How can Enel reach the moon? On the mini-series in Manga, Enel goes to the moon. It's a side story titled Enel's Great Space Operations. How can he reach the moon? 


He goes to space where there is no air, how can he breathe? 
It's a really long distance, the distance between earth and moon is 384,400 km (238,900 miles). And considering that One Piece takes place in the sixteenth century, the average speed of a ship at that time was less than 10 knot (18.52 kph/11.5 mph), it will take more than 2 years to reach the moon with a ship like that.
Can't he just use his power to reach the moon? (I don't know if lightning can go through space or not, maybe Kizaru can do it better?)
The problem I see is that you are using the logic of our history versus logic of One Piece.

In the 16th century, they did not have ships that could travel in the air nor in space for that matter.


According to the Wikia - Enel,


  His mini-series suggests he does not need to breathe and can survive outside of the atmosphere. However, his electricity-based powers have no effect on certain substances such as rubber, making Luffy a formidable enemy, as he is immune to Enel's powers.

You're using the logic of our history vs One Piece. The ship they used was the Ark Maxim which is a flying ship, stuff that we did not have in the 16th century. Also, Enel used his electricity powers to power the ship itself so it's possible to say that the ship could possibly reach very high speeds in space, there's nothing to slow down the ship. All they would need is a powerful thrust force to move in space, which can most likely be done using the chemical vats on the ship.

As the Wikia - Ark Maxim states,


  The Maxim is capable of flight, even in space. Enel's electrical energy is her main power supply, while two hundred Jet Dials are used as a backup power supply to keep the ark in the air for an hour if something were to happen to the Ark's mechanism. Enel used the gold from the Shandora ruins as a conductor for his electricity.


The Wikia states this about the vats


  Enel can use Maxim to create massive lightning storms via a combination of chemical vats that produce storms clouds and his own lightning abilities. One of these thunderbolts is enough to burn down an entire village, and Enel can create a giant thundercloud that can destroy an entire island.

I don't believe he can reach the moon just by his own power, wouldn't that imply that he could fly or teleport with his lightning powers. He does use his powers as previously stated only to fly the Ark Maxim to the moon though.