Minute to Mil

One arcminute of latitude was a convenient natural standard for sailors because it could be derived directly from celestial observations with a sextant. Measuring the Sun's altitude to the nearest arcminute and looking up the result in a table gave you your latitude to within one nautical mile — no sophisticated instruments needed. The modern nautical mile (1,852 m) is a standardized approximation of this relationship, and it still underpins all maritime and aviation distance calculations worldwide.

MOA stands for Minute of Angle. One MOA subtends about 29.1 mm (roughly 1.047 inches) at 100 meters, which conveniently rounds to "one inch at a hundred yards" for American shooters. Rifle scope turrets are typically calibrated in ¼ MOA clicks, so four clicks shift the point of impact about one inch at 100 yards. Competitive shooters obsess over MOA because a rifle that groups within 1 MOA is considered accurate enough for serious target work.

Divide arcminutes by 60 to get decimal degrees. So 30 arcminutes is 0.5°, and 7.5 arcminutes is 0.125°. Going the other way, multiply decimal degrees by 60. A GPS coordinate of 51.5074° means 51° plus 0.5074 × 60 = 30.444 arcminutes, or 51°30′26.6″. Most mapping software handles this conversion internally, but knowing it matters when reading older nautical charts or surveying records that use degrees-minutes-seconds notation.

The full Moon spans about 31 arcminutes (roughly half a degree). That means one arcminute on the lunar face corresponds to about 56 km of actual surface. The largest crater visible to the naked eye, Tycho, spans approximately 1.5 arcminutes. This is right at the edge of human visual resolution, which is why you can just barely make out the major dark maria (the "seas") but not individual craters without binoculars.

It really is a coincidence. The Sun is about 400 times the diameter of the Moon, but it also happens to be roughly 400 times farther away — so both subtend almost exactly 30 arcminutes (half a degree) as seen from Earth. This near-perfect match is what makes total solar eclipses possible, with the Moon barely covering the solar disc while leaving the spectacular corona visible. It won't last: the Moon recedes about 3.8 cm per year, so in roughly 600 million years total eclipses will no longer occur.

Because mils create a beautifully simple relationship: at 1,000 meters, 1 mil ≈ 1 meter of lateral distance. An artillery spotter who sees a shell land 30 meters left of the target simply radios "right 30" and the gunner adjusts 30 mils. No trigonometry, no calculator, no conversion tables — just a direct, linear approximation that works under fire. Degrees would require multiplying by 17.45 to get the same offset, which is exactly the kind of arithmetic you don't want to do while being shot at.

NATO uses 6,400 mils per circle because it divides evenly by many tactically useful numbers (2, 4, 8, 16, 32, 64). The former Warsaw Pact used 6,000 for simpler decimal arithmetic. Sweden historically used 6,300 (a closer approximation to 2,000π). The mathematically "pure" mil would be 6,283.19… (2,000π), making 1 mil exactly 1 milliradian — but nobody uses that because it doesn't divide evenly by anything. NATO's 6,400 won out as the global standard.

A true milliradian (mrad) is 1/1000 of a radian, giving 6,283.19… per circle. A NATO mil is 1/6400 of a circle, which is about 0.98 milliradians. The difference is roughly 2%, which matters in precision shooting but not in artillery. Long-range rifle scopes are increasingly calibrated in true milliradians (mrad), while military artillery sticks with NATO mils. If a scope says "mil-dot," it almost certainly means milliradians, not NATO mils.

A mil-dot reticle has dots spaced exactly 1 milliradian apart. If you know the size of your target, you can estimate distance: a 1.8-meter-tall person who spans 3 mil-dots is at 1,800/3 = 600 meters. The formula is target size (mm) ÷ size in mils = range (m). Snipers memorize common reference sizes — vehicle widths, door heights, shoulder widths — so they can range targets without a laser rangefinder. It's 18th-century trigonometry dressed up in modern optics.

A military lensatic compass reads 0 to 6400 mils instead of 0 to 360°. North is 0 (or 6400), east is 1600, south is 3200, west is 4800. Grid references and fire missions are called in mils because they plug directly into artillery calculations. To convert a mil bearing to degrees, multiply by 0.05625 (or divide by 17.78). Most soldiers never bother converting — they think in mils natively, the same way a pilot thinks in knots rather than converting to km/h.