Vernal Equinox

[LN400]

First, I have gathered there is no straight forward way of knowing where vernal equinox is in KSP using only stock game information but I was wondering:

KER has 2 items under Orbital:

Longitude of Ascending Node.

Angle To Equ. Ascending Node.

Both of these are in degrees.

To my brain, it sounds like one can use these 2 in tandem to determine where in the orbital path vernal equinox (VE) is. The Angle is the angle between the ship and AN. The LAN is the angle between VE and AN. When these 2 angles are equal, then you have found the  "VN spot" for your orbit. By subtracting angles, you can determine where you are relative to VE.

Am I totally wrong or would this make sense? Any thoughts?

[Reactordrone]

No axial tilt so no seasons. No seasons no equinoxes.

[LN400]

21 minutes ago, Reactordrone said:

No axial tilt so no seasons. No seasons no equinoxes.

Well yes, that is true but for calculating orbits, it would still be a factor. Vernal is perhaps the wrong word but what would one call the reference point in KSP?

[Red Iron Crown]

Are you referring to the reference direction, as shown in this diagram?

[Sharpy]

@Red Iron Crown: that would be it. In astronomy (and astronautics) it's about universally defined as the direction of vernal equinox - the point in the sky, where the Sun is on  March, 20th.

Interesting that they marked the Reference Direction with the Aries symbol, while the direction of Vernal Equinox of Earth is in the Pisces constellation.

But yeah, with Kerbin without axial tilt, it's hard to pick a non-arbitrary point.

[Kerwood Floyd]

Sharpy: My understanding is that because of precession, the vernal equinox has not always been in Pisces. It used to be in Aries, hence the symbol. Or were you asking about something else?

[LN400]

1 hour ago, Red Iron Crown said:

Are you referring to the reference direction, as shown in this diagram?

That would be it.

Perhaps it is arbitrary but it can still be fixed, and determined, or at least that's my original question.

Edited September 12, 2016 by LN400

[Red Iron Crown]

Seems like LAN, angle to AN, and inclination could be used to calculate that value.

[LN400]

4 minutes ago, Red Iron Crown said:

Seems like LAN, angle to AN, and inclination could be used to calculate that value.

I agree. It would allow for an interesting kOS orbit script.

[Snark]

The problem with Kerbin's orbit is that it's designed in such a way that there really aren't any "natural" reference points.

• There's no axial tilt.  So there's no AN/DN reference point, there's no "vernal equinox".  Or more to the point, everywhere all the time is the vernal equinox. 
• The planet has an absolutely perfectly circular, genuine-zero-eccentricity, never-varies-by-so-much-as-a-meter orbit.  So there's no solar periapsis or apoapsis to use as a reference point, either, so the argument of periapsis is effectively undefined.  All orientations are identical.

I assume that Squad set it up that way quite deliberately, to make it easy to navigate to/from Kerbin: i.e. they deliberately made it so that all points on the orbit are equal.  A side effect of doing that is that the orbit really has no "natural" defined reference points at all.

That said, the game does internally have a value used for these parameters-- if you play around with Kopernicus, for example, you can change values even for a perfectly circular-orbit planet to affect where its initial position is when the game starts.

But in terms of physical reality... no, there's really no one place in Kerbin's orbit that's different from any other, they're all absolutely identical.

[LN400]

52 minutes ago, Snark said:

The problem with Kerbin's orbit is that it's designed in such a way that there really aren't any "natural" reference points.

• There's no axial tilt.  So there's no AN/DN reference point, there's no "vernal equinox".  Or more to the point, everywhere all the time is the vernal equinox. 
• The planet has an absolutely perfectly circular, genuine-zero-eccentricity, never-varies-by-so-much-as-a-meter orbit.  So there's no solar periapsis or apoapsis to use as a reference point, either, so the argument of periapsis is effectively undefined.  All orientations are identical.

I assume that Squad set it up that way quite deliberately, to make it easy to navigate to/from Kerbin: i.e. they deliberately made it so that all points on the orbit are equal.  A side effect of doing that is that the orbit really has no "natural" defined reference points at all.

That said, the game does internally have a value used for these parameters-- if you play around with Kopernicus, for example, you can change values even for a perfectly circular-orbit planet to affect where its initial position is when the game starts.

But in terms of physical reality... no, there's really no one place in Kerbin's orbit that's different from any other, they're all absolutely identical.

That is all well and true. However, as I said in the original post, KER shows 5 of the 6 parameters:

Semi-major axis

Eccentricity

Inclination

Arguement of Periapsis

Longitude of Ascending Node

(in addition: angle to AN)

Only the reference point is missing. The point of this is that it appears that one can calculate where that reference is relative to your ship. The pondering is due to the uncertainty of how "fixed" this reference point is as far as KER is concerned but it does seem like it is fixed.

