Longitude of ascending node

[SixMarbles]

Since Kerbin's equatorial plane is aligned with the ecliptic plane, how do I determine the origin (zero degrees/reference point) for longitude of ascending node without using mods?

I understand that at UT = 0, a ray drawn from Kerbin to Kerbol is nearly aligned with it, but I don't know how to determine it in the map once a game has begun.

Thanks y'all.

Edited December 18, 2014 by SixMarbles

[The_Rocketeer]

Asked innocently, why would you want to?

The only Kerbin-centric body that has a real ascending node is Minmus, since Mun's orbit is also on the ecliptic plane.. The skybox is (as far as I know) not reliable as a constellation reference. The point of the origin for longitude of the ascending node is to provide an origin not relative to the various rotations of its own orbital system.

If that's what you really want, I would suggest using the ascending node of Minmus as your origin for all other ascending nodes. It's not exactly the same as the First Point of Ares because it's in-system, but I don't think KSP's orrery-model is so advanced that that is likely to be an issue, though I guess that depends on how you plan to use it.

How you'll actually measure any angle from it, though, is another matter. Come to that, so is what the hell you'll do with that information if you have it.

Edited December 17, 2014 by The_Rocketeer

[thereaverofdarkness]

I'm not sure why you'd want that information, nor am I exactly sure what you're asking for. You might consider KSC to be at the origin, since that would make it into a perfect equatorial origin marker.

You can probably measure what you're looking for precisely using nothing but your navball and map. I know that when you have an inclined orbit, you can find the ascending and descending nodes by finding the point where the plane of your orbit intersects with the plane of a non-inclined orbit, with both intersecting the center of the body you are orbiting. If you don't have a non-inclined reference body you can use the plane of the ecliptic or the Milky Way, but the Mün works much better.

[Red Iron Crown]

In stock: Put a satellite in equatorial orbit around Kerbin. Target Minmus. The ascending node show will be Minmus' ascending node, which is at 78 degrees. View the map from above and rotate it, using a real protractor to set Minmus' AN to 78 degrees from vertical. Voila, you're now oriented to celestial North, as long as you don't rotate the camera you can switch vessels or targets and measure from vertical with your protractor to get LAN for other orbits.

Much easier to use a mod that will give you the precise number, or poke around in the save file to find it (it's one of the orbital parameters for every object), but if you want to get it in-game without mods the above method works.

[SixMarbles]

The ascending node show will be Minmus' ascending node, which is at 78 degrees.

This is extremely helpful, thank you.

[TythosEternal]

One incredibly picky comment: the angle of ascending mode is usually right-ascension (the RA in RAAN), which is a measurement from the inertial reference vector you are looking for. Longitude is measured w.r.t. a point fixed to the rotating surface of the body you are orbiting (a la prime meridian). Ironically, Longitude of Ascending Node would be easier to eyeball in KSP without any other references--you'd just look at how far away you are from your reference, as measured by a projection into the equatorial plane (i.e., from a top-down view). Much less useful than RAAN, though, which is constant for a given orbital plane.

[OhioBob]

Another way to get a rough idea of the direction of 0-degrees celestial longitude is to zoom out on the big map a look at the orbit of Eeloo. Since Eeloo's orbit is so eccentric, it's not too hard to estimate the orientation of its major axis. Eeloo's apoapsis is at 130^o longitude and its periapsis is 310^o longitude. You can find 0^o from that easy enough.

[OhioBob]

Another way to find Minmus' nodes without using a satellite is...

(1) Go to the map and zoom out until you see Mun's orbit.

(2) Rotate the view top to bottom until Mun's orbit is seen edge on.

(3) Zoom out until you see Minmus' orbit.

(4) Rotate the view side to side until you see Minmus' orbit edge on (make sure to keep Mun's orbit also edge on).

(5) Where the orbits appear to cross is Minmus' line of nodes, which, in the current orientation, is perpendicular to the monitor screen.

(6) Rotate the view down until you are looking down on Kerbin's north pole (do not rotate side to side).

(7) Minmus' line of nodes now forms a vertical line on your screen.

[Geschosskopf]

Since Kerbin's equatorial plane is aligned with the ecliptic plane, how do I determine the origin (zero degrees/reference point) for longitude of ascending node without using mods?

Are you talking about for a contract satellite orbit? If so, just don't worry about it because it's not an independent variable. If you get your Ap and Pe in the right places and the have the correct inclination, then your LAN will be correct as well.

[Guest]

I just wish a Devs add a switchable additional hash-marks on orbits, like Ap and Pe, but AN/DN to the equator, and small 10 degree longitude marks. With ability to show/hide them when needed. Not only useful at some situations, but also I think that it will look cool.

Is there a mod for this?

[mjl1966]

I'm trying to figure out how to calculate when and where to burn for a transfer orbit without using maneuver nodes and it seems that "0" in the reference plane for Kerbal would be helpful so you can determine "where" your ship is relative to "where" the Mun is.

There has to be a zero somewhere so everything else means something. Where is "zero" degrees longitude on the Kerbal reference plane? How can you possible calculate anything without knowing that? (Honest question - I'm still just learning orbital mechanics and synchronizing transfer orbits mathematically is beyond me at the moment.)

[Red Iron Crown]

I'm trying to figure out how to calculate when and where to burn for a transfer orbit without using maneuver nodes and it seems that "0" in the reference plane for Kerbal would be helpful so you can determine "where" your ship is relative to "where" the Mun is.

There has to be a zero somewhere so everything else means something. Where is "zero" degrees longitude on the Kerbal reference plane? How can you possible calculate anything without knowing that? (Honest question - I'm still just learning orbital mechanics and synchronizing transfer orbits mathematically is beyond me at the moment.)

The zero longitude to which you refer is in the celestial frame of reference, in that it doesn't rotate with any body and is the same for all (this is important because orbits don't rotate with a body). For calculations involving transfer orbits, a different frame of reference is used, one that rotates with the body's revolution around the sun. This gives consistent measurement of "angle to prograde", the measure used to determine the correct timing of ejection burns. In this frame, 0 degrees is defined as the direction of Kerbin's motion around the sun.

[Geschosskopf]

I'm trying to figure out how to calculate when and where to burn for a transfer orbit without using maneuver nodes and it seems that "0" in the reference plane for Kerbal would be helpful so you can determine "where" your ship is relative to "where" the Mun is.

Just move the view to you can barely see Mun above Kerin's horison. Then burn prograde when your ship gets to that point until your Ap is anywhere from about 1/2 a finger's width short of Mun's orbit to the same as Mun's orbit. That's assuming you don't even have patched conics yet. If you do but don't yet have maneuver nodes, burn at the same place until the patched conics show what you want.

There has to be a zero somewhere so everything else means something. Where is "zero" degrees longitude on the Kerbal reference plane? How can you possible calculate anything without knowing that? (Honest question - I'm still just learning orbital mechanics and synchronizing transfer orbits mathematically is beyond me at the moment.)

There is a zero but it's totally invisible to players even with mods. Mods like MJ display things like your LAN but that's relative to some coordinate system that's under the hood. The mod talks to that and displays a number for LAN but it's useless because you have nothing to show you where zero is.