As I mentioned briefly in the post, light refraction effects make this difficult to do. According to Google maps, the nearby oil platform is about 2 miles offshore. It appears to be on or past the horizon when I measure from a crouching height of 4 feet. However, the horizon should be closer to 3 miles away according to the generally-accepted size of the Earth. So my photographs actually suggest a much smaller globe, maybe more in line with the size Columbus is reported to have believed in. It's possible this is caused by the ocean's swell, but I can't say for sure.

The main point is this: light often bends around the horizon due to temperature gradients, making the horizon appear more distant than it "should" and thus making the Earth appear larger than it is. I've actually witnessed this phenomenon; a few years ago, there was a partial lunar eclipse in Colorado which happened exactly at sunrise. As you know, a lunar eclipse happens when the Earth is positioned *exactly* between the Sun and the Moon. But on that morning, the Sun appeared in the sky at the same moment as the eclipsed moon. If light were not bending around the horizon due to atmospheric refraction, this would not have been possible.