(50g) Week day / Season begin / Mean / True ecliptic / Julian Day / Astron Nutation

[Gil]

Version #/label introduced.

Now this is the version Xe.

1)
Output for weekday improved with bugs fixed.

2)
JD conversion with calendar date "corrected" according to usual notion of calendar date :
no date of the format 0.mmdd is anymore admitted, as the year 0 has never existed.

Though now the negative years yyyy correspond to (yyyy+1) in Meus' Astronomical Algorithms (the strictly positive year remaining unchanged and 0.mmddd format, as mentioned, disappearing), for the rest the method employed to get JD or a calendar date follows rigorously the one proposed by Meeus in his book.

Two consequences:

a) Now JD=0 correspond to
date "-4713.0101, 12 h"
(Meeus: to date "-4712.0101, 12 h").

b) The day -1.1231 (December 31st, year -1)
is now followed, "more logically" (for most common users), by +1.0101 (January 1st, year +1).

A rough checking is made to check for non-allowed negative JD, dates before "-4713.0101, 12hours" or years=0.

3)
Introduction, on page 1.5 of the directory, of a preliminary version for the topical year calculation at an instant T.