Why does the Moon Look Flat


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I'm here to show you the moon. You’ve seen it before, but I’m here to tell you that there’s something wrong with it.

When the moon is full it is on the opposite side of the earth as the sun. So, when you look at the full moon the sunlight is coming from behind you, hitting the moon and reflecting back to your eyes.

This billiard ball is illuminated from the same perspective. The light is coming from behind the camera, reflecting off the ball and back to the camera.

Look at the edge of the ball. It is dark and the edge is indistinct. Why is this?

When a ray of light strikes the center of the ball it is  scattered, but much of the light reflects back in the direction it came from.

When a ray of light strikes away from the center, more light is scattered away, but a lot of the light still scatters back in the direction it came from.

However, when a ray of light strikes near the limb, most of the scattered light glances of away from its source and is lost. Little reflects back. This makes the limb appear dark and indistinct.

But look at the moon. It is evenly lit right up to the edge. How can that be?

One possible explanation could be that the face of the moon is flat. Then, no matter where the light strikes it will be scattered the same way. The face will be evenly illuminated.

If the moon were flat the light would reflect back the same way no matter where it hit.

So what else could cause the moon to be bright right up to the edge.

If the moon were retroreflective the light would reflect back the same way no matter where it hit.

Is there anything else that could cause light striking the moon to reflect back the way it came?

Light striking mirrors set at right angles always reflects back the way it came.

If you place two mirrors at a right angle to each other, any rays light that strike them from a reasonable range of angles will be reflected back the way they came.

This illustration show how this happens on a two-dimensional plane.

Here we have three flat metal surfaces, all at right angles to each other. This is a radar reflector and will reflect radio waves back the way they came in three dimensions.

This is why modern military aircraft and water-going vessels are designed to have no flat surfaces….

Intersecting each other at right angles. Eliminating such geometry makes the craft stealthy when it comes to radar detection.

A bicycle reflector is such a retroreflector--reflecting light back the way it came. It is made of many trios of flat surfaces intersecting at right angles. Each of these “corner-cubes” is a miniature retroreflector.

Could be moon be covered with retroreflectors? Maybe not corner cubes, but what about another shape?

Let’s take a look at how light travels through a sphere of glass. Here we see three rays of light entering a glass sphere from different angles to its surface. This light bends but continues on its way when it exits the glass. Nothing special to see here.

However, if there is something on the backside of the sphere to reflect the light back into it, each ray returns back the way it came.

Here I’m holding a glass sphere that is illuminated the same way as the billiard ball. If I put my finger in the right place on the back side of the sphere, reflecting the light back in, it becomes very bright. It becomes a retroreflector.

This is why when work crews dust the top of freshly painted lines on streets with glass beads. With the beads partially imbedded into the paint, the paint reflects exiting light back into the beads making them into thousands retroreflectors. The painted lines become brilliantly illuminated by vehicle headlights when seen from the driver’s perspective.

During the early 1960s NASA was preparing to land a robotic probe named surveyor on the moon in preparation for the Apollo Lunar Missions. NASA scientists, noting the even illumination of the face of the moon, speculated that the moon must be covered with tiny glass beads. The lunar dust on which these beads sat would reflect enough exiting light back into the beads to make them retro-reflective.

When samples were finally analyzed microscopically it was discovered that about 15% of the material on the moon’s surface consists of glass beads. They are dark. Some nearly black, but enough light passes through them to make them into retro-reflectors.

Much of the light striking the moon reflects back the way it came. When we see the full moon the sun is at our backs so this light reflects back to us.

So the moon, covered with these tiny glass spheres, reflects light back the way it came. So, to us, it appears bright right up to the edge.