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Measuring Time

September 8, 2015
By Robert Strong, Libby Strong and Richard Pollack - Contributing Writers , OVParent

Summer vacation always seems to fly by. Before we know it, it is back to school with the buzzing and or ringing of alarm clocks in the morning, time to catch the school bus, lunch time and even bed time.

We seem to need to know what time it is at all times.

How do we measure time?

Article Photos

Throughout history, people have measured time in various ways. One way has been by observing moon phases. When we talk about the moon, we mean the large natural satellite that orbits the Earth.

This observation helped with measuring the passing of longer periods of time than a single day. Even ancient people noticed that the moon seemed to change its shape and appearance over a period just over 29 days.

This is now known to be a predictable and repeatable pattern or cycle of moon phases that we can use to accurately gauge the passage of time.

The time for the moon to complete one cycle of lunar phases called the synodic cycle. The lunar synodic cycle is 29.53 days - this means it takes 29.53 days for the moon phases to go through one complete cycle before repeating.

Another astronomical way to tell time is with sun shadows.

Sun dials were used, but even a simple stick placed in the ground would show that the length of its shadow varied depending on the time of the day. At noon, the stick would cast the shortest shadow. The shadows were longest as the sun rose and set.

Humans have made mechanical devices to tell time in more recent history, but the basis of many of our time-keeping devices lies in an object called a pendulum (sometimes called a harmonic oscillator).

A pendulum is a simple device consisting of a mass (called a "bob") suspended by a string, cord, chain or rod usually having vary little mass.

The point from which the string is suspended has as little friction as possible to allow the pendulum to swing as long as possible without stopping.

If you have ever seen a grandfather clock, you may have noticed a brass or other metal "bob" swinging back and forth in the clock.

The pendulum as a time-keeping device was first scientifically investigated by Galileo in 1602. The inspiration came to Galileo while he was watching the swinging motion of the chandelier in the Pisa cathedral that had started swaying from a gentle breeze from an open window in 1582.

Pendulums continued to be the basis for the most accurate time-keeping devices until the early 1930s with the invention of the Quartz Clock in 1927.

Make Your Own Pendulum

To make your own Pendulum you will need:

A pendulum mass, the "bob" - a washer or ring magnet works great.

The center of mass of a washer or ring magnet is the exact center of the hole.

A piece of thread or thin string about 1.5 meter (59 inches) in length

Simply tie one end of the thread or thin string around the pendulum bob.

Hold the other end of the thread or thin string.

Pull the pendulum bob back from vertical about 10 cm (4 inches) and let the bob swing back and fourth through a small angle.

Investigate Pendulums

As you watch how the pendulum bob behaves, see if you can answer the following questions.

- Does the bob move at a constant speed?

- Does the bob stay the same distance from the floor as it swings back and forth?

- How can you increase or decrease the time for one complete back and forth cycle (period) of your pendulum?

Experiment with changing the length of the string or changing the mass of the bob (add more washers or ring magnets).

Make Your Own Pendulum


To make a pendulum stopwatch, all you need extra is a meter stick (or yard stick) to measure lengths.

To make a 1-second pendulum, a pendulum that completes an entire back and forth sway cycle (or one period) in exactly 1 second, measure the length of exactly 24.85 cm (9.783 inches) from the pendulum's pivot point (where you hold the string) to the center of mass of the pendulum bob.

To make a pendulum stopwatch that has a period of exactly 2 seconds, you need a length from pivot point to center of mass of 99.4 cm (39.13 inches).

Try it out for yourself. Check this with a clock with a second hand or a digital clock that shows seconds.

If you have some extra string, experiment with various lengths to find the length of string you need to make a 3-second pendulum.

Foucault Pendulum

Pendulums do more than tell time. In 1851, French physicist and clock maker Jean Bernard Foucault publicly demonstrated that a freely swinging pendulum also can reveal that the Earth spins on its axis.

Foucault suspended a 67-meter (220-foot) long pendulum from the dome of the Pantheon in Paris, France, creating a "pendulum mania" throughout Europe and North America. The period of his pendulum was a leisurely almost 16.4 seconds!



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