Have You Heard? The Earth is Round!

We spent the weekend in Rocky Point, Sonora, Mexico, visiting our friend Margie who lives a block from the beach on the Sea of Cortez. We had a great time - look for a couple more posts the next few days. We saw a neat effect that any casual observer could make that is presented here.

Her 3rd floor astronomy deck provides a great view of what looks to be an ocean, and to the southeast about 40km away (25 miles) is a stark white archipelago known as Bird Island. While an impressive set of peaks from the top of her house, from sea level there is barely a blip visible. The included images demonstrate - at left is the view from the roof, the middle shot about 40 feet (12 meters) lower, and the rightmost about that much lower again near sea level. The curvature of the sea when near that elevation blocks all but the highest peaks. A little higher and less of the island is obscured. These pics taken with a 500mm lens, taken within a few minutes of each other, and are reproduced at about the same scale.

Knowing the elevation of the island peaks, a more precise value of the elevation of my observations and a little trigonometry and you could estimate the diameter of the Earth. I'll leave that as an exercise for the reader...