<p>I know the answer now. Turns out that longitude of perihelion ($\overline\omega$) is defined as the sum of longitude of ascending node ($\Omega$) on the ecliptic plane and argument of perifocus ($\omega$) on the orbital plane: ($\Omega$ + $\omega$). It's strange to add up two non-coplanar angles, even when the inclination between the two planes is very small, but that's what it is.</p>

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2014-12-16T18:41:00.707Z
2014-12-26T19:26:17.897Z
2014-12-26T19:26:17.897Z
pkout
<p>I am learning orbital elements and one thing isn't clear to me. When I look at the Horizon's ephemeris data for Mars, <a href="http://ssd.jpl.nasa.gov/horizons.cgi#results" rel="nofollow">this</a> gives me argument of perifocus equal to roughly 286.53 degrees on epoch J2000. When I look up its longitude of perihelion for the same epoch <a href="http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html" rel="nofollow">here</a>, I get roughly 336.04 degrees. I thought these two parameters are the same thing. What is the difference between them? Why are the two numbers not the same?</p>

<orbital-elements>
Orbital elements: Difference between longitude of perihelion and argument of perifocus
pkout
# Orbital elements: Difference between longitude of perihelion and argument of perifocus

(Asked by: username_0 on 2014-12-16)

I am learning orbital elements and one thing isn't clear to me. When I look at the Horizon's ephemeris data for Mars, [this](http://ssd.jpl.nasa.gov/horizons.cgi#results) gives me argument of perifocus equal to roughly 286.53 degrees on epoch J2000. When I look up its longitude of perihelion for the same epoch [here](http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html), I get roughly 336.04 degrees. I thought these two parameters are the same thing. What is the difference between them? Why are the two numbers not the same?

**Tags:** orbital-elements

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## 1 answers

### Answer by username_0 on 2014-12-16. Score: 2 | Accepted answer

I know the answer now. Turns out that longitude of perihelion ($\overline\omega$) is defined as the sum of longitude of ascending node ($\Omega$) on the ecliptic plane and argument of perifocus ($\omega$) on the orbital plane: ($\Omega$ + $\omega$). It's strange to add up two non-coplanar angles, even when the inclination between the two planes is very small, but that's what it is.


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