Explanation For Silva Military Compass

Explanation For Silva Military Compass (How It Works)


Irish artillery originally used guns calibrated in degrees, minutes and yards but changed in the 1960s to Mils and meters.

A full degree is too big a step for accurate artillery fire and combining degrees and minutes can cause confusion.

A full circle = 360 degrees = 6400 Mils (the word comes from milliradian).




There are a number of reasons why 6400 was chosen:
1. It approximately matches the accuracy/resolution capability of artillery optical sights.
2. It divides by two multiple times easily - all points of the compass are multiples of 200Mils.
3. It makes it very easy for artillery observers to convert distance to angles.

 
Point 3 Continued:
Imagine that you are at the center of a circle of radius 1km (1000m).

The circumference of that circle is 2 x pi x 1000 = 6284m which is pretty close to 6400 so each Mil of angle represents 1 meter on the ground when viewed at a range of 1km.

If a round lands just to the left of the target that is 1km away and your binocular graticule tells you that it is 50Mils left, you know that that represents 50 metres on the ground.

In general, distance left/right = angle subtended in Mils x range in km.

If a target is 4km away and the round lands 70Mils to the right, that’s 70 x 4 = 280 meters on the ground.



 
All western countries use 6400Mils but countries that were part of the soviet bloc use a 6000Mils circle on all of their compasses, protractors and instruments.

It is still roughly 1m at 1km but they reckoned that people were more used to breaking a circle into 60, like on the face of a clock.

Sweden used a circle with 6300Mils for a while but now uses the 6400 NATO standard Mils.

In scientific work, there are exactly 2 x pi x 1000 (milliradian) in a full circle.
 

More Reading: https://en.wikipedia.org/wiki/Angular_mil