Discontinuous RotatedPole parameters produce inverted coordinate system

[reinderien]

Description

From the beginning, the implementation of Nightshade has included

        if lat > 0:
            pole_lat = -90 + lat
            central_lon = 180
        else:
            pole_lat = 90 + lat
            central_lon = 0

I have code that depends on that routine and the resulting RotatedPole to do a series of ecliptic calculations. Unfortunately, I've had to copy-and-modify, because the original code - due to the if above - produces discontinuous behaviour when lat (i.e. solar declination) crosses zero. In one half of the annual domain, the behaviour is correct; and in the other half the following happen:

• I believe the boundary of behaviour is at the September equinox
• delta_sun changes sign, which in itself is not a problem
• lat becomes positive; also not a problem
• the if expression above abruptly starts evaluating to True
• the entire coordinate system abruptly becomes inverted.

I have found that only the second case is correct (i.e. negative latitude). Copying the implementation and removing the conditional, preserving only the second case, produces correct, continuous behaviour.

I suspect that the reason this hasn't been noticed is that the nightshade curve is symmetric and so inversion is "invisible"; but anyone using the CRS owned by Nightshade for asymmetric curves is going to get a nasty surprise.

Code to reproduce

Compare behaviour before and after 3b77fe0, before and after an equinox.

Operating system

All

Cartopy version

0.22, but theoretically all versions

pip list

pip freeze
Cartopy==0.22.0
certifi==2024.2.2
charset-normalizer==3.3.2
contourpy==1.2.0
cycler==0.12.1
fonttools==4.49.0
idna==3.6
kiwisolver==1.4.5
matplotlib==3.8.3
mypy==1.8.0
mypy-extensions==1.0.0
numpy==1.26.4
packaging==23.2
pillow==10.2.0
pyparsing==3.1.1
pyproj==3.6.1
pyshp==2.3.1
python-dateutil==2.8.2
pytz==2024.1
requests==2.31.0
scipy==1.12.0
shapely==2.0.3
six==1.16.0
tomli==2.0.1
types-requests==2.31.0.20240311
typing_extensions==4.9.0
tzdata==2024.1
urllib3==2.2.1