Here is my answer to what was a

*very* astute question, posed almost ten years ago. Essentially, the question was:

*why is the longitude of the ascending node of the Earth's orbit 348.74 degrees, rather than zero?* The questioner clearly understood that Longitude angle (a.k.a. Right Ascension) is measured in the reference plane (taken to be Earth's orbital plane), between "the" reference direction (i.e. "the" vernal equinox, seen from Earth) and "the" ascending node. The questioner also clearly understood that, in the case of Earth's orbit, the ascending node is the vernal point direction and, by definition, the reference direction, so the longitude angle should be zero. And that, in principle, is true, hence the befuddlement. [NB I write "the" in quotes, because all "the" parameters change gradually with time, which is basically what complicated matters.]

Consider these data, reproduced from

https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html
Earth Mean Orbital Elements (J2000)

Semimajor axis (AU) 1.00000011

Orbital eccentricity 0.01671022

Orbital inclination (deg) 0.00005

Longitude of ascending node (deg) -11.26064

Longitude of perihelion (deg) 102.94719

Mean Longitude (deg) 100.46435

Key to answering the original question is to note that the inclination is given as 0.00005 deg, not zero. This means that we are dealing with two planes, inclined only very, very slightly to one another. Such a small inclination means that the nodes - both ascending and descending - could be almost anywhere; their longitude, almost any angle between 0 and 360 deg. Their position becomes increasingly sensitive and (here is the main point) increasingly

__irrelevant__, as the inclination decreases.

Precisely why it is that the above J2000 (simplified) orbital elements have this particular small inclination between the reference plane and "the" orbit plane, and this particular longitude, I am not entirely certain. I suspect it will be to maximize the usefulness ("average accuracy") in the current epoch of the

*static*, Keplerian orbits that the above,

*constant* orbital elements will produce. What I can say with confidence is that for school / college / amateur computations, the above elements can be used to satisfactorily propagate orbits. An informative exercize would be to investigate the sensitivity of the propagated orbits to changes in this longitude. For more serious calculations, known rates-of-change w.r.t. time of these elements are available. Even dealing with the above data properly requires some care, and I am happy to explain in more detail any of this. [As an aside, note that 360 - 11.26 = 348.74, which reconciles the two different longitude numbers mentioned above].

Incidentally, I came across this question and associated (growing) thread yesterday, almost ten years after it was posed, and so impressed was I by such a question from a then high-school 6th former that I got up early today to "sign up" on this forum, just so I could give what I think is a reasonable and long-overdue answer.

D.J. Walker, Wirral, UK.

29th Oct. 2022