# The Naked Scientists Forum

### Author Topic: Why is a centimetre this big? | |  (Read 3684 times)

#### dentstudent

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##### Why is a centimetre this big? | |
« on: 26/07/2007 15:08:36 »
Neils question triggered this one - who stipulated that a centimetre should be that size? Did it stem from a metre being about an arms length, or something more scientific?

#### another_someone

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##### Why is a centimetre this big? | |
« Reply #1 on: 26/07/2007 15:13:49 »
Although a metre is approximately a yard (a yard being the traditional measure of an arms length), I think the original metre was estimated (I think erroneously) to be 1/40000th of the Earth's circumference.

#### paul.fr

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##### Why is a centimetre this big? | |
« Reply #2 on: 26/07/2007 15:15:19 »
When the measurement was standardised in 1800ish. There were two  systems. One was the length of a pendulum with a half-period of one second, can't remember the other. The current (pretty accurate) system of measurement is the length traveled by light in vacuum during 1/299, 2972, 458 of a second.

#### dentstudent

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##### Why is a centimetre this big? | |
« Reply #3 on: 26/07/2007 15:17:03 »
ok. I think. But why 1/40000th? And what unit was used to measure it in the first place? Obviously 1 earth d isn't much use, so why the seemingly arbitrary fraction?

#### paul.fr

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##### Why is a centimetre this big? | |
« Reply #4 on: 26/07/2007 15:17:45 »
ooh, look. George knew the one i could not remember. I think we have that covered then. Quick q, was it the polar circumference?

#### dentstudent

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##### Why is a centimetre this big? | |
« Reply #5 on: 26/07/2007 15:19:23 »
When the measurement was standardised in 1800ish. There were two  systems. One was the length of a pendulum with a half-period of one second, can't remember the other. The current (pretty accurate) system of measurement is the length traveled by light in vacuum during 1/299, 2972, 458 of a second.
But again, these sound arbitrary. Why 1/2,992,972,458 of a second, and not 1/3,000,000,000? We could still use fractions of this smaller measure?

#### another_someone

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##### Why is a centimetre this big? | |
« Reply #6 on: 26/07/2007 15:28:13 »
When the measurement was standardised in 1800ish. There were two  systems. One was the length of a pendulum with a half-period of one second, can't remember the other. The current (pretty accurate) system of measurement is the length traveled by light in vacuum during 1/299, 2972, 458 of a second.
But again, these sound arbitrary. Why 1/2,992,972,458 of a second, and not 1/3,000,000,000? We could still use fractions of this smaller measure?

Because the measure of 1/2,992,972,458th is a standardisation on an existing measure, not a recalibration.  It used to be based on the number of cycles of radiation from a particular emission from a caesium atom, and before that the swing of a pendulum with certain characteristics, and before that, simply by observing the motion of the Sun through the heavens.  In each case, there was no desire to invalidate previous measurements, but merely to create greater consistency in measurement, so they did not invent a new scale but recalibrated a new measuring tool against an existing measure, and just redefined the measure by the new tool.
« Last Edit: 26/07/2007 15:40:05 by another_someone »

#### dentstudent

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##### Why is a centimetre this big? | |
« Reply #7 on: 26/07/2007 15:43:00 »
Thanks Paul, thanks George!

#### Bored chemist

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##### Why is a centimetre this big? | |
« Reply #8 on: 26/07/2007 20:38:41 »
1/40000 seems odd but really it was 1/10000 od the meridian from the north pole through Paris and down to the equator. 1/10000 gave a reasonable sized unit and powers of ten seem to have been popular at the time. The story of measuring this bit of the earth to a great accuracy notwithstanding that it ran through several countries that were at war with one another and during the revolution in France is an amazing tale of, if nothing else, bloodymindedness.

#### lightarrow

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##### Why is a centimetre this big? | |
« Reply #9 on: 27/07/2007 22:16:44 »
When the measurement was standardised in 1800ish. There were two  systems. One was the length of a pendulum with a half-period of one second, can't remember the other. The current (pretty accurate) system of measurement is the length traveled by light in vacuum during 1/299, 2972, 458 of a second.
But again, these sound arbitrary. Why 1/2,992,972,458 of a second, and not 1/3,000,000,000? We could still use fractions of this smaller measure?
It would be very interesting to know how our world would be if that was light's speed.
The actual value is 299,792,458 m/s.

#### paul.fr

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##### Why is a centimetre this big? | |
« Reply #10 on: 27/07/2007 22:22:06 »
When the measurement was standardised in 1800ish. There were two  systems. One was the length of a pendulum with a half-period of one second, can't remember the other. The current (pretty accurate) system of measurement is the length traveled by light in vacuum during 1/299, 2972, 458 of a second.
But again, these sound arbitrary. Why 1/2,992,972,458 of a second, and not 1/3,000,000,000? We could still use fractions of this smaller measure?
It would be very interesting to know how our world would be if that was light's speed.
The actual value is 299,792,458 m/s.

it would appear that i stuttered on my number pad. Thanks for pointing that out, kind sir  [:I]

#### The Naked Scientists Forum

##### Why is a centimetre this big? | |
« Reply #10 on: 27/07/2007 22:22:06 »