Why is `C-C` bond length in graphite shorter than in `C-C` bond length in diamond?

Explanation

Graphite is `sp^(2)` hybridised and `C-C` bond involves `sp^(2)-sp^(2)` hybrid carbons. Diamond is `sp^(3)` hybridised and `C-C` bond involves `sp^(3)-sp^(3)` hybrid carbons. Moreover, greater the `s`-character in hybridised atom, the smaller is the size of hybridised orbital and greater is the overlapping (hence shorter is the bond length). `s-`character in `sp^(2)` orbital is more than that in `sp^(3)` orbital.