TIL breasts have roots . I did not research this but it sounds very reasonable . <| sep |> Consider a clock face with the diameter 1 . No , imagine a clock hand , which starts at 3 o ' clock and moves counterclockwise Why start there and that direction ? Just convention . . Now , the sine of an angle is if you look at the Y axis where the end of the clock hand is . So , sine moves from 0 1 0 1 and back to 0 . The cosine is very much the same , but you look at the X axis . So , the cosine moves from 1 0 1 0 and back to 1 . so , sine and cosine are the same , just 90 degrees offset . Tangent is a bit harder . From the end of the clock hand , draw a line at a right angle to it . Where that line hits the Y axis , that ' s the tangent . So , why are these useful ? Well , these are very , very useful when working with triangles and every form of wave , and most practical geometric problems can be broken down to triangles . So , basically , they are tools useful to handle the most common problems . Edit Radius 1 , not diameter 1 , of course !