Over the Moon

Ben Rubel, 15, is our resident astronomer.  Over the past couple of years he has become very interested in science, mathematics, and astronomy.  Last year, he and Calvin taught an introductory astronomy and astrophysics class.  After Ben and Calvin watched the much-anticipated lunar eclipse together, we asked Ben if he would write a post about it.  This request also triggered an additional idea: a regular series of videos, posted to our Facebook page, where Ben will tell viewers what to look for in the sky over the coming week. Please check out Rubel’s Rambles: Episode 1.

You may have seen, or read about, the “total super blood moon lunar eclipse” on the night of September 27-28. But what does this actually mean?  Let’s look at each part separately.

What is a lunar eclipse?  A lunar eclipse occurs when the surface of the full moon, usually illuminated by the light of the sun, is covered by the shadow of the earth.  

Why was it “total?”   It was a total lunar eclipse because the moon was fully covered by Earth's shadow.  

Why was it “super?”  The moon has a slightly elliptical orbit, so it has a nearest point, or perigee, and a furthest point, or apogee, from earth.  It was a supermoon because the full moon coincided with a lunar perigee: the point at which it was closest to the earth, so it appeared larger.   

Why was it a “blood moon?”  You may have noticed that when the moon or the sun is near the horizon, it appears reddish.  You may have wondered why this is so.   To answer that question we have to understand the nature of light.  Light is both a wave and a particle.  If you picture light as a wave, a wavelength is the distance between two successive peaks. The shorter the wavelength of light, the bluer it is, and the more area it covers.  When the sun is on the horizon, its light has to pass through more atmosphere than when the sun is directly above. If there's is a particle in the way of a light wave, then a bluer (shorter) light wave, because it has a greater area, has a better chance of hitting it than a redder (longer) light wave.  As the sun gets closer to the horizon, its light has to pass through more and more atmosphere, filtering out most of the green light, then most of the yellow light, leaving just the red and orange light, making it look red. The only light that can reach the eclipsed moon is the light that gets refracted (bent) by earth’s atmosphere. Since this light that hits the eclipsed moon has to pass through twice as much atmosphere than at sunset (or sunrise), it looks really red!

I would encourage you to watch future eclipses. The next total lunar eclipse will be in 2018. I like to use http://www.timeanddate.com/eclipse/list.html to check when there are upcoming eclipses in my area.