Update / Erratum on "What IS that below?" (height above the horizon; photos of an arrow)

Ooops.

I forgot about something in my answer yesterday. When you look for the Farallons, you have to think about WHERE you’re standing.

In the original Challenge I asked: And where should I stand?

That’s an important part of the question. If you’re standing on the beach at the westernmost edge of San Francisco, your eyes might be only 5 feet above sea level. A quick search for information about the horizon leads us to this equation to compute the visual distance to the horizon. Here, d is the distance of the horizon (in miles) if your eyes are h feet above the ground. (The squiggly line symbol means “nearly equal to.”)

So, if your eyes are 5 feet above the ground, the horizon seems to be 2.72 miles away. That is, your 5-foot-high eyes can see an island on the sea that’s 2.72 miles away.

As I noted, this picture of the islands was shot from Twin Peaks, a local mountain in the western part of San Francisco.

If you’re standing on Twin Peaks (at a height of 922 feet), the apparent horizon is 37 miles. So how far are the Farallons from Twin Peaks? I used Google Maps to measure the distance and found this:

Since the islands are 30 miles from Twin Peaks, it’s easy to see them on the horizon (with a decent telephoto lens and clear air).

To see the Farallons from the closest beach (which is 27 miles from the Farallons), you’d have to have your eyes at (27/1.22)² = 489 feet in the air. That’s a mighty tall ladder.

Okay. I admit my blunder, and have set the record straight. (Tip of the hat to Don Norman for pointing this out!)

In other news…

I just coincidentally (really) happen to be St. George, Utah to give a talk today at Dixie State University.

As I mentioned in yesterday’s post, one of the Transcontinental Airmail arrows happens to be here. So I popped out there today to take a few pics.

Note the steel L-beam still sticking up out of the concrete.
It was one of the legs of the light tower over the concrete arrow.
St. George is off in the distance.