egg changed the topic of #principia to: Logs: https://esper.irclog.whitequark.org/principia | <scott_manley> anyone that doubts the wisdom of retrograde bop needs to get the hell out | https://xkcd.com/323/ | <egg> calculating the influence of lamont on Pluto is a bit silly…
<queqiao-->
<thunk> i assume principia does not have general relativity, and so no anomalous precession of Mercury. does this pose issues for long-term stability of the solar system as suggested here? https://academic.oup.com/mnras/article/521/3/4349/7075888
<queqiao-->
<thunk> I assume not because this would be a millions-of-years thing, not the 100-year or so timescale relevant to KSP
raptop is now known as umbralraptop
<queqiao-->
<Smrtihlav> Hello sorry if this is a common question but is there like a beginner's guide to principia?
<queqiao-->
Reply to "Smrtihlav: Hello sorry if this is a common question but is there like a beginner's guide to principi..."
<queqiao-->
<Smrtihlav> Things to watch out, timeskips, saves, kraken conditions etc.
<queqiao-->
<GoForPDI (less drag=more faster)> There is a wiki but it’s more focused on general concepts and how to do x than that stuff
<queqiao-->
<Smrtihlav> ah okay thanks
<queqiao-->
<GoForPDI (less drag=more faster)> Principia will break most saves, especially if you have probes en route or around other planets
<umbralraptop>
I want to say that the big thing to watch out for is to not run a prediction too far into the future or with too short a timestep?
<queqiao-->
<GoForPDI (less drag=more faster)> it re-integrates from Day 1 of a save, so your planetary encounter will suddenly become an encounter with the planet is no longer there
<queqiao-->
<Smrtihlav> that's good to know 🙂
<queqiao-->
<GoForPDI (less drag=more faster)> Yeah long and precise computations generally will bring your PC to its knees, as will >~300000x time warp
<queqiao-->
<Smrtihlav> haha no i was just curious if i should be on my toes even with interplanetary missions while using princip but apparently not
<queqiao-->
Reply to "thunk: i assume principia does not have general relativity, and so no anomalous precession of Me..."
<queqiao-->
<Clayel> principia does have precession in general though, right? since general relativity isnt the whole picture
<queqiao-->
Reply to "Clayel: principia does have precession in general though, right? since general relativity isnt th..."
<queqiao-->
<thunk> yes, most of it is outer-planet perturbations and quadrupole/J2 terms from celestial oblateness.
<umbralraptop>
Orbits yes, axial unsure
<queqiao-->
<GoForPDI (less drag=more faster)> Yeah it precesses a bit from N-body but a lot more than it should just from that iirc
<queqiao-->
<Clayel> i imagine the values are so small that floating point might be an issue
<queqiao-->
<GoForPDI (less drag=more faster)> Relativity I think causes most of it
<queqiao-->
<GoForPDI (less drag=more faster)> Ah, ok, I stand corrected
<queqiao-->
<thunk> for most spacecraft purposes, the quadrupole effect is the most important.
<queqiao-->
<Clayel> oh so only like 7% is general relativity
<queqiao-->
Reply to "thunk: for most spacecraft purposes, the quadrupole effect is the most important."
<queqiao-->
<GoForPDI (less drag=more faster)> Earth’s J2 is not real and cannot hurt you
<queqiao-->
<Clayel> why is it called a quadrupole moment btw?
<queqiao-->
<GoForPDI (less drag=more faster)> Neither are lunar mascons
<queqiao-->
Reply to "Clayel: why is it called a quadrupole moment btw?"
<queqiao-->
<thunk> spherical harmonics or something idk. the most relevant one here is the base oblateness term that is positive at the equator and negative at the poles
<umbralraptop>
No,no, J2 helps you! (get into SSO)
<queqiao-->
<thunk> so as you go around the poles, there's a + - + - pattern, hence 4 poles
<queqiao-->
<Clayel> sneaky raptop switching between discord and irc
<queqiao-->
<GoForPDI (less drag=more faster)> Oh, so THAT’S what they’re doing!
<queqiao-->
<thunk> the term goes as 1/radius^3, so it's far more relevant in low orbits
<queqiao-->
<GoForPDI (less drag=more faster)> I was always confused lol
<queqiao-->
<thunk> the next most important for Earth is the J3 term that arises from the southern hemisphere being more massive than the northern, and this is a tiny correction to the 'J2 force' for nodal precession as seen here https://en.wikipedia.org/wiki/Geopotential_spherical_harmonic_model
<queqiao-->
<thunk> the Moon has far lumpier gravity with a lot of large high-order terms, which causes much pain for newcomers
<queqiao-->
Reply to "thunk: the next most important for Earth is the J3 term that arises from the southern hemisphere..."
<queqiao-->
<sichelgaita> Actually, the (2, 2) tesseral terms tend to be more important because (1) they correspond to a potential in r⁻³, just like J₂; (2) they are numerically larger than J₃; (3) they introduce new effects, like inhomogeneities in longitude. Their effect is particularly pronounced for Phobos and Deimos.
paculino has quit [Ping timeout: 198 seconds]
paculino has joined #principia
<queqiao-->
Reply to "sichelgaita: Actually, the (2, 2) tesseral terms tend to be more important because (1) they correspond..."
<queqiao-->
<thunk> the best way to learn something new, say something completely inaccurate 🌿
<queqiao-->
<thunk> i assume that would cause GEO longitude drift?
<queqiao-->
<sichelgaita> Not sure, the longitude drift may require more than the (2, 2) term. But surely the tesseral terms are the cause of the drift.
<queqiao-->
<thunk> > There are two stable equilibrium points sometimes called "gravitational wells"[66] (at 75.3°E and 108°W) and two corresponding unstable points (at 165.3°E and 14.7°W).
<queqiao-->
<thunk> yeah that looks to me like a (2,2) potential