Skyfield >= 1.50 can cause "OverflowError: Python int too large to convert to C long" #1086

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1. aendie commented on Jun 10, 2025

   

   aendie

   on Jun 10, 2025

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   I have whittled this issue down to a small program (which I don't really want to call an MWE) that illustrates two problems which occur with Skyfield versions 1.50 to 1.53 inclusive.

   Running the program scans Saturn RISE and SET events per day in the year 1980.
   Using Skyfield 1.49 there are no errors or warnings: it merely lists each day, i.e. "it works".

   Using Skyfield 1.50 it complains about a divide by zero warning on June 15th 1980:

   

   ... and it crashes on December 27th with "OverflowError: Python int too large to convert to C long"

   

   Here is my small test program (it only requires de421.bsp in the same folder:

   #!/usr/bin/env python3
   # -*- coding: utf-8 -*-
   
   from datetime import date, datetime, timedelta
   from skyfield import VERSION
   from skyfield.api import Loader, Topos, wgs84, N, S, E, W
   from skyfield import almanac
   
   def DOY(d00, daysinyear, idx):
   # format day of year like 'Jan 1'
       if idx is None: return "-NONE-"
       if not 0 <= idx <= daysinyear:
           return "idx={}".format(idx)
       if idx == daysinyear:
           return "Dec 31 24:00"
       dt = d00 + timedelta(days=idx)
       return "{dt:%b} {dt.day}".format(dt=dt)
   
   print("\nSkyfield version = ",VERSION)
   d = date(1980, 1, 1)
   load = Loader("./")
   ts = load.timescale()
   eph = load('de421.bsp')
   earth   = eph['earth']
   saturn = eph['saturn barycenter']
   planet = saturn
   
   lat = 0.0
   latns = 'N' if lat >= 0.0 else 'S'
   d00 = date(d.year, 1, 1)
   daysinyear = (date(d.year+1, 1, 1) - date(d.year, 1, 1)).days
   daystoprocess = daysinyear + 1
   ts = load.timescale()	# timescale object with built-in UT1-tables
   
   print("planet:  Saturn in 1980 at latitude {:.1f}°{}".format(abs(lat),latns))
   
   #---------------------------------------------------------------------------
   topos = wgs84.latlon(lat, 0.0 * E, elevation_m=0.0)
   observer = earth + topos
   
   dt = datetime(d.year, d.month, d.day, 0, 0, 0)  # convert to datetime
   t0 = None
   idx = 0
   while idx < daystoprocess:
       print(DOY(d,daysinyear,idx))
       
       if t0 is None:
       # Note: do NOT use 'ts.ut1' as the date returned can be outside the requested boundary
       #       e.g. Saturn 2042 Feb 13 60°N returns RISE at 14 Feb 00:00:00
           t0 = ts.utc(dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second)
       t1 = ts.utc(dt.year, dt.month, dt.day+1, dt.hour, dt.minute, dt.second)
   
       planetrise, iR = almanac.find_risings(observer, planet, t0, t1)
       planetset,  iS = almanac.find_settings(observer, planet, t0, t1)
   
       # ------ assemble True events in chronological order ------
       nR = len(planetrise); nS = len(planetset)
       ndxR = ndxS = 0
       riseset_time = []; isrise = []
       while ndxR < nR or ndxS < nS:
           if ndxR < nR and ndxS < nS:     # if a RISE and a SET is available
               if iR[ndxR] and iS[ndxS]:   # if both are True events
                   if planetrise[ndxR] < planetset[ndxS]:
                       riseset_time.append(planetrise[ndxR]); isrise.append(True)
                       riseset_time.append(planetset[ndxS]); isrise.append(False)
                   else:
                       riseset_time.append(planetset[ndxS]); isrise.append(False)
                       riseset_time.append(planetrise[ndxR]); isrise.append(True)
               elif iR[ndxR]:              # if RISE is a True event
                   riseset_time.append(planetrise[ndxR]); isrise.append(True)
               elif iS[ndxS]:              # if SET is a True event
                   riseset_time.append(planetset[ndxS]); isrise.append(False)
               ndxR += 1; ndxS += 1
           elif ndxR < nR:                 # if only a RISE is available
               if iR[ndxR]:                # if RISE is a True event
                   riseset_time.append(planetrise[ndxR]); isrise.append(True)
               ndxR += 1
           elif ndxS < nS:                 # if only a SET is available
               if iS[ndxS]:                # if SET is a True event
                   riseset_time.append(planetset[ndxS]); isrise.append(False)
               ndxS += 1
       # ----------------------------------------- end of 'while'
   
       if len(riseset_time) >=1:
           dt0 = riseset_time[0].utc_datetime()
       if len(riseset_time) >= 2:     # this happens most often
           dt1 = riseset_time[1].utc_datetime()
   
       t0 = t1
       idx += 1
       dt += timedelta(days=1)
   # ----------------------------------------- end of 'while'
   

   I hope this is useful in reproducing and diagnosing the error.