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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; 1cm=.1mrad Because the length of the radian is equal to the radius. At 100 meters the length of arc is 100meters, divided by 1000 equals ...


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Old December 30th, 2016, 11:02 PM   #31
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1cm=.1mrad

Because the length of the radian is equal to the radius. At 100 meters the length of arc is 100meters, divided by 1000 equals 10 cm and .1 being 1 cm on the money.

I just needed some scotch

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Old December 31st, 2016, 07:41 AM   #32
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CORRECTION:

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

It's largely a French invention.
Another "fact" in my head that has become sadly twisted.

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Old December 31st, 2016, 05:49 PM   #33
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Given a accurate quadrant or sextant, you can measure the angular distance from various stars to the moon and provided you have charts that has these distances from a known position on the Earth and from a known time, you can figure out how far you are displaced from from that known point
No wait, you just proposed a solution where you have a known position, and a known time.

A Frenchman trying to locate his position in London has neither the correct time, nor a known position. Therein lies the problem. Determination of longitude requires accurate time stamp, or a known position. If you have two unknown variables, you cannot solve the problem.

The entire issue of navigation is linked to an accurate determination of what time it is at the location where the angular measurement is being taken, and that was a feat that was not accomplished until the 1750's, where several engineering issues with clocks were resolved, like bearings, temperature compensation in the flywheel, bimetallic strips in springs.

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Old December 31st, 2016, 07:49 PM   #34
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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.
You mostly have this correct....except, 1 cubed centimeter (cm, not mm) equals 1 milliliter (ml)
And 1 ml or 1 cubic centimeter (cm) equals 1 gram (not mg)
https://en.wikipedia.org/wiki/Cubic_centimetre

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Old December 31st, 2016, 08:07 PM   #35
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No wait, you just proposed a solution where you have a known position, and a known time.

A Frenchman trying to locate his position in London has neither the correct time, nor a known position. Therein lies the problem. Determination of longitude requires accurate time stamp, or a known position. If you have two unknown variables, you cannot solve the problem.

The entire issue of navigation is linked to an accurate determination of what time it is at the location where the angular measurement is being taken, and that was a feat that was not accomplished until the 1750's, where several engineering issues with clocks were resolved, like bearings, temperature compensation in the flywheel, bimetallic strips in springs.
The correct time stamp can be obtained via the distance of several stars to the moon. The rotation of the stars about the poles and the moon about the earth are very regular, more regular than any man-made clock.

If one has a chart of the stars motion relative to the moon, you can figure the time, and thus your distance from any point of origin, (ie, "1) at 12:00 PM at Greenwich the star Regulus will be at X degrees from the horizon and Y distance from the moon and the moon will be Z distance from the horizon, 2) at 12:05 PM..." it's a very thick book).

Lunar distance (navigation)

Such charts were published starting in 1763, available at a purchase price of a few pounds. Compared to the cost of one of the three of four existing chronometers that cost £500 in 1763 (£100,000 today, or $120,000, almost a third of the cost of a fully outfitted frigate), it was a bargain.

During Captain Cooks' second voyage around the world in 1772, one of the tasks he was given was to compare the times displayed on some two dozen chronometers, of different design and manufacture, against the times calculated by lunar distance, and gauge their accuracy for navigation. Of that two dozen over half had ceased to work altogether or were highly inaccurate before he reached the mid point of his journey. Only four or five gave good result the entire trip, Larcum Kendall's K1 was one of them and embodies most of the refinements of modern mechanical naval chronometers. However, it would be another thirty or forty years before the cost of such instruments became reasonably affordable, around ten thousand dollars each.

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Old December 31st, 2016, 09:02 PM   #36
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"Longitude" is a good flick. http://www.imdb.com/title/tt0192263/

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Old December 31st, 2016, 10:33 PM   #37
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"Longitude" is a good flick. http://www.imdb.com/title/tt0192263/
It introduces more antagonism between Maskelyne and Harrison than I think really existed. Maskelyne was not dead set opposed to the use of a chronometer for navigation as depicted in the film (and book), Maskelyne actually encouraged several watchmakers to make chronometers along the designs of Harrison's H4 chronometer. Further, Maskelyne's appointment to the Board of Longitude made him ineligible for the £20,000 prize, so he had no reason to oppose Harrison on anything other than what he felt were technical grounds.

Besides, Maskelyne was a heavy set man....

