[arbuthnot (OP)]

Why are there two semitones in a major tonic scale,say from c to c,when each note sounds equi distance from one another.Do our ears here something different?If eadh note of the scale were a full tone then c to c would be from c to d strange music

[Make it Lady]

Why are there two semitones in a major tonic scale,say from c to c,when each note sounds equi distance from one another.Do our ears here something different?If eadh note of the scale were a full tone then c to c would be from c to d strange music

In this section you could play 'hair on a G-string'

[lyner]

It just sounds nice, I think.
There are a lot of other scales and 'modes' used in music from other cultures, remember.
If you listen to all the overtones from a fixed length brass instrument (i.e. no valves or slider) played in a high enough register, you get an approximate major scale. This, sort of, sounds normal and presumably relate to our analysis of sounds with the cochlea.
Ideas of harmony and discord are based on harmonic relationships between notes; we recognise things like an octave or a fifth or a third. These are what a major chord consists of. The third and fifth 'sound' right. Other intervals 'sound' less and less 'right' and modern western music has more unconventional intervals; any two notes of the twelve note scale are allowable to our ears. 
There are quarter tone intervals in some Eastern music.
It's complicated!

[DoctorBeaver]

...when each note sounds equi distance from one another.

I'm not sure what you mean by that. I can certainly tell the difference between a tone and a semitone. In fact, in a couple of my compositions I use quarter tones by bending a guitar string through half a semitone.

[lyner]

They don't sound so much 'equal spaced' as 'right'.
Quarter notes and wierd intervals are more adventurous and people need to get used to them.
Of course, many of the discords are resolved, later, by something more familiar. Even if they aren't resolved, there is often an 'implied' resolution.

[Soul Surfer]

There are in fact twelve semitones to an octave in a truly equally tempered scale. The normal seven white notes on the piano miss out five of them to produce the normal C major scale the five black notes fill in the gaps the use of which allows a piano to produce all the keys.

The reason for this is rather obscure and boils down to, that's what we've got used to, and the numbers fit together in more simple ratios if you do it that way.  This is because twelve is divisible by two three and four.

Other musical traditions use different numbers of notes in a scale notably the five note scale of scottish and many other ancient instruments

[lyner]

The Five note scale is about what you get from a brass horn in a high registger. The Eight note scale is what you get in the next register up. The 'scale' which a bugle uses has fewer notes in it because it is blown in a lower register.
 The even tempered scale is an attempt to enable a keyboard instrument to play in any key, using the same 12 notes.
The ratio between each semitone is 'the twelfth root of two'; after twelve semitones you have an octave.
 It's an approximation to a natural scale and is what we have got used to. For brass instruments to stick to the even tempered scale, they need to 'pull' some of their notes slightly. This is second nature for a good musician and not even noticed by a bad one!

[Atomic-S]

See http://en.wikipedia.org/wiki/equal_temperament