Planetary orientation

[mcirish3]

Hi, Here is a really challenging question which to be honest probably can only be answered by squad and could as far as I know only be estimated by players.

 Is there any source available to players or mod makers that  states the exact initial orientation of the various planets and moons in KSP relative to Sol?

[Tatonf]

Yes, just check the Wiki of each planet. For exemple, Moho is inclined by 7° :

http://wiki.kerbalspaceprogram.com/wiki/Moho

[mcirish3]

1 minute ago, Tatonf said:

Yes, just check the Wiki of each planet. For exemple, Moho is inclined by 7° :

Inclination is not the same as orientation.  Orientation is tells you what parts of a planet can see the sun and what parts cannot for a given time, in this case I am looking for time 0 since it is the only time the orientation given to Unity.

[Tatonf]

Orbits are defined by 6 parameters, and "orientation" isn't one of them. Maybe you're looking for Mean anomaly ?

Edited January 23, 2016 by Tatonf

[mcirish3]

On 1/23/2016 at 4:47 PM, Tatonf said:

Orbits are defined by 6 parameters, and "orientation" isn't one of them. Maybe you're looking for Mean anomaly ?

Nope, I mean orientation.  In other word the prime meridian at T=0 and its angle relative to the sun at that time.  Of course inclination angle matters too for some of the moons in KSP.

[Streetwind]

That's unfortunately not a parameter for placing a planet into its orbit, no. It's instead defined by how the surface map and texture is wrapped around the spherical model.

In an imaginary example: the artist sculpting the planet could decide to put an ocean on the left and a continent on the right, and then the ocean would be in the sun at epoch and the continent wouldn't be. Or the artist could decide to place the continent on the left and the ocean on the right, and then the continent would see the sun at epoch. Ultimately though the artist doesn't know where what part of his map will end up except through trial and error.

[PLAD]

A good question. If you are using the Real Solar System mod, for instance, you can find the initial orientation of 0 degrees latitude  of a body in the RSSKopernicus.cfg file as "initialRotation=". I do not know where to find the same data for the bodies in stock KSP. However it is not too hard to figure it out. Do as follows:

1) Install Mechjeb and Hyperedit.

2) Place a craft on the surface of a body (this is where you use Hyperedit, but you could drive or fly there without it) at 0 degrees longitude and, say, 20 degrees South latitude.

3) Now find the Mechjeb data item "LAN" and put it in a display somewhere. It will show the longitude of ascending node as if your craft was to start flying from where it is sitting, at the orbital speed that the rotating surface of the planet gives you. You will notice that the LAN is rising at a steady speed all the time as your ship moves with the rotating surface. Since you are South of the equator you would cross the equator 90 degrees after your current location (if you were to start flying due East from there), so your absolute current direction, and thus the direction of 0 degrees latitude of the planet, is indicated LAN minus 90 degrees. (See why you can't just sit at the KSC, it is on the equator so the LAN is undefined there.)

Note that KSP does have an absolute '0' direction that everything is referenced to, but there is nothing to indicate that direction from just looking at the system. You can figure it out by noting that Kerbin starts at a mean anomoly of 3.14 radians, which is roughly 180 degrees, and since its orbit is circular and it moves at a constant rate you can always compute its position at any given time based on Kerbin's constant angular speed around Kerbol.

In case you are wondering I have only done this with RSS Earth and Moon, to verify the meaning of the initialRotation parameter, so I can't give you the numbers for stock.

[ElWanderer]

The kOS mod is able to return the rotation angle of each body, that is the angle between the solar prime vector and the body's zero longitude/prime meridian. A kOS script run just after starting a game would return something very close to the starting values. I wouldn't be surprised if they're all started at zero. I'm not at my PC so I can't test this.

If kOS can get this data from the game, other mods should be able to as well.

From the wiki, as Kerbin's mean anomaly at epoch is 180°, but the longitude of the ascending node and argument of periapsis are both 0°, I think that means the prime vector as seen from Kerbin points directly at the sun at the start of the game (which fits with KSC at longitude -74° starting off in sunlight: it'd be early in the day about 1h15m from noon. Again, I've not tested  that).

[Dunbaratu]

17 minutes ago, ElWanderer said:

If kOS can get this data from the game, other mods should be able to as well.

In the KSP API, the vector called Planetarium.right is essentially the solar prime meridian from which values like Longitude of the Ascending Node are measured against.  If an orbit's LAN is zero, then it's ascending node will be along the Planetarium.right vector out from the planet's center.

Nothing in the game exposes this information to the user though, so without a mod to show it for you, you end up having to reverse-engineer it from what you do know.  If you know an orbit's LAN, and where its An is on the map, you can use that to find the Planetarium.right vector.

The objects' starting positions at time=0 are referencing this Planetarium.Right direction.

[mcirish3]

2 hours ago, ElWanderer said:

The kOS mod is able to return the rotation angle of each body, that is the angle between the solar prime vector and the body's zero longitude/prime meridian. A kOS script run just after starting a game would return something very close to the starting values. I wouldn't be surprised if they're all started at zero. I'm not at my PC so I can't test this.

If kOS can get this data from the game, other mods should be able to as well.

From the wiki, as Kerbin's mean anomaly at epoch is 180°, but the longitude of the ascending node and argument of periapsis are both 0°, I think that means the prime vector as seen from Kerbin points directly at the sun at the start of the game (which fits with KSC at longitude -74° starting off in sunlight: it'd be early in the day about 1h15m from noon. Again, I've not tested  that).

YOU NAILED IT!!!!  that is it!   I was thinking it was pointing at Kerbin but of course that explains everything I already of the rotational offsets for all the planets in the stock game (Hyperedit as well as some other mods like KOS provide that)  But what I was missing is the orientation of the X axis.   Thank You  

7 hours ago, PLAD said:

A good question. If you are using the Real Solar System mod, for instance, you can find the initial orientation of 0 degrees latitude  of a body in the RSSKopernicus.cfg file as "initialRotation=". I do not know where to find the same data for the bodies in stock KSP. However it is not too hard to figure it out. Do as follows:

1) Install Mechjeb and Hyperedit.

2) Place a craft on the surface of a body (this is where you use Hyperedit, but you could drive or fly there without it) at 0 degrees longitude and, say, 20 degrees South latitude.

3) Now find the Mechjeb data item "LAN" and put it in a display somewhere. It will show the longitude of ascending node as if your craft was to start flying from where it is sitting, at the orbital speed that the rotating surface of the planet gives you. You will notice that the LAN is rising at a steady speed all the time as your ship moves with the rotating surface. Since you are South of the equator you would cross the equator 90 degrees after your current location (if you were to start flying due East from there), so your absolute current direction, and thus the direction of 0 degrees latitude of the planet, is indicated LAN minus 90 degrees. (See why you can't just sit at the KSC, it is on the equator so the LAN is undefined there.)

Note that KSP does have an absolute '0' direction that everything is referenced to, but there is nothing to indicate that direction from just looking at the system. You can figure it out by noting that Kerbin starts at a mean anomoly of 3.14 radians, which is roughly 180 degrees, and since its orbit is circular and it moves at a constant rate you can always compute its position at any given time based on Kerbin's constant angular speed around Kerbol.

In case you are wondering I have only done this with RSS Earth and Moon, to verify the meaning of the initialRotation parameter, so I can't give you the numbers for stock.

Someone had already suggested this to me and I was ready to try and do this but  I was hopping for a less time consuming option.