As we approach block 800,000, the question is begged - why did Satoshi choose halvings every 210,000 blocks? Where did he pull that number from?

It just, really seems arbitrary. Why 210,000? Why not make it a round number and just go with 200,000 if he wanted to do it that way? Why not make them more frequent to help create a little more buzz around them, and go with just 100,000 blocks between each halving. Moreover, how did he come up with these numbers - did he just throw darts at a dartboard, or maybe just dream them up? Maybe Satoshi himself was 21 years old at the time he created Bitcoin?

I'm just trying to think where he got these amounts from, as they seem like they were just pulled out of the air.