How do you calculate the position of the True Node

Question

this is the code i am using for the "Mean Node" even tho even this one is off by a couple of minutes

import datetime
import swisseph as swe

swe.set_ephe_path('/usr/share/ephe')

# Get the current date
current_date = datetime.date.today()

# Calculate the Julian Day
jd = swe.julday(current_date.year, current_date.month, current_date.day)

# Calculate the position of the North Node (10 represents North Node)
pos = swe.calc_ut(jd, 10)

# Extract the ecliptic longitude of the North Node
ecliptic_longitude = pos[0][0]  # Access the correct value within the tuple

# Calculate astrological sign and degrees
zodiac_signs = ['Aries', 'Taurus', 'Gemini', 'Cancer', 'Leo', 'Virgo',
                'Libra', 'Scorpio', 'Sagittarius', 'Capricorn', 'Aquarius', 'Pisces']

# Calculate degrees, minutes, and seconds
degrees = int(ecliptic_longitude)
remaining_degrees = (ecliptic_longitude - degrees) * 60
minutes = int(remaining_degrees)
seconds = (remaining_degrees - minutes) * 60

# Calculate astrological sign index
sign_index = (degrees // 30) % 12

print(f"Current Date and Time: {datetime.datetime.now()}")
print(f"Longitude of North Node: {degrees}° {minutes}' {seconds:.2f}\"")
print(f"Astrological Sign: {zodiac_signs[sign_index]}")

swe.close()

I am yet to find any info on how to calculate this thing. The one in the example uses python by any language is fine. Also even tho this code uses "swisseph" skyfield is also fine as long as it works

Answer 1

The short answer is to call swe.calc_ut with body swe.TRUE_NODE (11) instead of swe.MEAN_NODE (10).

The underlying library documentation explains the difference between the two. If I understand correctly, the "true" node is that of an osculating orbit, and the "mean" node uses a semi-analytical expression in the form of ELP 2000-85 but fitted to JPL DE431.

I also suggest passing the current UT hour to swe.julday instead of the default 12:00.

current_date = datetime.datetime.now(datetime.timezone.utc)

jd = swe.julday(current_date.year, current_date.month, current_date.day,
        current_date.hour + current_date.minute / 60)

pos = swe.calc_ut(jd, swe.TRUE_NODE)