0 is the freezing point and 100 is the boiling point at normal pressure. How is that arbitrary?
LOL. Let me help you:
1. the freezing point (arbitrary but easily observable state)
2. of pure water with no dissolved substances (arbitrary but common chemical compound)
3. at sea level (arbitrary but easily located place)
4. at normal atmospheric pressure
5. on earth (arbitrary but very convenient location)
6. is 0 degrees (arbitrary value which kind of makes sense until you realize that you can still get colder)
7. and the boiling point of water at sea level on earth at normal atmospheric pressure (previous comments still apply)
8. is 100 degrees (arbitrary number chosen for convenience of the units - "10" would be too course grained and "1000" would be too fine grained)
So, yes, the celsius scale is arbitrary, the Fahrenheit only slightly more so. At least the celsius scale can be kind of reproduced in a pinch if you're at sea level and normal pressure and you have water and the ability to freeze and heat it. But, then, if you have all that you can reproduce the Fahrenheit scale, too.
For an idea of a less arbitrary scale look at the Kelvin scale. On it, "0" is the absolute lowest temperature where matter has absolutely no heat content. Of course the scale is the same as celsius so it still ends up being arbitrary in scale, which *any* temperature scale will be. But "0" being "absolute 0" is what sets it apart.