Moon provides a visual treat to our eyes. Some times it appears deep reddish orange color and most of the times its white in color. So what is the actual color of moon, and why the apparent change in color at different times? To answer these questions, first lets glimpse through governing fundamental principles and reason out from there.
Property of gray color:
Gray color has a flat reflection spectrum meaning that all colors that are incident on it are reflected equally. All the wavelengths in the visible spectrum of incident solar radiation is reflected back to give a white color perception .
Sky is filled with particles of varying size. Scattering depends on the size of the particles. When the particle size is small, the scattering is wavelength dependent. Smaller wavelengths like blue are scattered more and longer wavelengths are scattered less.
An incident light ray has three different ways : Transmission, reflection and absorption. Reflectance is the amount incident light reflected back to the incident medium. The surface of moon reflects only ~12% of incident radiation.
Retroreflectance is a property of reflecting the light in the same direction of the incident direction. For example Cat-eye, road signs and danger marks are retroreflective surfaces. The surface of the moon is retroreflective.
Armed with the underlying principles, lets reason out. When the moon rises in the evening (near horizon), it is at a greater distance from us. The light coming from moon is scattered by atmospheric particles. The smaller wavelengths like blue, violet are scattered in the atmosphere and only red color reach us. Hence moon appears to be reddish-orange
Actual color of moon:
Photograph of moon1
The actual color surface of moon is grey (grey has flat reflectance spectra!). As moon rises directly above us , atmospheric scattering effect is reduced. All colors reach our eye approximately equally yielding a white color perception. The bright white color is also attributed to the high contrast of moon.
During New Moon, moon is in between sun and earth. Hence none of the reflected light reaches us and it appears dark or invisible.
The surface of the moon is retroreflective in nature. It reflects most of the light back in the direction of incidence. At the time of full moon, earth lies exactly at the middle of sun and moon. Since most of the reflected light is directed to earth and appears brighter than other phases of moon. Upto 40% increase in brightness is observed from 1st quarter phase to full moon phase.
During Lunar eclipse, the shadow of earth falls on moon and it appears dark for a while. However during that time, it appears red in color briefly. This is because, scattered red color light from earth's atmosphere reaches moon and is reflected back to us.
Blood red color of moon at one stage during eclipse3
At times you could see colored rings around moon. This is termed as a halo. There are two types of halos formed around moon, viz. Lunar corona and the 22 degree halo.
Lunar Corona, shot from my phone
Lunar Corona is formed due to refraction and diffraction of moon light by thin cloud containing water droplets and ice crystals.
22 degree halo of moon4
22 degree halo is caused by the refraction of moon light by the ice crystals present in the cloud. 22 degree signifies the angle between the ends of the ring with respect to the observer in earth.
If you look carefully at the above pictures, the innermost color is blue and outermost color is red in case of Lunar corona and vice versa incase of 22 degree halo. This is because of single refraction through the water droplets in case of Lunar corona similar to primary rainbow and double refraction through the hexagonal shaped ice crystals in case of 22 degree halo, similar to secondary rainbow.
Sources of Photographs:
- Photograph by my friend, Gaurav Shroff (https://instagram.com/p/0BACPHDPFY/)
- Photograph by Stephen Balaban (http://www.stephenbalaban.com/)