Suspend several corks from strings of different lengths, and tie identical weights to them. Arrange the corks in a line, in an aquarium filled with water.

Look straight down through the water at a cork and, while looking, place a mark on the outside of the aquarium with a grease pencil to show the point where you think the top of each cork is.

When the brilliant scientist says "where you THINK the top of each cork is," it tells us that we will be mistaken. Read on.

Compare the marks with the actual heights of the corks when they are viewed through the side of the aquarium. (The marks will tend to be above the actual heights of the corks.)

You will realize that water affects depth perception. (Visibility is distorted because of the refraction of light in the water.)

Perhaps that is why, in life-guard classes, we were taught to swim under water for the last 10 or 12 feet to the struggling victim. (Another reason, of course, is because if the victim sees the rescuer, the victim will grab the rescuer and they will both drown.) However, it is easier to judge the distance to a victim if you look through the water laterally, as opposed to looking into the water from the surface.

Air also affects depth perception, although to a lesser extent. Because we become adjusted to the effect, we find it difficult to judge distances when air is absent. (Astronauts report that they had some difficulty judging distances while they were on the moon.)

So let's go to the moon and check on that.