Edited September 12, 2016 by LN400

[Red Iron Crown]

It's fixed. IIRC it's set as the line through Kerbin and the sun at the start of the epoch.

[LN400]

Got some testing done with KER + a satellite, 0.00005 degrees inclination relative to Mun so for all practical purposes of this test, this is an equatorial orbit.

The reference is fixed but boy was this confusing. True anomaly in radians and all other angles are in degrees, including all the other anomalies. Took a few moments before it dawned on me. WHY?

So:

Semi-major axis and eccentricity check out fine when I calculate them based on Apo/Peri.

True anomaly checks out with 2*pi at peri and pi at Apo.

Eyeballing Arguement of Periapsis and it seems to check out fine. Since the orbit is slightly inclined, the AN stays put and everything looks ok.

Longitude of AN checks out, it seems, in that the angle changes predictably when I move the AN around. If I don't burn, it stay constant. Same for AoP.

Angle to Equ. AN is a bit of a mystery though. I haven't been able to make any sense of what I read out of KER, increasing and decreasing in a way I can't suss out.

 

 

[Red Iron Crown]

1 hour ago, LN400 said:

Angle to Equ. AN is a bit of a mystery though. I haven't been able to make any sense of what I read out of KER, increasing and decreasing in a way I can't suss out.

Should be zero when the craft is at AN, then cycle through the angle range of 0-180-0.

[Skalou]

11 hours ago, Reactordrone said:

No axial tilt so no seasons. No seasons no equinoxes.

tilt is not the problem, inclinaison of the orbit is, but as said it's just an arbitrary direction in an arbitrary reference plane to define orbital parameters ( not so arbitrary for earth).

this point could be "marked" in the skybox by drawing a remarquable star/galaxy/whatever... even in stock ( used in contract).

Edited September 12, 2016 by Skalou

[LN400]

7 hours ago, Red Iron Crown said:

Should be zero when the craft is at AN, then cycle through the angle range of 0-180-0.

That's the puzzling bit. It isn't and doesn't. I wonder if KER's AtEAN is what it says it is. As I approached and passed the AN, that angle read some 154 degrees in this case. In fact, the angle maxes out at 157.38-some at periapsis, reaches about 154 degrees at AN and is down to 151.08-some as it approaches the periapsis.

Edited September 13, 2016 by LN400

[LN400]

Looking at the KSP Wiki to see what it has to say about Kerbin:

At the start of the game (UT = 0), Kerbin is at its apoapsis [mean anomaly (at UT = 0) = pi] (eccentricity = 0 but the apo is still defined apparently), in its orbit around Kerbol.

For Kerbin's orbit, the Angle to Ascending Node is 0 degrees.

That means, if one draws a straight arrow from the center of Kerbol through the center of Kerbin, pointing away from Kerbol, that arrow is the reference direction, since the Angle to Ascending Node is the angle between the reference direction and the ascending node.

This does not add up to what KER says about the AtEAN nor to what my Mk I Eyeball sees. If KER is correct, then the reference direction would be the opposite direction. Either KER is wrong, or the Wiki is wrong or I need to go back to elementary school.

If I am right so far, then it is possible that either KER or the Wiki wrongly uses pi as the mean anomaly at periapsis.

 

EDIT: Thinking about it, it could be a workaround for the situation where e = 0 leaving AtAN and AoP undefined. Still confusing trying to work with this.

Edited September 13, 2016 by LN400

[OhioBob]

On ‎9‎/‎13‎/‎2016 at 5:57 AM, LN400 said:

That means, if one draws a straight arrow from the center of Kerbol through the center of Kerbin, pointing away from Kerbol, that arrow is the reference direction, since the Angle to Ascending Node is the angle between the reference direction and the ascending node.

It's just the opposite of that.

Although Kerbin as no vernal equinox, the Kerbal celestial coordinate system does have a direction that defines zero degrees longitude.  Kerbin's longitude of ascending node and argument of periapsis are both assigned values of zero.  This means that Kerbin's ascending node and periapsis (even though it really doesn't have either) and the origin of celestial coordinate system all point in the same direction.  Kerbin's starting mean anomaly is 3.14000010490417 radians (not pi), which means its starting longitude is 179.908753681645 degrees.  That is the longitude from the center of the Sun looking outward toward Kerbin.  Looking from Kerbin, the Sun is located at approximately 0 degrees at the start of the game.

 

Edited September 16, 2016 by OhioBob