*** Posted by run ***
Hi Chaco

The reason why you see different numbers is because the Speed of sound changes depending on altitude. You can use the following page to give you the correct number for a certain altitude:
<a href='http://www.desktopaero.com/stdatm.html' target='_blank'>http://www.desktopaero.com/stdatm.html</a>
This of course is only for standard atmosphere. As soon as you have a 'non-standard' day these numbers change.

Regards John Raahauge RUN
[I have a question about Mach numbers. I am wondering, what is the exact number for Mach of Mach 1. At what speed does it start? Also, why is it that when I read books on plane specs that the Mach numbers are always different from book to book. It seems that some are just guessing. Thanks for any responses. ]