Why did 360° suddenly become 2π? Who approved this? 😭 You get comfortable measuring angles in degrees, open a trigonometry or calculus textbook, and suddenly there are πs everywhere. A quarter turn is π/2. Half a turn is π. Somehow the whole circle is 2π. Okay. But what are we actually COUNTING? Start with the radius: the distance from the center of a circle to its edge. Take a piece of string exactly that long. Lay it along the curved edge of the circle. Draw a line from the center to each end. The angle you just made is ONE RADIAN. That is what the animation is doing. The orange length bends around the circle while keeping the same length as the blue radius. We have used the circle’s own radius as our measuring stick. One radius of curved distance → one radian. Two radius lengths → two radians. Keep wrapping until you have gone all the way around. Now the familiar circumference formula actually tells us something: C = 2πr The distance around the circle is 2π times its radius. Divide by one radius to count how many radius lengths fit: C/r = 2πr/r = 2π So a full turn contains 2π radians. About 6.283185… radius lengths. You can fit six whole lengths around the edge, with a little bit still left over. That leftover matters. Six radians gets you CLOSE to one full turn. It does not get you all the way back to the start. And now the conversions have a picture behind them: 360° = 2π radians 180° = π radians 90° = π/2 radians 60° = π/3 radians 45° = π/4 radians 30° = π/6 radians One radian is about 57.3°. It is the angle made by one radius worth of ARC length. Measure along the curve. Drawing a straight shortcut between the endpoints gives you a different distance. “Fine. But what happens if I make the circle bigger?” The radius gets bigger. The circumference gets bigger by the same factor. Their ratio stays 2π. Double the radius and you double the distance around it, so you still need exactly the same number of radius lengths to complete a lap. For any angle, the general rule is: θ = s/r θ is the angle in radians. s is the arc length. r is the radius. You are asking how many radius lengths fit into that particular stretch of the circumference. Then the UNIT CIRCLE makes the arithmetic especially simple. Its radius is 1. That means s/r becomes s/1, so the numerical arc length equals the angle in radians. Walk 1 length unit around its edge: 1 radian. Walk π length units: half a lap. Walk 2π length units: one whole lap. That is also why sine and cosine repeat every 2π. After one full lap, the dot is back at the same horizontal and vertical positions. On the unit circle, those coordinates are cos θ and sin θ. The same position gives you the same answers again: sin(θ + 2π) = sin θ cos(θ + 2π) = cos θ And yes, degrees still work. Nobody confiscated your protractor. The angle itself has not changed; we are measuring it with a different unit. This is a real textbook definition. The book shown is OpenStax’s Calculus Volume 1, section 1.3, “Trigonometric Functions.” You can read it free on the OpenStax website and follow the same connection from radians to the unit circle. A quick way to convert: multiply degrees by π/180 to get radians. Multiply radians by 180/π to get degrees. For example, 90 × π/180 = π/2. The conversion agrees with the quarter turn we can already see. Also check your calculator’s DEG/RAD setting. Typing 30 in degree mode and typing 30 in radian mode asks about different angles. For the same 30° angle, the radian input is π/6. Both give sine equal to 1/2. That setting changes what your input means, so it can completely change your answer. Next time you see π/2 in a question, picture a quarter turn. Next time you see 2π, picture the dot completing a lap. Give the symbol something physical to mean. What did you think a radian actually was before this? #GravityAssist #Math #Radians #Trigonometry #Calculus - @gravityassistus