You can access external data from Ethereum contracts via what they call an "Oracle" which is an intermediary between a third party service and the blockchain.

If the connection to the third party service is cryptographically secure, the Oracle can even provide proof the results were not tampered with, and if multiple data sources are used it can provide a consensus answer based on them.

Otherwise, the smart contract only has access to what is in the blockchain, but Oracles are pretty easy to write and use.