Conversely to the question, the moon is actually not that bright, especially when we compare it to other astronomical objects in space. The reason that the moon appears so bright in our night sky on earth, is because we are comparing the brightness of it to the object around us that are receiving absolutely no direct sunlight. So, the moon appears brighter to us on earth when it is receiving direct sunlight, but that is only because the part of the earth that is not receiving sunlight, and is therefore in night-time, is not as bright.
The earth’s moon is one of the least reflective astronomical objects in our solar system, and if you were to float in front of the sun as an astronaut (not really recommended) and see the moon aligned with the earth, you would see that the earth is far brighter than the moon, even when the two of them receive the same amount of sunlight.
The way that the human eye sees things is heavily reliant on light entering them directly. The reason that we can see things in the world is because they themselves are reflecting that same light into our eyes.
Or they can also produce their own light which is, again, directed into our eyes. So, an object will either reflect borrowed light or create its own in order for our eyes to see it.
The objects that create their own light will tend to be the brightest object nearby in relation to the objects that do not create and give off their own light. examples of this on earth are light bulbs, fires and screens. Examples of this in space and on an astronomical scale are stars.
These are the main producers of their own light, hence why the sun is the main producer of most of the light that we see and use to see other things on earth.
Stars are obviously some of the brightest things in the universe. And it just so happens that we have a star in our solar system. It would not, in fact, be called a solar system if we did not have a star.
And we would not be alive to make that statement if we did not have a star. Our sun is the reason for life on earth, we are orbiting the sweet spot where life can be sustained thanks to the sun. Not only because of its warmth but also because of its light.
The sun produces most of the light that we encounter on earth, it allows us to see most things on earth’s surface in the daytime, so what happens at night?
Well, planets and moons, including earth and its moon, do not produce their own light (obviously not including the unnatural light pollution that us humans produce). If they suddenly started glowing, it would mean that they were probably large enough to carry out nuclear fusion, something that stars do, thus it would make them a star and no longer a planet or moon.
So, how does the moon light up some of earth at night? Well, they reflect light, or borrow it from other astronomical bodies. In this case, the moon reflects the sun’s light when half of the earth is in darkness and facing away from the sun.
The moon is not the only astronomical object that reflects the sun’s light causing us to be able to see it. Sometimes we are able to see other planets in our solar system, simply because they are lined up in such a way on their orbits around the sun that they can reflect the sun’s light to earth and allow us to see them.
The reason we associate this light reflection more with the moon is because we can see it every night, the moon is constantly reflecting the sun’s light.
The brightness, or amount of light that these planets and moons reflect, depends on a lot of factors. It depends where the planet is in orbit around the sun, where it is in its own axis spin, and what the planet or moon is made of.
Earth appears bright because the light is reflected by the clouds in our atmosphere, as well as the shiny surface of the water on our planet. Planets that are covered in ice will have a similar effect, but planets or moons with an exclusively rocky or dusty surface will not be as reflective.
Therefore, the moon that orbits our earth, is not very bright in comparison to, say, Venus. So, the reason that we can see the moon is because it reflects the sun’s light, just like every other astronomical body in the solar system.
But it is not a simple as that: there are in fact two types of reflectivity. The first is called specular reflectivity. With specular reflectivity, it measures how much of the original light gets reflected by the object that is borrowing the light back along the mirror line/angle. The other type of reflectivity is called diffuse reflectivity.
This type measures how much light gets reflected in any direction. Interestingly, an actual mirror has a high specular reflectivity but a low diffuse reflectivity. Something like dust or sand will have the opposite: a low specular reflectivity, but a high diffuse reflectivity.
A human in normal day to day life will experience specular reflectivity in things like mirror images or even spots of glare on objects when they reflect the sun’s light.
But we experience diffuse reflectivity more often. This is the reflectivity that allows us to see things with the sun’s light. Diffuse reflectivity presents itself in the form of a uniformity in light, brightness and color in the world. For example, if you are looking at a bright red apple that is sat on the grass on a sunny day, no matter which angle you look at the apple from, it will be just as bright and the same uniform color thanks to diffuse reflectivity.
But there may be a spot on the apple that shines brighter than the other red parts, this sheen can be blamed on specular reflectivity, since it is the only point on the apple that the light from the sun is reflecting straight back.
A rough surface tends to have are greater amount of diffuse reflectivity than specular reflectivity. This is simply because specular reflectivity works better on a smooth reflective surface. If you take a ceramic pot for example: an object with a smooth, shiny surface that will reflect light in a specular way.
If you take some sandpaper to it and rough up the surface, you are essentially creating many smaller reflecting planes that will reflect light in multiple different directions instead of just straight back in a specular fashion.
Now think about how rough the surface of the moon is. The roughness of the earth’s moon means that it has a greater diffuse reflectivity than a specular one. The sun’s light is diffused when it reflects off the surface of the moon. This is why the moon is not the brightest moon in the solar system, nor is it that bright in terms of astronomical body standards.
This overall brightness can be defined in this way: by looking at its ‘bond albedo’. The bond albedo of an object is simply the average amount of total light that is scattered in any direction by the object. Or essentially its measured diffuse reflectivity.
A perfectly black object would have a bond albedo of 0% for reference. And the moon that orbits our earth has a bond albedo of 12%, compared to that of 85% for Triton, one of Neptune’s moons. The reason Triton is so bright is because it is covered in ice that is rough enough to reflect the sun’s light in a heavily diffused way.
To summarise then: the moon is bright in our night sky because everything else seems so dark in comparison. But in the greater scheme of things, the moon is one of the dimmest astronomical bodies in our solar system. To put a number on it, the moon as a bond albedo of 12%, less than earth’s and less than all of the other planets in the solar system.
It reflects the sun’s light, mainly in a diffuse reflectivity way, but this is still not enough to make it as bright as other bodies.