animating the image rotation smoothly..?

Question

i want to rotate the image while animating it..i am making analog clock in which i want to rotate the seconds hand continuously...(animating)..i am little sucessfull in it..for half of minute it rotate smoothly and after that it starts ticking...(as normal clock) why this happens..?

i am calling this function after every sec to rotate the seconds hand.

    {

        [self addSubview:imgSecView];

 [UIView beginAnimations:nil context:NULL];
 [UIView setAnimationDuration:1.0];
 [UIView setAnimationCurve:UIViewAnimationCurveLinear];
 trSec = CGAffineTransformMakeRotation(sAlpha);

 imgSecView.transform = trSec;

 [UIView commitAnimations];


 }

where sAlpha is the angle rotated every second by hand. why it slows down the animation by time?enter code here