Day-night terminator and twilights fails around equinoxes

[mikez]

Description of the problem

I posted this issue also directly in the gmt project; however, since I'm actually using pygmt, maybe it's of interest here as well.

Running your solar example around equinoxes results in the terminator circles unexpectedly getting clamped to 90º.

Minimal Complete Verifiable Example

import datetime

import pygmt

fig = pygmt.Figure()
# Create a global map using the Mollweide projection, centered at 0°E, with a width of
# 15 centimeters.
fig.basemap(region="d", projection="W15c", frame=True)
fig.coast(land="darkgreen", water="lightblue")

# Set a time for the day-night terminator and twilights to 17:00 UTC on January 1, 2000
reference_time = datetime.datetime(
    year=2026, month=3, day=20, hour=12, minute=0, second=0
)

# Plot the day-night terminator and twilights
for terminator in ["day_night", "civil", "nautical", "astronomical"]:
    fig.solar(
        terminator=terminator,
        terminator_datetime=reference_time,
        # Set the fill for the night area to navy blue with 85 % transparency
        fill="navyblue@85",
        pen="0.5p",  # Set the outline to be 0.5-point thick
    )

fig.show()

Full error message

System information

PyGMT information:
  version: v0.18.0
  …
GMT library information:
  version: 6.6.0
  …

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[yvonnefroehlich]

Hi @mikez,
thanks for reporting this 🙂. As far as I understood the issue it is an upstream issue and was / is currently fixed in the GMT repo. So there is nothting directly to do from the PyGMT side and this issue can be closed.

[mikez]

@yvonnefroehlich Thank you for your work here and the handy solar.py gallery example. :)

What I find weird is that the day-night terminators have seemingly been wrong for over 8 years and nobody noticed.
Hopefully, we can now find a fix in GenericMappingTools/gmt#8938

[yvonnefroehlich]

@yvonnefroehlich Thank you for your work here and the handy solar.py gallery example. :)

Really happy to hear that the gallery examples help you with learning and using PyGMT in your work! 🙂.

What I find weird is that the day-night terminators have seemingly been wrong for over 8 years and nobody noticed.

Yep, eight years is a long time; but not everybody reports potential bugs 🙁.

And partly, the terminator lines do not look very different for the different terminator types. So, in case only one type is plotted, it's likely that users do not recognize that there is an error.
Example for year=2026, month=month, day=20, hour=12, minute=0, second=0 with month=[1, 2, ..., 7]:

Click for code

import datetime
import pygmt

terminators = ["day_night", "civil", "nautical", "astronomical"]


fig = pygmt.Figure()

for month in [1, 2, 3, 4, 5, 6, 7]:

    reference_time = datetime.datetime(
        year=2026, month=month, day=20, hour=12, minute=0, second=0
        # year=2026, month=month, day=21, hour=12, minute=0, second=0
        # year=2026, month=month, day=22, hour=12, minute=0, second=0
    )

    for terminator in (terminators + ["all"]):

        fig.basemap(region="d", projection="W10c", frame=0)
        fig.coast(land="gray", water="white")

        if terminator != "all":
            fig.solar(
                terminator=terminator,
                terminator_datetime=reference_time,
                fill="red@85",
                pen="0.5p,red",
            )
        elif terminator == "all":
            for tt in terminators:
                fig.solar(
                    terminator=tt,
                    terminator_datetime=reference_time,
                    fill="red@85",
                    pen="0.5p,red",
                )

        if month == 1:
            fig.basemap(frame=f"+t{terminator}")
        if terminator == "day_night":
            fig.text(
                text=f"month: {month}", position="LM", justify="BC",
                no_clip=True, font="20p,1", angle=90,
            )

        fig.shift_origin(xshift="+w+0.5c")

    fig.shift_origin(yshift="-h-0.5c", xshift="-52.5c")

fig.show()

[mikez]

@yvonnefroehlich What you write makes a lot of sense. I had noticed something was uncanny at times, but couldn't quite pin it down. In retrospect, I now realize the uncanniness was likely in March and September of each year.

Thank you also for the code; very helpful to see how you can arrange plots into a spreadsheet.