Things people know: why is the moon white?

The darling of the section, Naama Erzi, challenges us with a new question: Why is the moon white?

We all learned that the moon has no light of its own and it reflects the sunlight back to us. But wait: the sun is yellow and yellow light should also look yellow when reflected in a mirror.

So that's it, Naama, that the sun isn't really yellow and the moon isn't really white.

In 2015, an innocent picture of a dress became a viral web phenomenon when it became clear that some viewers see stripes in blue and black and others in gold and white. The phenomenon exposed the public to the understanding that color is not a property of an object but an interpretation given by the brain to information received from the visual system and this information may be ambiguous. Therefore, the first question is definitely meaningful:

Is the sun yellow?

In a word: no. The sun emits light in all colors, meaning it is white. The world around us is colorful because different objects reflect light in different colors to our eyes which all originate from sunlight. When the sunlight is refracted: in a rainbow, in a prism or in the spray of water drops in a sprinkler, you will be able to see that the yellow stripe is not as strong as its red and green neighbors. Scientists on the International Space Station cruising at an altitude of about 400 km see a white sun at all hours of the day.

So why do children come back from kindergarten with drawings of a smiling yellow sun in a blue sky?

Color of light is the way our brain interprets the wavelengths of the radiation focused on the retina of the eye. 4 types of receptors are waiting for the light waves that reach there. One family of cells is responsible for night vision and they are called cones because of their elongated shape. These cells do not distinguish colors at all and transmit information to the brain that is translated into shades of gray. Meters: 3 groups of color-sensitive cells are responsible for the reception of the primary colors (RGB): blue (short wavelengths), green and red (long wavelength radiation). A body will appear yellow if the light coming from it activates the red and green receptors. The peak of the sun's emission is at a wavelength of 503 nm: somewhere between blue and green. So where did the blue of the sun go? to the sky The Earth's atmosphere scatters some of the sunlight and this scattering (named after the physicist Rayleigh) gets stronger as the wavelength is shorter, meaning that a lot of excess blue is "lost" along the way and what remains in excess are the wavelengths of red and green (ie yellow). The dispersion is proportional to the wavelength to the power of 4 so that blue light (wavelength 450 nm) has a 6 times greater chance of scattering and coloring the sky than red light (700 nm). Therefore, the maximum of the light that is not scattered is at a wavelength of approximately 560 nm, which is both the color of tennis balls and the peak sensitivity of the measuring cells. At noon, when the sun is "above us" and its rays travel a short distance through the atmosphere, only a little light is lost and the sun's hue is very bright, but our impression of the sun's color comes mainly from those parts of the day when the intensity of the radiation is weak enough for us to see it directly: early in the morning or late in the evening, and then the light rays have More distance to travel through the atmosphere, more blue light is scattered and the sun appears yellow and then orange and red.

Is the moon white?

No, the moon is gray. A white body reflects the radiation that hits it, the degree of reflection of light from the surface is called albedo. The moon's albedo is 0.12, which means it reflects 12% of the solar radiation hitting it and absorbs 88%: close to the albedo of an asphalt road. But the reflection of sunlight from the moon depends on its wavelength and the longer wavelengths (red) are reflected more and absorbed less than the shorter wavelengths (blue). In addition, the moonlight also goes through the same dispersion in the atmosphere that the sunlight goes through and we would expect the moon to be yellowish and even tend to be more orange than the sun and it does look like that sometimes when it is close to the eastern or western horizon. And yet, when the moon hangs above us, it appears silver or white to us. The answer, Naama, to the riddle of the moon's whiteness is not in the sunlight that hits the moon, nor in the lunar soil that returns it to us, but in the human eye. The moon is the little light for the night kingdom. And so his light mainly activates the canes that give a grayscale image. In addition, the color of the moon is affected by the Purkinje effect, a phenomenon that results from a different sensitivity of the medullary cells (cells sensitive to color) that are active in strong light to the cells (that provide black-and-white vision) that are active at night. When the light begins to dim, the reeds take the place of the gauges, but the transition is not sharp and there is a range of light intensity where the 2 systems work side by side and the brain adds the information received from the reeds to that received from the gauges. Moonlit nights are in this intermediate range. The reeds are more sensitive to short wavelengths (peak sensitivity at 500 nm) and therefore the red shades are more affected than the blue ones in weak lighting. The Purkinje effect can be seen by those who observe the flowers, even if during the day the red flower is bright and stands out against the background of the leaves, in the twilight of the sunset the flowers lose their red color while the leaves are still green. The moon sends more red light to our eyes and the vision system in night conditions is more sensitive to the green-blue part, the result is a tie and in the world of colors equality between the shades means white. 

What is the difference between the dark side and the far side which is sometimes (erroneously) called the dark side?

The moon also arouses Oved's curiosity: "At the beginning of the month I was able to notice the bright sickle side of the moon also in its dark part. Is this an optical illusion or is it possible to see the dark side of the moon"?

The "dark side" of the moon can never be seen. Not because it is particularly dark - it receives exactly the same amount of sunlight as the visible side to us, but because it always faces away from us. And yet it is not an optical illusion: it is indeed sometimes possible to distinguish, albeit dimly, the part of the moon's wheel that is supposed to be dark. The phenomenon is mentioned in the ballad "Sir Patrick Spence” which won, translated by Natan Alterman, to be included in the middle school literature curriculum. The ballad, for those who missed literature lessons, tells about the king who sends the hero on a sea voyage from which he will not return. One of the harbingers of disaster is exactly what piqued your curiosity:

" Jose I Saar and Black.
Because I saw the birth of the living moon
and in his lap the dead moon."

or in the original language (medieval English)

Yestreen I saw the new moon,
Wi the auld moon in her arme,

 The first to document and explain the phenomenon was Leonardo da Vinci at the beginning of the 16th century. The moon receives its light from the sun and returns a small part to us, but we are also illuminated by the sun and return a part to the moon. When the moon is a thin sickle in our sky on the moon the earth shines full (and of course the opposite: our full moon night is the time when a viewer of the moon will not see an earth in the sky). The faint moon seen next to the bright moon is the reflection of the earth's light that illuminates it. The blurry image of the moon you saw was created by sunlight that went through a particularly challenging obstacle course: it came out of the sun, splashed from clouds on the bright side of our world to the moon and from there was returned to your eyes again. Since only a few percent of the sunlight that hits us reaches the moon and only 12% of this light is reflected from it, the intensity of this radiation is very low and can be observed under conditions where the earth reflects a lot (relatively) of the light that hits it. The earth is a rather weak mirror and the sea reflects even less of it, but certain types of clouds (cumulus) are effective reflectors of radiation. The intensity of the light coming from the "dead moon" is a measure of changes in the earth's cloud cover.

Did an interesting, intriguing, strange, delusional or funny question occur to you? sent to ysorek@gmail.com

More of the topic in Hayadan:

• A simple experiment: watch the total white eclipse tonight, compare it to the scale in the article and report the color of the moon to the scientist who studies the atmosphere of the Earth
• "The red moon did not lie tonight" Spectacular photos from the eclipse that took place early in the morning
• NASA photographed the far side of the moon completely illuminated and hiding the Earth

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