Modelling with an apple and an egg
Last Tuesday, on July 16th 2019, we had a partial lunar eclipse which was visible in Germany. Unfortunately it was too cloudy for me to see but when I was 8, there was a solar eclipse right over Germany (in 1999) and I loved the event. I didn’t quite understand how rare solar eclipses are when you don’t travel to see one. I guess I’d appreciate it so much more now. Lunar eclipses are more common but still events for themselves. And what I also love about eclipses is that there are so many ways to visualize and understand these phenomena. I’ll explain three in this post.
Ever kid has seen the moon phases. They have probably learned about them in Elementary school. At least on a very basic level. And learning about light and shadows in physics education almost certainly leads back to moon phases and, in the next step, to the phenomena of eclipses. Basically, an eclipse happens when sun, moon and Earth are aligned. The type of eclipse depends on the order of the three. Let me refresh your memory on each of them.
When the moon blocks the sunlight from getting to Earth, we have a solar eclipse. It looks a lot like this:
Now from the picture, you can probably already tell why solar eclipses are so rare for one place on Earth. The shadow of the moon is even smaller than the moon itself, because the sun is just so BIG. NASA has a really cool website for solar eclipses and where they’ll occur. The next solar eclipse will happen on December 26th and will probably be seen best from Saudi Arabia, India, Sumatra, and Borneo.
The picture above is pretty static, how exactly does the solar eclipse happen? The moon moves slightly across our vision of the sun, blocking at first only part of the sun and, in the event of a total eclipse, blocking the whole sun for a couple of seconds. You can replay a solar eclipse with an apple and an egg (even though I used an Kinder egg capsule due to a lack of real eggs).
Very briefly, you can see the darker spot on the apple before the “moon” (aka the capsule or egg) then moves even further. From this video it becomes clear, that we can only see a solar eclipse when we have a new moon because its side turned to Earth does not reflect any light from the sun.
WARNING: Please make sure to protect your eyes when looking at a solar eclipse! Don’t look directly into the sun.
A lunar eclipse happens way more often than a solar eclipse. This is because the moon enters the shadow of the Earth, which is bigger than the moon itself. That’s also the reason why we have way more total lunar eclipses than partial lunar eclipses.
This can also be modelled with an apple and an egg. Even without holding the egg akwardly in front of the lamp that represents the sun.
And as the solar eclipse can only happen at new moon, the lunar eclipse only happens with a full moon. The moon has to be on the direct opposite of Earth, faced towards the sun, for a lunar eclipse to happen. Furthermore, it’s safe to watch, as we won’t look directly into the sun.
Now these were only two representations?
Right. Drawing the shadow lines like I did with the pictures is one way to understand the phenomena. Modelling them with an apple and an egg is a second representation. The third one is a little mind game:
Think of yourself as a little ant, able to move quickly and wherever you like. Now in the exact time of a solar eclipse, the sun is being covered by the moon. And we, as the ant, can move directly from where the sun still hits the earth into the shadow of the moon. This is a nice way to understand why we can only see solar eclipses in certain places and why some people go “eclipse hunting”.
And if you’re having trouble with the mind game, you can always hold your egg out in front of you towards the lamp, pinch you eyes and move your head a little from one side to the other. See? Just remember to be careful with your eyes. How would you be able to see all these great phenomena the world is made of if you couldn’t see anymore?