hadec crashes: 'int' object has no attribute 'longitude'

[ion1]

The following code crashes with the following output:

import skyfield
from skyfield.api import Star, load
from skyfield.data import hipparcos
from skyfield.named_stars import named_star_dict

print(skyfield.__version__)

eph = load("de421.bsp")

with load.open(hipparcos.URL) as f:
    stars_df = hipparcos.load_dataframe(f)

ts = load.timescale()
t = ts.utc(2022, 1, 1, 0, 0, 0)

earth = eph["earth"]
regulus = Star.from_dataframe(stars_df.loc[named_star_dict["Regulus"]])
print(earth.at(t).observe(regulus).hadec())

1.42
Traceback (most recent call last):
  File "...\crash.py", line 18, in <module>
    print(earth.at(t).observe(regulus).hadec())
  File "...\.venv\lib\site-packages\skyfield\positionlib.py", line 302, in hadec   
    ha = self.center.longitude.radians - sublongtiude
AttributeError: 'int' object has no attribute 'longitude'

[brandon-rhodes]

Thanks for reporting this, I think it suggests that this error could be made clearer. But, first: could you explain what value you would like to be returned for the hour angle here? If you could share the number, then it might help me suggest the correct Skyfield code to get the value you need.

[ion1]

could you explain what value you would like to be returned for the hour angle here? If you could share the number, then it might help me suggest the correct Skyfield code to get the value you need.

I'm just getting to know Skyfield and I was looking into the differences between radec, hadec and geographic_position_of when I encountered the crash. I think geographic_position_of will suit my current needs.

[brandon-rhodes]

I think geographic_position_of will suit my current needs.

Great, I'm glad you've found a way to move forward! In the next few days I'll see about getting that error message made more specific.

[brandon-rhodes]

There — I've landed a fix which should print a more interesting error message in the future:

to compute a body’s hour angle, you must observe it  from a specific latitude and longitude on Earth

Hopefully that will help make the error clearer for the next person to run into it!