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 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, 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?
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.