Below is a screen shot from the amazing Gravity Simulator driven by our own @HappyKoala. If you haven't seen it recently go check it out now, then come back in 30 minutes and read the rest of my question :-)

I clicked on TESS, left the Physics section on "System barycenter" which since the Sun is shown, I assume is close to the Sun, and the Graphics section rotating reference frame on "Moon".

But I need more help understanding what that means, why the orbit looks like a pretzel, and what to do to unpretzefy it.


  1. What does Graphics section rotating reference frame = "Moon" actually mean? A rotational frame can't be referenced to a point, it needs a direction. So is this rotating with the line connecting the Moon with the Earth or with the Sun?
  2. There is no non-rotating frame option, yet this seems like what one would click in order to "de-pretzefy" the orbit and reveal it's lovely 2:1 lunar-resonant elliptical nature.

Gravity Simulator website screen shot of TESS

  • 1
    $\begingroup$ Comment instead of answer b/c unsourced, but: 1: The moon isn't a point, it's a body with an orientation. That orientation is somewhat close to fixed wrt to the line between Earth and Moon, and depending on the fidelity of this simulation, might be identical with it, or might not. 2: You're getting pretzels because TESS is orbiting the Earth, not the moon, so you're getting a sort of 3D cousin of a lissajous figure. The 2:1 resonance manifests as the two lobes of the pretzel. If you use Earth as the graphical reference you get a non-pretzely view, but you can't see the 2:1 part as well. $\endgroup$ Commented Oct 27, 2020 at 0:39
  • $\begingroup$ @RussellBorogove These are all good points! I just noticed that the Sun doesn't pass by every month, but instead still takes a year to move out of and back into the field of view. Am I confused, or does that seem erroneous? $\endgroup$
    – uhoh
    Commented Oct 27, 2020 at 0:49
  • $\begingroup$ 1. Setting the rotating reference frame to be the Moon simply holds the Moon fixed at the center of the coordinate system. $\endgroup$ Commented Oct 28, 2020 at 18:56
  • $\begingroup$ 2. Set the rotating reference frame to be the Earth and let the camera focus be Earth and then you will see the familiar elliptical pretty thing of an orbit that you tend to see on infographics about TESS $\endgroup$ Commented Oct 28, 2020 at 18:58
  • 1
    $\begingroup$ @uhoh Right you are... Will do first thing in the morning, right now I'm watching an episode of Midsomer Murders, hehe! $\endgroup$ Commented Oct 28, 2020 at 20:16


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.