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In the SpaceX BulgariaSat-1 Launch Webcast there is a 3D animation of the orbits/trajectories of the first and second stage showing most of the Earth. There is a blue line that starts out over Florida, bur later "goes south" both literally and figuratively. At T +00:27:20 it starts sliding south faster and then disappears.

What does this line represent?

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The blue line represents the part of the satellite's trajectory after it traveled once around the earth, so it's basically the continuation of the 2nd stage track (except that the payload will have been deployed by then).

If you don't mind my paint skills, I've connected the two tracks in this image.

Because the stage/satellite is still being accelerated the track is at a higher altitude and because the earth rotates beneath it, it has shifted to the west from the earth's frame of reference.

Why does it suddenly go south and disappears?

It doesn't really go south, it just appears to do so, because the virtual camera is moving. As the track approaches the camera the faster it seems to be moving, until the camera flies right over it and it gets out of sight.

Shortly before it disappears you can even see the track being cut off. This is caused by the "near clipping plane" during the 3D rendering process, in which elements that are too close (in front of the viewing frustum) get discarded. In the animation more and more of the track exits the frustum until it's gone completely.

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    $\begingroup$ +n for the thorough, educational, and colorful answer! I'm not sure why I didn't figure this out myself, but it's probably a combination of the front clipping with the lack of any depth cues for the line. Drawing traces for orbits that are both technically accurate and simultaneously easy for the public to grasp is probably quite a challenge. $\endgroup$ – uhoh Jun 26 '17 at 5:39
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That should be the projected future path on the second orbit. It looks like it is drawn relative to Earth surface (otherwise it would connect directly to current position of the second stage) and that only limited part of it is computed (it ends abruptly even before it vanishes). Unfortunately the ground track (the shadow) is only drawn for past (real) path as can be seen so this second end looks weird because without the shadow we are not able to properly assess it's actual position in the perspective projection from that 2d video (and because it does not help that is an "infinitely thin" line drawn with constant width line). It imho does not actually go so much south during the second stage2 burn as it goes up and at the same time gets shorter because the limited number of computed steps is taken by the long way to the raised apogee. I remember on some previous launch you could clearly see the inclination change the second stage performed during second burn by watching this line. Will try to locate it when I am on PC.

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  • $\begingroup$ +n also! Yep, I see what you mean. I think that it might have been easier to understand if the ground track line had also been extended the same way that the orbit was extended. That might have been a cue for depth that would make the whole thing easier to understand in 3D. Since the "near future" part of the initial orbit has a ground track, but the later part (after returning from behind the Earth) looses it's ground track, it's more difficult to associate the two segments as different parts of the same curve. $\endgroup$ – uhoh Jun 26 '17 at 8:18
  • $\begingroup$ If you can find the video with an easier-to-understand 2nd burn, that would be fantastic! $\endgroup$ – uhoh Jun 26 '17 at 8:18
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    $\begingroup$ @uhoh will check. But the "near future" has no ground track as far as I can see. It ends at the vertical line from the 2nd stage. $\endgroup$ – jkavalik Jun 26 '17 at 8:23
  • $\begingroup$ Ya you are right. it terminates at the current position with a single "pin" from the spacecraft to the surface. I guess I projected it forward without realizing it. I appreciate your help, drawing orbits in 3D for the public in a way that's both easy for everyone to understand and technically correct is a really challenge. Thank you for your help with this!! $\endgroup$ – uhoh Jun 26 '17 at 8:33
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    $\begingroup$ @uhoh I found multiple webcasts (GTO) where you can see the inclination change during the burn, but none of them gave better view on that "second line". You can see the circularization in Iridium-1 webcast which shows regular "next orbit" blue line which does not vanish anywhere. $\endgroup$ – jkavalik Jun 26 '17 at 18:37

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