[ProtectOurFreedom]

For a visual area roughly one-quarter the size of a full moon, there are only about 10 nerve cells connecting the retina to the visual cortex.

I thoroughly HATE stupid analogies. What the hell does "one-quarter the size of a full moon" mean? Why can't a somewhat technical journal just specify area in square inches or millimeters or use some technical measurement?

The moon has a total surface area of 14,647,440 square miles. At full, half of it is illuminated. One-quarter of that is 1,830,929 square miles. Is the author saying our eyeballs have a visual area of 1.8 million square miles?

Or is Hartnett saying that the subtended arc of our vision is 1/4 of the arc of the full moon? That can't be as we would all have tunnel vision.

[DUMBGRUNT]

What the hell does “one-quarter the size of a full moon” mean?

I took that as, the equivalent fraction of our field of vision. With or without the moon illusion, I don’t know?

[Cboldt]

The claim is that there are 10 nerve cells in the retina servicing the fraction of a person's field of vision that occupies the area of an object that is 1/4 the size of the full moon.

It's not an analogy. It's meant to be a precise statement, but made in terms that people who are clueless about arc-seconds can comprehend. Most people can picture "a full moon" without knowing the diameter of the moon either.