strange result of acos

Hi all,

I have to calculate an expression using acos -function.  A strange 
result of acos appears

*1. case with error*

ss <- sin(10.74*pi/180)**2 
+(cos(10.74*pi/180)*cos(10.74*pi/180)*cos(0*pi/180))
 ss
 acos(ss)


[1] NaN
Warning message:
NaNs produced in: acos(ss)

*2. case without  error*

 ss <- sin(10.7*pi/180)**2 
+(cos(10.7*pi/180)*cos(10.7*pi/180)*cos(0*pi/180))
 ss
 acos(ss)



In both cases the variable ss equals 1.   I  think there is a problem 
with precision. If I multiply ss  by 0.999999999  (ss*0.99999999) no 
error occurs.  How  the results can be explained ?

Regards

Klaus-P.

Dr. Klaus-Peter Pleissner
Max Planck Institute for Infection Biology
Core Facility Bioinformatics
Campus Charit? Mitte
Schumannstr. 21/22
D-10117 Berlin
Germany

*Phone:* +49-30-28460-119
*Fax:*   +49-30-28460-507
*URL:*   http://web.mpiib-berlin.mpg.de/bioinformatik <http://web.mpiib-berlin.mpg.de/bioinformatik/>
*Email:* pleissner at mpiib-berlin.mpg.de  <mailto:pleissner at mpiib-berlin.mpg.de>