Degree to Radian

The Babylonians used base-60 arithmetic and noticed the year was close to 360 days — convenient because 360 divides evenly by 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, and more. That ridiculous number of factors makes slicing a circle into halves, thirds, quarters, fifths, sixths, and eighths all come out to whole numbers. A base-100 system would only divide cleanly by 2, 4, 5, 10, 20, 25, and 50 — far less flexible for geometry and land surveying.

Navigation requires quick mental arithmetic with headings, bearings, and wind corrections. "Turn right to heading 270" is instantly understood in a cockpit; "turn right to 4.712 radians" would get someone killed. Compass roses, runway numbers (which are headings divided by 10), and nautical charts all assume a 360° circle. The entire global aviation and maritime infrastructure — from VOR stations to AIS transponders — is calibrated in degrees.

The degree symbol is a small raised circle placed immediately after the number with no space (45°, not 45 °). On Windows, hold Alt and type 0176 on the numpad. On Mac, press Option+Shift+8. On phones, long-press the zero key. In HTML, use ° or °. A common mistake is using a superscript letter "o" or the ring-above diacritic — these look similar but are different Unicode characters and can break search or data parsing.

One degree equals 60 arcminutes, and one arcminute equals 60 arcseconds — so 1° = 3,600″. To convert DMS to decimal, divide minutes by 60 and seconds by 3,600, then add them to the degree value. For example, 40°26′46″N becomes 40 + 26/60 + 46/3600 = 40.4461°. Google Maps uses decimal degrees internally but lets you enter either format. At the equator, one degree of longitude spans about 111 km.

Yes — the French Revolutionary government introduced the gradian (grad), dividing a circle into 400 parts to match the metric system. A right angle became exactly 100 grad. They also tried decimal time (10-hour days) and a decimal calendar. The time and calendar experiments died within a few years, but gradians survived and are still used in French and German surveying. Every scientific calculator has a GRAD mode because of this 230-year-old French experiment.

Radians make calculus work cleanly. The derivative of sin(x) is cos(x) — but only if x is in radians. In degrees, the derivative picks up an ugly π/180 factor that contaminates every formula. Angular frequency (ω = 2πf), rotational kinetic energy, wave equations, and Euler's formula (e^(iπ) = −1) all assume radians. Degrees would litter physics with conversion constants the way imperial units litter engineering. Radians aren't a preference — they're the unit that makes the math not lie to you.

Imagine wrapping the radius of a circle along its curved edge — the angle that arc subtends at the center is one radian. It works out to about 57.3°, which is a little less than the angle of an equilateral triangle's corner (60°). A pizza slice cut at one radian would be a generous but not absurd portion — wider than a sixth of the pie but narrower than a quarter. It looks unremarkable, which is ironic given how fundamental it is.

Every major language — C, Python, JavaScript, Java, Rust — uses radians in Math.sin(), Math.cos(), and related functions because the underlying floating-point hardware and Taylor series expansions assume radian input. The Taylor expansion of sin(x) is x − x³/3! + x⁵/5! − … and only converges correctly when x is in radians. Feeding in degrees without converting first is one of the most common bugs in student code and game physics.

Memorise that π radians = 180°. From there: multiply radians by 180/π (roughly 57.3) to get degrees, or multiply degrees by π/180 to get radians. The common angles are worth memorising outright — π/6 = 30°, π/4 = 45°, π/3 = 60°, π/2 = 90°, π = 180°, 2π = 360°. If you forget, just remember that 1 radian ≈ 57° and estimate from there.

Officially dimensionless. The radian is defined as arc length divided by radius — meters over meters — so the units cancel. The SI classifies it as a "supplementary unit" turned "derived unit with the special name radian." This dimensionlessness causes genuine headaches: torque (N·m) and energy (J = N·m) have identical SI dimensions, and only the implicit "per radian" distinguishes them. Some physicists argue the radian should be treated as a base unit to avoid exactly this confusion.