Tuesday, January 1, 2019

Why does the Moon turn red?

.....I'm going to try to start from as close to basics as I can get, so if I start at an a level that's obvious to you, please give me a moment to catch up to you.  Let's start with the most basic building blocks.

.....Light is energy moving from one place to another.  Anything that has a temperature above absolute zero (anything that exists) will give off energy, and therefore will give off light.  (Human beings have a body temperature of 98.6 degrees Fahrenheit which is 310 K in the Kelvin scale, and human beings do give off light.)

.....All light is not equal, it exists on a spectrum in which the amount of energy carried by a given "piece" of light energy* defines where it falls.  The visible spectrum of light is just a part of this, but the principles that the define the whole spectrum are shown here.  In addition to pretending that light is a particle, we can also pretend that light is a wave*.  (Yes, both notes reference the same end note.)  The more energetic the wave, the shorter the wavelength.  An object emitting energy emits energy along the entire spectrum, but what type of light  is mostly emitted depends on the temperature of the object.  NOW I'm getting to the point where this becomes germane.

.....One last bit of backstory: the Sun has a temperature of 6000 K (around ten thousand Fahrenheit), and emits most of its light in (surprise not surprise) the visible spectrum, and that combination of every color of light results in the Sun emitting white light. (At 310 K, the human body emits light in the infrared range.)



.....We can see through air, but that does not mean that nothing is there; it is made up mostly nitrogen and oxygen molecules.  As these waves pass among the atoms and molecules in air, the waves are scattered, and the shorter the wavelength, the MORE the light is scattered.

.....When that white light hits the Earth's atmosphere, the blue light, with the shortest wavelength gets scattered first, and it's scattered a LOT.  Across the entire sky, as a matter of fact.

.....The more air the light passes through, the more light is scattered.  Violet, indigo, and blue very quickly, and then as the Sun gets lower in the sky green light is removed, then yellow, leaving only orange and red.  When the Sun is lower still, only the red is left.  The red light is scattered least, but still scattered, and in fact the red light is scattered so little that it leaks into the shadow of the Earth, so that what would be blackness in the shadow of an airless planet becomes a cone of red.

*This is not physically "real", but it is a very useful and helpful way of looking at the universe despite not being, for the lack of  a better word, "true".

(As it turns out, the Moon will also be on the nearer part of its orbit to the Earth, meaning it will

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