Planning the Perfect Date…Astronomy Style

Relax…I promise there’s an astronomical connection here! Every semester just after the activity on lunar illumination (my way of saying lunar phases), I give a short lecture on eclipses and then ask the class if they would like to know how to plan the perfect date. This surprising question gets a lot of interested looks, and they sometimes as if I’m serious. Of course I’m serious, and of course I’m setting up something interesting as well. The target audience is an introductory general astronomy class. Here’s what I do.

To plan the perfect date you need a clear night, some wine (substitute the beverage of your choice to suit your audience, but I work at a college so…), a blanket, some music (I usually suggest smooth jazz but again, substitute for your audience), and…this is the most important thing…a total lunar eclipse.

Having assembled the necessary components, you then call your significant other or intended companion and ask, “What if I told you I could show you all the world’s sunrises and sunsets in just one night?” The spectrum of responses ranges from “no” to “I wanna see you try!”. Fair enough, but it’s totally possible.

Now the science. We’ve been using our head to represent Earth, a styrofoam ball to represent Moon, and a light bulb to represent Sun in our recent classroom activities. Earth, like me, has hair (I slowly run my fingers through my hair when I say this…long hair helps with the theatrics). We don’t call it hair; we call it Earth’s atmosphere. Our atmosphere is composed mainly of nitrogen, and nitrogen likes to scatter blue light out of Sun’s otherwise white light. (At this point, I sometimes digress into a preview of second semester astronomy by introducing photons, light’s spectrum, Rayleigh scattering, etc.) So when Sun’s white light (beware of astronomy books claiming Sun is a yellow star…”color” has multiple meanings in astronomy) passes through Earth’s “hair” that light has the blue component scattered out of the way, and we see that scattered blue light as the blueness of the sky. (Incidentally, I’ve always wondered whether or not the sky looks blue looking down to Earth’s surface from above the atmosphere and I believe sometime in the past year I saw a discussion about this somewhere online but can’t remember where. Also, this brings up the distinction, to me, between the “atmosphere” and the “sky.” I don’t think they’re the same thing any more. The sky appears blue when seen from Earth’s surface or just above Earth’s surface, but the atmosphere is transparent. This may be the basis for some good discussion at another time.) White light minus its blue component leaves the light looking reddened.

So now I stand between the light bulb and the white board (now representing Moon’s surface) so “Earth’s” shadow is projected onto the white board. I ask the class to tell me where Earth’s terminator is as I stand there, and they correctly point out that it’s in the plane perpendicular to the line connecting Sun, Earth, and Moon. Sunlight reaching Moon’s surface to illuminate it has to pass through Earth’s “hair” (I run my fingers through my hair again here) or atmosphere, and this does two things. First, as described above it reddens the light. Second, it refracts the light slightly toward the interior of Earth’s shadow, somewhat concentrating it on the lunar disk. This reddend light accounts for Moon’s color during a total lunar eclipse.

Now here’s the big reveal. At any given moment on Earth, sunrises and sunsets are happening along the terminator (the boundary between Earth’s illuminated side and unilluminated side) and as we generally know, sunlight is usually reddened at sunrise and sunset. Those sunrises and sunsets contain light that is reddened by Earth’s atmosphere as described above. That same reddened light, having passed tangentially through Earth’s atmosphere along the terminator, is the combined light of all the sunrises and sunsets happening on Earth at that very moment. Yep! You’re seeing all of the sunrises and sunsets on Earth illuminating Moon’s disk during totality! How romantic is that? In my opinion, very!

So there you go. It is indeed possible to make good on the initial promise, and I think this constitutes the perfect date…astronomy style. Your mileage may vary.

Comments and feedback are welcome.

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