Time period misnomers

[Alexander Wolf]

Original by Anonymous from:

https://sourceforge.net/p/stellarium/discussion/278769/thread/49cb1b7d/

Pressing F1 reveals some shortcuts, notably such to manipulate time by adding/subtracting specific periods of time - umfortunately there's some problems:

* "Add 1 sidereal month" - what this (unexpectedly) actually does is advancing time by 1/12 of a sidereal year (approx. 365.256363 mean solar days divided by 12, i.e. 30.436849 mean solar days). However, you can't just say "let's call 1/12 of a sidereal year a sidereal month", because that latter term has a very distinct meaning - namely, one revolution of the Moon around the Earth against the fixed stars, which is approx. 27.321662 mean solar days.

* "Add 1 sidereal week", "Add 1 solar week" and somesuch - this seems just wrong. There is no concept of "week" in astronomy, as there is no corresponding period of one of the fundamental celestial bodies (i.e. Earth, Moon, or Sun). The term "week" is a purely cultural invention, with a seven days week most likely stemming from the ancients dedicating one day each to a deity being associated with one of the seven major solar system celestial bodies known at that time, i.e. the Sun, the Moon, and the five planets visible to the naked eye).

Generally, one should probably better refrain from "inventing" periods like "sidereal week" (much rather strongly preferring to say "7 sidereal days" instead) and especially take care not to use existing, well-defined terms like "sidereal month" in an improper and misleading way.

[gzotti (georg-zotti)]

I fully agree, sidereal month is distinctly linked with a lunar revolution.

[Alexander Wolf (alexwolf)]

A fix has been committed as revision 7734 of the trunk branch of Stellarium's Bazaar repository at Launchpad: http://bazaar.launchpad.net/~stellarium/stellarium/trunk/revision/7734