user3125: I was looking thru Block explorer at an address with a balance.  It has never been spent from thus the public key is unknown.  If the public key is unknown, how can Block explorer calculate the hash160 of the public key?

user208: Because an address is the Hash160 (=RIPEMD160(SHA256(x))) of the public key. The transaction output lists that 160-bit hash directly.
When redeeming such an output, you provide both the public key (which must hash to the value given in the output) and a signature (which must be valid for the given public key).

3125: <p>I was looking thru Block explorer at an address with a balance.  It has never been spent from thus the public key is unknown.  If the public key is unknown, how can Block explorer calculate the hash160 of the public key?</p>
 208: <p>Because an address <strong>is</strong> the Hash160 (=RIPEMD160(SHA256(x))) of the public key. The transaction output lists that 160-bit hash directly.</p>

<p>When redeeming such an output, you provide both the public key (which must hash to the value given in the output) and a signature (which must be valid for the given public key).</p>
