Magic number in boost::hash_combine

Question

The boost::hash_combine template function takes a reference to a hash (called seed) and an object v. According to the docs, it combines seed with the hash of v by

seed ^= hash_value(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);

I can see that this is deterministic. I see why a XOR is used.

I bet the addition helps in mapping similar values widely apart so probing hash tables won't break down, but can someone explain what the magic constant is?