How do I calculate a forward Sun–Moon arc across zero degrees?
Subtract the Sun’s longitude from the Moon’s and wrap the result into the interval from zero to less than 360 degrees.
Understand the idea
The forward arc has a direction. It is measured from the Sun’s longitude onward through increasing longitude until the Moon’s longitude is reached. Use D = (Moon − Sun + 360) modulo 360. If the Sun is at 350° and the Moon at 10°, subtraction gives −340°, but wrapping gives a forward arc of 20°. The shorter separation is also 20° in this example, yet that agreement does not make the two calculations interchangeable. Reverse the positions and the forward arc becomes 340°, while the shorter separation remains 20°. Babylon’s collective station rule uses the forward arc. Aspect geometry commonly uses the shorter separation. Retain the direction and label the value before using it in either layer. Exact conjunction should use the numerical tolerance specified by the implementation.
An example to work with
Compare Sun 350°, Moon 10° with Sun 10°, Moon 350°. Divide each forward arc by 36 and take the floor plus one. The results are Stations 1 and 10. This is a worked example, not a current sky reading.
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Are you measuring a directed arc or the shortest separation?
Part of Babylon’s authored teaching framework. Symbolic correspondences offer a language for reflection and learning.