General Question Earth's Orbit

[fullarmor2]

I'm in LEO in the Arrow Freighter looking at Orbit MFD. I referenced the Sun and I think what I see it showing is my orbit around the Sun. And my problem is this. The MFD is showing my PeA distance to be 67.86 G ???? The ApA looks right. At 148.3 G . The Ecc is .3639?? I don't understand! I thought Orbit MFD would show the Earth's orbit to be closer to being circular at around 149G (93 million miles)?

Now I just did a check of this from a vessel on the surface of Earth, and Orbit MFD is showing correct information regarding Earth's ApA and PeA . So I suppose everything is ok. I was worried that my Earth was in a scewed up orbit somehow! I thought maybe because my Orbiter copy is too old! But its ok. lol

 

[blixel]

I'm in LEO in the Arrow Freighter looking at Orbit MFD. I referenced the Sun and I think what I see it showing is my orbit around the Sun. And my problem is this. The MFD is showing my PeA distance to be 67.86 G ???? The ApA looks right. At 148.3 G . The Ecc is .3639?? I don't understand! I thought Orbit MFD would show the Earth's orbit to be closer to being circular at around 149G (93 million miles)?

Now I just did a check of this from a vessel on the surface of Earth, and Orbit MFD is showing correct information regarding Earth's ApA and PeA . So I suppose everything is ok.

Everything is ok. The problem is that Orbit MFD can only calculate a 2-body solution.

When you are on the ground, you are "attached" to Earth. If you reference the sun in Orbit MFD while on Earth's surface, you are seeing the correct ApA and PeA for Earth - and your vessel as well, since your vessel is fixed on the ground.

When you get into orbit, you're no longer "attached" to Earth, but Orbit MFD is still limited to solving the orbital elements for your vessel based on 2-bodies. But the equation is quite a bit more complicated because you have the Earth as one body, the moon as another, the Sun, and Venus and Mars, and maybe even Jupiter could potentially throw things off a bit as well.

With a 2-body solution, as I understand it, the orbit is calculated based on your ship and the reference body you selected. That's it. So when you are in Orbit around the Earth and you have Referenced the Sun, Orbit MFD is trying to calculate your orbit around the sun and is oblivious to the fact that the Earth and moon are there.

Hopefully that makes sense. Someone smarter than me (anyone else on O-F) can explain it better.

 

[asbjos]

In Low Earth Orbit you travel at speeds close to 8 km/s. If you look at orbital elements with the Sun as reference, your orbit around the Sun will cycle between [math]v_e+v_o[/math] and [math]v_e-v_o[/math], where [math]v_e[/math] is Earth's orbital velocity around the Sun (roughly 30 km/s) and [math]v_o[/math] is your orbital velocity around the Earth (roughly 8 km/s). Therefore, your velocity relative to the Sun will vary between 22 km/s and 38 km/s.

I made a quick-and-dirty MS Paint drawing of the situation.

You can see the red orbit of the ISS go around the light blue Earth. The black arrows are the velocity vectors for the total velocities relative to the Sun.