# Argument of periapsis

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Hi KSP colleagues,

I would like clarification on something please.

Wikipedia explains that the argument of periapsis is "the angle from the body's ascending node to its periapsis, measured in the direction of motion."  Now we all know that Kerbin's orbital inclination equals zero degrees, and the KSP wiki notes that Kerbin's argument of periapsis also equals zero degrees.

Since Kerbin's orbital inclination is zero, Kerbin does not actually have ascending or descending nodes, right?  Moreover, since Kerbin's orbit is circular, it has neither a periapsis nor an apoapsis, right?

Thus, only by convention -- to get a reference point -- can we speak about Kerbin's argument of periapsis.  Again, isn't this correct?

Thank you.

Stanley

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The argument of periapsis is useful when you analyse a set of orbits around the same reference body, say, the Solar System.

Then it's handy to use some single base plane (say, the Earth orbit plane, the ecliptics), and measure all planetary periapses from the points of intersection of their orbits with the base plane (i.e. the ascending nodes).

For a single orbit it isn't useful very much, but sometimes is used from personal habit.

Edited by kerbiloid
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Hi KSP colleagues,

I would please like to follow up on this issue of the Argument of the Periapsis (Arg Pe), and introduce the issue of Longitude of the Ascending Node (LAN).

I have a rocket on the launch pad at Kerbin Space Center.  Its Arg Pe reads 90.0 °, no matter the time of day.  When I time warp, however, the LAN keeps changing.

Could someone be so kind as to explain what is going on.

Stanley

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11 minutes ago, MetricKerbalist said:

Hi KSP colleagues,

I would please like to follow up on this issue of the Argument of the Periapsis (Arg Pe), and introduce the issue of Longitude of the Ascending Node (LAN).

I have a rocket on the launch pad at Kerbin Space Center.  Its Arg Pe reads 90.0 °, no matter the time of day.  When I time warp, however, the LAN keeps changing.

Could someone be so kind as to explain what is going on.

Stanley

The issue here is that as you noted in the opening post, the Longitude of the ascending node is undefined for an equatorial orbit, and since LAN defines argument of periapsis this would also be undefined.

Since computers don't like undefined numbers KSP does things a little backwards to prevent them, it assumes that the Longitude of Periapsis is on the other side of the planet from where you are now, and that argument of Ascending node is -90 degrees from that, then swaps them about for the more conventional system

If you were to launch into inclined orbit, you would see you LAN change by 90 degrees, but if you launch from a non equatorial launch site the LAN readout on the launchpad will give an accurate reading

Longitude changes over time because it is celestial longitude rather than geographic.

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Thank you, @Rhomphaia.  Let me think about what you are saying.  This is deep.

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