Monday, August 5, 2019

Stacking all the Planets

You've probably come across this idea that all the planets could fit between Earth and the Moon. The usual representation looks like this image I found on Google:

It turns out, it's not entirely true. Here's a good article about this, published in Slate a few years ago. The planets can fit, but you have to make a lot of adjustments.

What got me thinking about this recently was an amazing video I found on YouTube by yeti dynamics (here's YD's channel). He has made a number of what-if? astronomy videos. The video that astonished me was a simulation of the Earth-Moon system with all the planets fitted inside the Moon's orbit. The view is from the Earth's surface, and the speed is greatly increased. It makes your head swim. But there's something spell-binding about these gigantic orbs circling so close to the Earth (that is, if it doesn't give you motion sickness, like it does for my wife).

What really astonished me is how much work it must have taken YD to construct this. He created his assets (images of planets, background landscape, 3-D modeling), programmed the animation, and created the video using Blender, 3dsMax, and Natron.

I was contemplating this Herculean task when I realized that I already had an application designed for astronomical simulation. It's called Celestia, and I've worked with it for years. It comes pre-loaded with visual assets (and you can simply add more), and the animation programming is done with script files, also included, which are easily modified. Celestia's basic job is to model the known universe, but you can also create alternative worlds, alien star systems, and break the laws of physics.

So I made a copy of Celestia's basic solar system script, and started modifying. I didn't want to disturb our solar system, so I chose a new Sun - 18 Scorpio, a star about the same size and composition as our own Sun. Then I started modifying the planetary data. First, I created a spreadsheet to help me work out the distances and orbital times (also called periods) for the planets. This is where I had to work out the adjustments I mentioned above to fit (or stack) the planets. Here's the list of adjustments:
  • The Moon is permanently at apogee (greatest distance from Earth)
  • All planetary orbits are circular (zero eccentricity)
  • All planets are perfectly spherical (mean radius)
  • Pluto is included even though it's not a planet anymore (it fit!)
  • All bodies are evenly spaced (1666 km gap between bodies)
  • Saturn is tilted 45 degrees so the rings won't slice through other planets
  • Planets are not in their traditional order, but in order by size.

I took that last point from YD's video. I did try putting the planets in their traditional order, but the visual result was not impressive. This was an inspired move by YD.

Data was obtained from NASA's Planetary Fact Sheets.

Here's a screenshot of my spreadsheet:

This is a 7½-minute video of the final simulation recorded from Celestia. I've positioned the viewpoint in geosynchronous orbit about 8 miles above the Earth's surface, facing northeast, a 50-degree field of view, with the rate of time speeded up a thousandfold.

In case you download and install Celestia, here is a link for downloading a version of the script file I created. You can put it in Celestia's Extras folder, and modify as you wish.

I have shown this simulation to several people. It's quite mesmerizing. As another physics teacher told me, if this is what the sky looked like, we'd never get anything done. My students like it when I project it onto the big whiteboard in my classroom. I'm not sure there is much educational value to it, though. Students seem to recognize that it's "not real," but do understand that the planets would look like that up close. They don't get right away that it's speeded up, and the idea that the planets have been fitted into the Moon's orbit is pretty abstract. Not many people even spot the Moon. Hardly anyone realizes that there's no gravity in the simulation. With gravity, the whole system would collapse pretty quickly. There's no way this could have formed naturally.

But interesting questions do come up, and students like to guess which planet is which, and they sometimes just watch, like you would watch fish in a fish tank. Lankshear & Knobel, in their book New Literacies, describe the role of the teacher as elicitive. In this case, I suggest that, as a teacher, I am being evocative. And maybe that's OK.

Cross-posted to Teaching Is . . .

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