I’ve been kind of depressed/busy/depressed/who knows lately, which has meant no posting either here or on the phunsics blog. Basically, I haven’t been having a good time with physics recently, for the usual reasons that grad students are totally miserable. Things have been so yuck that I actually got even more serious about my persistent to threat to apply to law school than I usually do. I actually picked out schools I might want to go to!
This has made me thoroughly depressed about life. As I approach my 26th birthday, I am approaching the anniversary year of my first foray into academia. What have I done with myself? Who I have I become? Where am I going? Nowhere good if the demoralization keeps up. So, in an effort to curb the Great Sadness, I have decided to try to write entries on a regular basis about something in physics or science that is cool.
Tonight’s entry is about why the moon looks orange near the horizon. So, what’s the deal with that?
First of all, where does moon light come from? The sun! The only objects in the universe that are energetic enough to produce light are highly energetic ones like stars, gases near the centres of black holes, [whatever makes cosmic rays], etc. The sun shines light onto the moon just like it shines on earth, and in turn some of the light bounces off of the moon and lands on the earth.
However, before the light gets to our beautiful eyes, it must travel through earth’s atmosphere. The atmosphere is made of a lot of (increasingly man-made) gases. Generally speaking, when light passes through a gas, the gas acts like a filter. What kind of filter depends on the properties of the gas. Thus, the atmosphere acts like a filter, which happens to discriminate against bluer wavelengths. Everything is a little bit redder when it gets to us!
But if the atmosphere is essentially the same in all directions, why do we see the moon in different shades depending on where it hangs in the sky? This is actually a geometry problem. When the moon is at the horizon, the light from the moon has to travel through the atmosphere longer than when the moon is overhead. The longer the light travels, the more it scatters inside the atmospheric gases, and the more the poor blue gets filtered out! When the light finally does get to your eyes, it’s red.
So, why is this cool to know? Well, light is always a cool thing to think about. It is ubiquitous in the human universe — essential for survival from the physical plane to the psychological. And it’s fun to think about this thing we observe regularly, the moon rising, and how we can understand its red appearance with the same math and physics that we use to understand all sorts of stuff, for example the electrical properties of a material. (Look up mean free path on google if you want to know more.)
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