A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2006 Apr 14, 04:06 -0700
Gary LaPook wrote:
The U. S Army uses what we call the "artillery mil" of which there
are 6400 in a circle so there are 17.777777777 mils in a degree and
each mil equals 3.375 minutes of arc. This is an approximation of a
milli-radian. Since there are 2 Pi radians in a circle there would
actually be 2000 Pi milli-radians in a circle or 6283.185307
milli-radians, approximately. The artillery mil approximates the true
milli-radian within 2%. The Russian approximation of the milli-radian
with 6000 units approximates the true milli-radian within 5%. They are
both used the same way and each has compasses marked in these units.
There are two uses of the mil (in addition to stating direction), one to get a quick approximation of range or dispersion, and the other use is precision measurement of angles for aiming the cannons and for surveying. The first use works with the relationship that one mil has a tan (or sin, since they are virtually the same for small angles) of 1/1000. This is true within the 2% accuracy mentioned above which is closer than you can measure with the reticule in a binocular. Think of the mil scale reticule as a scale on tans. An object one meter high will subtend an angle of 1 mil at 1000 meters, 1/2 mil at 2,000 meters, 2 mils at 500 meters, etc. So the formula you use to determine the range to an object is to divide the known height of the object by the number of mils it subtends and then multiply by 1000. For example, you see a russian T-72 tank coming at you. You know it is 3 meters tall. You look through your binocs and see it subtends and angle of 2 mils. Three meters divided by 2 mils equals 1.5 times 1,000 means the tank is 1,500 meters away. Set your sights to a range of 1,500 meters and open fire.
This approximation is also used to adjust artillery fire. The
observer is calling in artillery onto an enemy position located at a
road junction that he has determined from reading his map is 4, 200
meters away form his observation post. He rounds this to 4,000 meters.
He knows that at this distance a mil on the horizontal reticule of his
binocs represents 4 meters. Looking through his binocs, he sees the
first rounds land 15 mils to the left of the target. He multiples 15
mils by 4 meters per mil in his head and calls in the adjustment of
"right six zero." The next rounds land on the target.
The other precise use of the mil is for precision aiming of the
guns. The onboard fire control equipment mounted on the cannon allows
you to aim the tube right and left to a precision of 1 mil which means
that if you are firing at a target 22,000 meters from the gun you can
expect the rounds to land within 22 meters right or left of the target.
An 8 inch howitzer projectile (which weighs 200.0 lb..) has a bursting
radius of 40 meters, so close counts in horseshoes, hand grenades, and
howitzers. Usually elevation is set to a precision of 1 mil. If
precision fire is needed a gunner's quadrant allows you to adjust the
elevation of the tube to one-tenth of a mil.
Artillery units also have survey sections and their transits are
calibrated in mils and their trig tables and computers use trig
function calibrated in mils.
And the compass ("bussole" in Soviet artillery,
but compass in the Soviet Navy)
was divided into 60 parts, correct?
Do US and British Armies use degrees?
On Fri, 7 Apr 2006, jean-philippe planas wrote:
> The French artillery (and infantry)
> uses division called "thousandth" (millième") as well. "One Thousandth"
> is the angle of an object one meter high seen from a 1000m (1 km
> distance) with the same conclusions as Alex.
using compasses/optics that are graduated in degrees and mils.
In the US equipment there are 6400 mils to 360 degrees. And on long range
optics (rifle scopes, arty optics etc) there are markings on the horizontal
and vertical stadia for range finding. These markings (round dots) are 1 mil
in diam. Because 1 mil subtends a certain area at certain ranges these
optics give good results in range finding.
I had a Russian (Soviet actually) compass a few years ago. It, as you
stated, was marked in 6,000 units instead of the 6,400 units I was used to
using. It was confusing as I always had to think about the difference when
using it instead of just using the equipment. I got rid of the Soviet
compass. Not because of inferior quality of the compass but I am comfortable
using 6400 mils to a circle.