The moon appears white primarily because its surface reflects sunlight, which contains all colors of the spectrum, equally.
How Sunlight Interaction Causes the White Appearance
The surface of the moon is composed of rocks and dust (regolith). Unlike substances that absorb certain colors and reflect others (which gives objects their specific color), the moon's surface material doesn't selectively absorb colors from sunlight.
Think of sunlight as a mixture of all colors – red, orange, yellow, green, blue, indigo, and violet. When this light hits the moon's surface, the surface reflects virtually all of these colors back towards space and towards Earth without favoring any particular wavelength.
Based on the reference: When the moon is up in the sky during the daylight, it reflects all sunlight equally.
The Lack of Significant Scattering
For us to see an object's color, we perceive the wavelengths of light it reflects or emits. Because the moon reflects all wavelengths of sunlight relatively equally, the combination of these reflected colors appears white to our eyes.
Unlike how Earth's atmosphere scatters sunlight (making the sky blue and sunsets red), the light reflected from the moon is not significantly scattered before it reaches us, especially when viewed from space or under certain conditions.
Why It Looks White During the Day
The appearance of the moon can be influenced by the surrounding environment, particularly the Earth's atmosphere and the presence of sunlight.
According to the reference: During the day, the reflected light not being scattered enough contributes to its white appearance. Furthermore, during the day the moon has to compete with sunlight, whose color is also scattered by the atmosphere, so it looks white.
Here are the key factors contributing to the moon's white appearance during the daytime:
- Equal Reflection: The moon's surface reflects the full spectrum of sunlight uniformly.
- Limited Scattering of Reflected Light: The light reflected from the moon doesn't undergo significant scattering before reaching our eyes.
- Atmospheric Context: The daytime sky itself appears bright and often whitish due to atmospheric scattering of sunlight. The white appearance of the moon during the day is seen in the context of this bright, often hazy or white sky, making its own white color less distinct than it might be against a dark night sky.
Essentially, the moon reflects white sunlight, and when viewed against the bright backdrop of the daytime sky (which is also a product of scattered white sunlight), it appears white because that's the color of the light it's reflecting.
While the moon can sometimes appear yellowish or reddish during moonrises or moonsets due to light passing through more of Earth's atmosphere (similar to sunsets), its fundamental reflected color, especially when high in the sky or viewed away from atmospheric effects, is white.
