The apparent brightness of the Moon as seen from Earth is primarily due to its reflection of sunlight. The Moon's surface reflects a significant amount of sunlight back towards Earth, making it appear relatively bright in our night sky. However, when astronauts were on the Moon during the Apollo missions, they experienced a different perspective that can explain why it didn't seem as bright.
The primary reason is that the Moon's surface is covered in a layer of fine dust called regolith, which is made up of small, highly reflective particles. These particles can reflect sunlight very effectively, creating a bright environment when standing on the lunar surface. When sunlight interacts with the regolith, it scatters and bounces off in various directions, illuminating the surroundings and making the landscape appear relatively bright.
In contrast, when astronauts stood on the Moon's surface during the lunar daytime, they were surrounded by a barren, airless environment. The sunlight directly illuminated the astronauts and their immediate surroundings without any atmospheric scattering or diffusion that occurs on Earth. This lack of atmospheric scattering made the overall scene appear more stark and the surrounding landscape appeared relatively darker compared to what we observe from Earth.
Furthermore, the spacesuits worn by the astronauts had tinted visors on their helmets to protect their eyes from the harsh glare of the Sun. These visors may have further diminished the apparent brightness and contrast of the lunar surface.
So, while the Moon appears bright from Earth due to its reflection of sunlight and the scattering of light in Earth's atmosphere, the experience of being on the Moon's surface during a lunar daytime is different. The direct illumination and lack of atmospheric scattering contribute to a perception of relatively less brightness and a darker appearance of the lunar landscape during the Moon landing.