It was the 15 October 1502, just a few hours before midnight in an old tavern at the docks. Emerald raised two fingers, after repeatedly looking at his three cards, before finally answering the awaiting dealer. "Stick!" His voice was a bit shaky, since his last savings were at stake. As each player started to reveal his cards, Emerald quickly counting their points - and with 28, he was the highest so far. But this just wasn't his lucky day, and just as he revealed his cards, the dealer started to smile - revealing his 31 points.
"I can't believe it! Again!" Emerald dug his nails into his wooden chair, looking up to the dealer. "I don't have more with me. I'll pay the rest next month, alright?"
The dealer nodded, before asking with another smile "Tell ye what... I'll give ya chance ta 'win it back. With a simple lil' bet."
Emerald raised an eyebrow, "A bet?"
The dealer calmly nodded once more while reaching for the other players' cards. "Yeah. Remember that ol' bookworm th'was here earlier?"
Emerald shrugged. "The guy that blabbered about a round world?" he scratched his chin. "What about him?"
The dealer's smile turned into a greedy grin. "Prove me that th'world is a ball, and you get m'winnings. If ya can't, I get double. 'til Friday. Deal?"
Emerald had the luck to talk with the scholar from before and smiled, quickly standing up to shake the dealers hand. "Deal. But I think I can prove it to you tonight, if you still stay sober for another hour or two."
How can Emerald prove that the world is round without leaving the city? And how can he do it in the same night?
For extra fun: What's the game they're playing?
(This is my first riddle here, and I wanted to add a bit of atmosphere to it - but since I'm not a native English speaker, my writing style is probably a bit weird. ;) Still, good luck 'solving' it. It's not too hard. Also, tagging puzzles is a puzzle by itself!)