Celestia Forums

Chris, when you have time do you think you could give us a rundown on how to compile xyz files into flight paths?

Im not sure of the data...I believe the first entry is Julian date? the last three are galactic coordinates of sorts? Ra and Dec?

Rassilon wrote:Chris, when you have time do you think you could give us a rundown on how to compile xyz files into flight paths?

Im not sure of the data...I believe the first entry is Julian date? the last three are galactic coordinates of sorts? Ra and Dec?

Each row in a .xyz file is a Julian date followed by a Cartesian coordinate. The units of the coordinates are kilometers. The coordinate system is astrocentric; in our Solar System, this means that the center of the Sun is at (0, 0, 0). The xy plane is the ecliptic plane of the star system at the reference epoch J2000.0; in our Solar System, this is the Earth's orbital plane on Jan 1, 2000. The positive z axis is perpendicular to this plane in the directional sense of Earth's north pole. The x axis points from the Sun to Earth's ascending node, and the y axis completes the right handed coordinate system.

--Chris

Rereading the above message, it's pretty clear that I need a diagram. Sheesh.

--Chris

Gee. Where the axes point is perfectly clear to me, anyhow!
Getting all the dot products right is another matter entirely...
(It's been a long time since I actually had to do any coordinate transforms.)
Maybe you could publish on SourceForge the program that you've been using?

From your discription I take it these will only work in our Solar System and not around another sun?

selden wrote:Gee. Where the axes point is perfectly clear to me, anyhow!
Getting all the dot products right is another matter entirely...
(It's been a long time since I actually had to do any coordinate transforms.)
Maybe you could publish on SourceForge the program that you've been using?

What I said was actually not quite correct . . . The origin is the center of the parent object, whether it's a star or a planet. If the parent is a planet, the +z axis is the planet's north pole and the xy plane is the planet's equatorial plane at the reference epoch J2000. The x axis points at the ascending node of the equatorial plane on the ecliptic.

The good thing is that these are exactly the same conventions used by JPL's Horizons system by default, so very little processing of Horizons generated ephemerides is necessary. For Sun orbiting objects, just select a center of @10 (Sun), vector output, and eclip for the reference plane.

--Chris

Hmm this will work rather well for plotting moons of moons orbits...eg intrasystem star with planets may have moons...Even of some parent planets may have moonlets of moons...Though we have yet to find anything more than asteroids doing this...

nice...