[AstroPy] BCE Time & Dates?

Frederic V. Hessman hessman at astro.physik.uni-goettingen.de
Wed Aug 14 10:47:19 EDT 2019


I'm simulating Kepler's "Harmony of the Spheres" using the extended JPL ephemerides not usually available so that I can go back to Bischop Ussher's and Kepler's creation dates, thereby using astropy.time.Time for convenience functions.  However, although

	>>> from astropy.time import Time
	>>> t = Time('4444-10-03T09:00:00')
	>>> str(t)
	'4444-10-03T09:00:00.000'
	>>> t.jd
	3344473.875

obviously works, this doesn't:

	>>> t = Time('-4444-10-03T09:00:00')
	Traceback (most recent call last):
	  File "<stdin>", line 1, in <module>
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/astropy/time/core.py", line 399, in __init__
	    precision, in_subfmt, out_subfmt)
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/astropy/time/core.py", line 454, in _init_from_vals
	    precision, in_subfmt, out_subfmt)
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/astropy/time/core.py", line 505, in _get_time_fmt
	    raise ValueError('Input values did not match {0}'.format(err_msg))
	ValueError: Input values did not match any of the formats where the format keyword is optional ['datetime', 'iso', 'isot', 'yday', 'datetime64', 'fits', 	'byear_str', 'jyear_str']

nor does this:

	>>> t = Time(259237.875,format='jd')
	>>> t.datetime
	Traceback (most recent call last):
	  File "<stdin>", line 1, in <module>
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/astropy/time/core.py", line 1542, in __getattr__
	    value = tm._shaped_like_input(tm._time.to_value(parent=tm))
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/astropy/time/formats.py", line 682, in to_value
	    out[...] = datetime.datetime(iy, im, id, ihr, imin, isec, ifracsec)
	ValueError: year is out of range

and other attempts simply return garbage along with the message

	WARNING: ErfaWarning: ERFA function "d2dtf" yielded 1 of "dubious year (Note 5)" [astropy._erfa.core]

Why is a BCE date garbagge or "dubious"?

Rick



More information about the AstroPy mailing list