The goal is to find precessed coordinates for BCE dates.

from astropy.time import Time
equinox = Time('-0500-01-01') 

is just not working, though '0500-01-01' works just fine. Is there a time format that allows BCE dates or years before present or anything like that?

  • 1
    $\begingroup$ What does the documentation say? $\endgroup$ Commented Apr 22, 2023 at 2:20
  • 2
    $\begingroup$ The documenation docs.astropy.org/en/stable/time/index.html It says "iso 8601 compliant date" but iso just says implementations may use an expanded date format, by prior agreement. $\endgroup$
    – James K
    Commented Apr 22, 2023 at 6:13

1 Answer 1


Yes. Use the format='fits' to the time function.

FITS format: “[±Y]YYYY-MM-DD[THH:MM:SS[.sss]]”.

ISOT but can give signed five-digit year (mostly for negative years);



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