, down to one hundredth or one thousandth of a second. They are used in laboratories and sports.

 

5. Atomic Clocks

An atomic clock measures time using the extremely regular vibrations (radiation) of atoms, usually caesium-133. The first accurate caesium clock was built in the 1950s, and the second has been defined using caesium since 1967.

The best atomic clocks gain or lose less than one second in many millions of years. They are used in satellite navigation (GPS), telecommunications, and to set world standard time.

 

6. Time from Networks and Satellites

Mobile phones, computers and GPS receivers get time signals from atomic clocks over networks or from satellites. As a result, their time is kept accurate automatically.

Please read on the concept of direction: scalers and vectors here.

 

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Old versus Modern Methods of Measuring Time

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Exam Tip. When asked to distinguish between two things, always state a point about each side. A table, with matching points for both methods, earns full marks.

Please read the continuation of this topic (simple pendulum and ticker-tape timer) alongside its calculations and practice questions here.