Longitude of the ascending node of the Moon

[salvomic]

hi,
I need to calculate the longitude of the ascending node of the Moon (not only the passage through the node). Is it possible to do with Skyfield?
thank you

[brandon-rhodes]

Skyfield is only designed to produce physical coordinates, like the longitude of the Moon each time it crosses the equator. Since the Moon is not located on the ascending node in between actual ascents past the equator, Skyfield will instead return the Moon's actual position.

[salvomic]

Thanks, it's true what you say, anyway, in the meantime I found the answer using the API the orbital elements (https://rhodesmill.org/skyfield/api.html#searching), in particular the function [.longitude_of_ascending_node].

[brandon-rhodes]

Where you generate the orbital elements using the position and velocity of the Moon?

[salvomic]

with this data: https://rhodesmill.org/skyfield/api.html#searching
I think to use this code (where t = ts.now() )
[code]

# ...
position = (moon - earth).at(t)
elements = osculating_elements_of(position)
# ...
node = elements.longitude_of_ascending_node
print ('Longitude of ascending Node ', node)
#passage through the node (within 15 days)
t0 = t - 15
t1 = t + 15
ti, y = almanac.find_discrete(t0, t1, almanac.moon_nodes(planets))
print('Moon transit through the nodes')
print(ti.utc_iso(), ' ')
print(y)
print([almanac.MOON_NODES[yi] for yi in y])

[/code]

is it correct?

[brandon-rhodes]

I'll try drawing a graph of the output and I'll see what I think of the results! My guess is that the result may be monotonic (ever-increasing in longitude), but will probably exhibit swings in the rate at which longitude increases, which won't be what most people expect. I suppose it depends on the project for which the ascending node longitude is being used, whether the caller will expect a uniformly-increasing number or one that varies in its velocity.

[salvomic]

Great, thank you very much. Keep me informed, then.

The idea came to me these days, while the Moon is passing through the node (at point γ, exactly this evening: 2025-01-05T19:45:30Z - the 19th will pass through the descending node) and I don't find many sites that accurately describe the longitude of the node and the Moon in this regard.
Wolfram Alpha report always 125.08 as "ascending node longitude", that is always as J2000...