function drawHour(hour){

ctx.save();

ctx.beginPath();

ctx.lineWidth = '5';

//设置线条末端为圆形

ctx.lineCap = 'round';

//每小时弧度 2*Math.PI/12 ==>Math.PI/6

var radHour = hour * Math.PI/6;

//旋转

ctx.rotate(radHour);

ctx.moveTo(0,0);

ctx.lineTo(0,-radius/2);

ctx.stroke();

ctx.restore();

}

function drawMinute(minute){

ctx.save();

ctx.beginPath();

ctx.lineWidth = '3' ;

var radMinute = minute * Math.PI/30;

ctx.rotate(radMinute);

ctx.moveTo(0,0);

ctx.lineTo(60,0);

ctx.stroke();

ctx.restore();

}

drawBackground();

drawHour(6);

drawMinute(30);

指向6点45分，这样看的话drawMinute的旋转是以3点为起始值得