Front Door

My front door is 110 years old and is starting to crack on the outside. The front of my house faces west so we get the setting sun on the front of the house year round. The outside of the door has no like varnish finish left on it and it has turned almost black. It is starting to crack and it has like pit marks where the grain is starting to separate. It is also very dry. The inside is just fine. Is there a way to "re-hydrate" the wood? What do I use to fix the cracks and pitting? Can you suggest anything that I can use while refinishing to better protect the door from the sun in the future?