Just to expound on my last post a bit...

No mechanical time keeper was ever accurate enough to just have the time read off it and that was that, even in WW2. All mechanical clocks, watches, etc, have some drift, they gain or lose some fraction of a second each day, even the best Hamilton Model 21, made in the 1940s (probably the ultimate Naval Chronometer), measures one second any where from 0.999958 second to 1.000042 second. You may not think that’s much but over three months it took to sail from Portsmouth to Kingston, Jamaica in the Revolutionary War period, that adds up to a full minute and a half, or about 1.25 nautical miles. A round trip journey to the far flung outposts of the British Empire could take a year, this would have them crashing into the rocks off the coast of Ireland or other such calamity instead of dropping anchor in Plymouth or Portsmouth.

So, how do you know where you are if the clock times drifts?

Because the accuracy of a nautical chronometer, is not how closely it measures a second, but how uniform the drift is. If it drifts 0.5 seconds toady and 0.5 seconds the next day and so on and so on, all you need to know is how many days has in been since the chronometer was synchronized. If you synchronized your chronometer 90 days ago at the Royal Observatory in Greenwich, and it now reads 11:59:21 AM, and the drift is -.43 seconds per day (it loses .43 second each day), you know it is high noon in London (90 x [-0.43] = - 38.7, so add 39 seconds to the displayed time).

If you have ever seen a naval chronometer, they have a card with the measured drift (known as the “Going Rate”) and the time and date of the last synchronization for these calculations.

One of the reasons the Board of Longitude did not accept Harrison's clock was the fact that they expected the time read directly off the clock would be accurate enough (as many people still believe today), and did not allow for the estimation of error from drift to be used in calculating the correct time.

One of the jobs of the ship’s navigator in the late 18th and early 19th century was to periodically check the ship’s chronometer against Lunar Distance calculated time to gauge if the going rate was indeed stable and the predicted error was the same as the actual error.

With the invention of radio, the naval chronometer became less important for two reasons, 1) navigation by radio beams became possible, and 2) synchronization of the chronometer by radio signal made the time between synchronization shorter (you didn’t have to bring the clock to the observatory to sync it, saving much time), so less accurate clocks were usable. Some Liberty ships had nothing more than a really good pocket watch as a chronometer (Hamilton Model 22, normally a “deck watch” a watch set to the primary chronometer daily and used for day-to-day navigation). Radar simplified navigation even more.

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Old January 20th, 2017, 04:37 AM   #38
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I have a few words of caution when ordering or purchasing Schmidt & Bender scopes and a funny story to go with it.

2 years ago I attended a precision rifle clinic & competition.
I attend this clinic with my shoooting partner/spotter several times a year.
It is put on by a friend and we assist with running the rifle butts ensuring proper target presentation, scoring etc.

When you are in the rifle butts you are downrange, below ground so you have no idea what's going on at the firing line.
You just see the impact of the bullet, you pull down the target, mark the target with an indicator and send it back up for the next shot.

The shooters are split into 4 relays, relay 1&2 in the butts for target duty while relays 3&4 shoot and spot.

So my buddy and I are in the butts and we are paired up with 2 guys from south Asia.
It is their first time shooting at any distance and they showed up in full kit eager to learn.
They come equipped with a pair of brand new FN SPR AG3's each topped with an S&B 3-27 x 56 PMII.
These rifles are so new they have not yet been zeroed.

From my perspective down in the butts the first shooter gets in position to zero and everything is good first shot is a little high and to the left 1 correction and he's all set.

The second shooter comes on and he is maybe 6" low and slightly to the right.
The second shot come in 12" too low.
I indicate the hit and put the target back up.
The next shot is off the bottom of the target in the dirt.
I get on the radio and call a dirt shot on target 4
By this time all the other targets have finished their zeroing and everyone is waiting for this guy to be done.
Down in the butts we are all gathered around target 4 trying to guess where his next shot is going to be.
We have no idea what is going on at the firing line but it appears that by this time the 2 doctors, shooter and spotter are yelling at each other due to the second shooters inability to zero his rifle.

Instead of telling the shooter to dial up or down however many mils, the spotter was telling the shooter to turn the elevation turret to the left or to the right.

What happened was that when purchasing their scopes one purchased the scope in clockwise rotation and the other in counterclockwise rotation.

The majority of scopes out there are CCW to be very clear.

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