Over the years, I've seen lots of people trying to give examples of how we know the Earth is round. You've seen 'em: ships sailing over the horizon disappear from the keel up (which is actually a good one), or shadows cast at different parts of the Earth have different angles (which is how the Greek Eratosthenes determined the size of the Earth to excellent accuracy).
Those are fine, but to me unsatisfying. I haven't observed the Sun shining straight down a well at noon in Egypt, for one, and who has the patience to watch a ship sail away?
Ah, but here I sit in my mom's sunroom in Sarasota, Florida. The temperature is peaking near 80 F, while at home I believe the high will be closer to 35. That's certainly an indirect indication that the Earth is not only round, but also tilted!
But even that won't do it. After all, a clever (if there is one) geocentrist might say, maybe the climate is simply different on different parts of the disk-shaped Earth.
But in this day of jet travel, direct evidence of a globular Earth is trivial. The night before I left Boulder, I got a good look at Orion hanging in the sky over my southern horizon. Last night, after US Airways had spent an entire day bending me over, I went outside to stretch, and saw my old friend the Mighty Hunter once again. Orion was noticeably higher off the horizon.
Boulder is at a latitude of 40 degrees north. Sarasota is at 27*, a difference of 13 degrees south. From Boulder, the belt of Orion is about40 50 degrees off the horizon when it reaches its highest point in the sky in the south (called the culmination), so a change of 13 degrees is pretty obvious. Orion is about 15 degrees high, so that puts Rigel (Orion's knee) where I'm used to seeing Betelgeuse (his armpit). Any amateur astronomer worth their salt would notice that!
This is pretty conclusive that we live on a round planet. Unless we live on the edge of a geocentrist's disk, that means the Earth is a sphere. Or close enough, anyway.
*I'll add that the first time I ever saw Canopus was from Sarasota. It never gets above the horizon for people living north of about 38 north latitude, and from Florida it gets just high enough to see. Canopus is the second brightest nighttime star (after Sirius), so that's a pretty cool thing to see. Took me a few minutes to figure out the first time I saw it, too.