I am currently in the process of writing a program that generates a GURPS Space star system according to the RAW. For the final output format I intended to make plots of the orbits, thus this question came to me.

The argument of periapsis basically describes which way the pointy ends of an orbital ellipsis points (see figure below). I have found the rules for calculating the orbital eccentricity (GURPS Space p. 116), however the book makes no mention of the argument of periapsis. Have I missed something?

Digram illustrating and explaining various terms in relation to Orbits of Celestial bodies.


I realize that there probably is a very good way to find this out using real-world astronomical know-how (which I don't possess) but I thought it would be nice if one could characterize (i.e. personalize) the star system one generates in a little more detail.

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    \$\begingroup\$ This question appears to be off-topic because it is about orbital physics. \$\endgroup\$ – SevenSidedDie Aug 5 '14 at 16:02
  • \$\begingroup\$ I was contemplating taking this question to Physics.SE. However, I wanted to know whether I have missed some addendum or user-written guide about this matter, specifically for a GURPS star system. \$\endgroup\$ – tschoppi Aug 5 '14 at 16:05
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    \$\begingroup\$ Similar question asked here. This can be closed. \$\endgroup\$ – tschoppi Aug 5 '14 at 16:45
  • \$\begingroup\$ This is a really neat idea for program. Do you have a forum post on the official forums I can track this project through? What language and platform are you building it for? \$\endgroup\$ – JMD Aug 5 '14 at 17:49
  • \$\begingroup\$ @JMD It is currently not in a state where I feel comfortable showing it to the community. It's written in Python (primarily 2.7, but it should be fully compatible with 3.x) and thus platform independent. The code is hosted on GitHub, feel free to watch it there :) \$\endgroup\$ – tschoppi Aug 5 '14 at 17:53

The argument of periapsis is mainly important for calculating the timing of trajectories to shift between one orbit and another. It likely not included in GURPS Space because they opt to express travel terms of delta-vee and largely ignore specific times.

In comparison, eccentricity is a useful detail because it generate planets with interesting environments particularly those with highly eccentric orbits, so calculating it is included in the text.

  • \$\begingroup\$ Thank you for addressing why there are no instructions for generating the argument of periapsis for the orbits. \$\endgroup\$ – tschoppi Aug 6 '14 at 8:41

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