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Is the metric system based off a circle?

This is a discussion on Is the metric system based off a circle? within the Optics forums, part of the Gun Forum category; I was looking at buying a scope which led me to looking at schmidt and bender. As I have yet to figure out how to ...


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Old December 29th, 2016, 10:25 PM   #1
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Is the metric system based off a circle?

I was looking at buying a scope which led me to looking at schmidt and bender. As I have yet to figure out how to grow and harvest money and have to work for it but still believe in quality over quantity and at least trying to be frugal I decided the 10x42 pmII would do just fine. After looking at it I saw that it is 1click=1cm. I do understand the basics of MOA and MRAD. I understand that MOA at 100yds is 1 inch though this is a nominal value as it is 1.047 to be exact and that is about 3.6" MRAD along with that, Is 1cm at 100 meters exactly .1 MRAD? I do not understand why schmidt and bender would have 1click=0.1 MRAD on some turrets and 1click=1cm on others? If it is not exact for 1cm/.1MRAD at 100 meters then it would not exactly match a milldot reticle and if it does I find it not only interesting but that the metric system must be based on a circle...

Please help if you can as I am tired of reading about math


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Old December 29th, 2016, 11:14 PM   #2
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Here's a link that has a reasonable explanation for your question.

http://www.scout.com/military/sniper...-right-for-you

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Old December 30th, 2016, 05:15 AM   #3
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double post

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Old December 30th, 2016, 05:17 AM   #4
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To answer your question, no, the metric system is not based off a circle.

From my understanding, 1 MILRAD = 10 cm at 100 meters, 20 cm at 200 meters, 30 cm at 300 meters, etc, etc. Most scopes using a mil dot have adjustment increments of 1/10 a MIL per click. If your scope is marked as moving 1 cm per click at 100 meters, it means that 10 clicks, or one full MIL, will move your POI 10 cm. 1 click on a scope marked 1/10 mil per click will move it 1 cm at 100 meters, with 10 clicks moving it 10 cm at that distance. So .1 MIL or a 0.1 MIL or 1 cm on your scope turret is the same thing (or the same difference) as between 6 and a half dozen eggs.

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Old December 30th, 2016, 06:29 AM   #5
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Is the metric system based off a circle?

A really big one. A meter was originally defined as one ten-millionth of the distance of a line through Greenwich, England, that ran from the equator to the north pole.

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Old December 30th, 2016, 06:35 AM   #6
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The metric system is a base 10 system of measurements. Measurements of volume, length and weight are all related, and in units of ten unlike our yards, feet, inches, gallons, quarts, ounces, pounds and tons. 1000mm=1meter. 1000ml=1liter, and this is the neat part, 1mm cubed equals 1ml, and 1 ml of pure water equals one milligram. So units of length, volume and weight have a direct relationship.

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Old December 30th, 2016, 08:38 AM   #7
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The 'idea' about MRADs starts with how much change of angle is needed to give a movement of 1/1000 the distance between the end points of a line.
It doesn't depend on what method is used to measure the distance.
But it works nice with the metric system because it's easy to convert the units.
So, it also works nice when the clicks of a turret are in 1/10 MRAD to maintain that ease of converting units.

That amount of angular change is called the milliradian (MRAD) and it doesn't depend on angular degrees, moa, etc.

Having a turret marked '1 cm / click @ 100m' just makes it easier for people who want to know 'how much' and aren't concerned with the details about MRADs. For those who do know about MRAD, seeing the '1cm / 100m' tells them the scope is calibrated in MRADs.

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Old December 30th, 2016, 08:51 AM   #8
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Quote:
Originally Posted by Rich D View Post
The metric system is a base 10 system of measurements. Measurements of volume, length and weight are all related, and in units of ten unlike our yards, feet, inches, gallons, quarts, ounces, pounds and tons. 1000mm=1meter. 1000ml=1liter, and this is the neat part, 1mm cubed equals 1ml, and 1 ml of pure water equals one milligram. So units of length, volume and weight have a direct relationship.
This

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Old December 30th, 2016, 11:05 AM   #9
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One-tenth of a millirad is very, very close to 1 cm at 100 meters because of the definition of a radian, not anything specific to the metric system.

I say very, very close because the length of an arc subtended by one radian is equal to the radius of the circle. Since, the measurement of 1 cm at 100 meters is actually the secant of a circle, not an arc, that angle is not exactly one-tenth milliradian. But, it's real close, you have to go several decimals before the difference shows up.

(EDIT: The distance perpendicular to your LOS at 100 meters subtended by .1 mrad is actually .999999958... cm.)

If you really wanted to use millirads in the English system, it's 1 inch at 1000 inches....

(Ever wonder why the machine gun qualification range was at thousand inches?)

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Last edited by lysander; December 30th, 2016 at 11:24 AM.
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Old December 30th, 2016, 11:07 AM   #10
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Quote:
Originally Posted by Samuel Seeley View Post
Is the metric system based off a circle?

A really big one. A meter was originally defined as one ten-millionth of the distance of a line through Greenwich, England, that ran from the equator to the north pole.
CORRECTION:

A meridian from the pole to the equator running through Paris.

It's largely a French invention.

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Old December 30th, 2016, 11:38 AM   #11
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More useless information:

The use of Greenwich meridian as the "Prime Meridian" is actually quite new, prior to 1884, there were several "prime meridians", each based on the country defining it. The British used the one through Greenwich, the US used the one running through the Naval Observatory in Washington D.C., the French used the one running through Paris, the Dutch, Amsterdam (used by them for time until 1937, BTW), etc, etc.

Needless to say this made locating things on the planet difficult when more than one country was involved, mainly international boundaries.

Nobody wanted to give up their own proprietary "prime meridian", so a "neutral" prime was proposed running through the Bearing Straight. Eventually, in a conference held in Washington D.C., in 1884, the meridian running through Greenwich was chosen as the "Prime Meridian", much to the disgust of the French . . .

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Old December 30th, 2016, 12:23 PM   #12
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Good thread thank you

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Old December 30th, 2016, 12:40 PM   #13
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Quote:
Originally Posted by lysander View Post
(EDIT: The distance perpendicular to your LOS at 100 meters subtended by .1 mrad is actually .999999958... cm.)
This is incorrect.

The distance perpendicular to your LOS at 100 meters subtended by .1 mrad is 1.0000000833 cm.

I forgot to take into account that R cos θ does not equal R . . .

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Last edited by lysander; December 30th, 2016 at 12:54 PM.
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Old December 30th, 2016, 12:59 PM   #14
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The answer is that scope adjustments are based on a circle.
This is why a 1minute of angle translates to approximately 1inch at 100yds, 2inch at 200yds, etc.
Two common ways to describe this is as degree's or as radian's. Two different systems to define the same thing.
1degree of angle = π/180 radians

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Old December 30th, 2016, 01:05 PM   #15
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This thread is making my head hurt!

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