I looked into the old app, espesially for Chai Bu and it seems the library for calculating the Suns apperent ecliptical longitude, that is basically the position of the Sun in relation to Earth during the year(and also what we are basing the Ju on, skipping entirely the Solar Terms) is not very good.
While the idea to base the Ju directly on the longitude is something I stand by and do enjoy as an idea, the longitude in the old apps is not very accurate.
Yet as it allow the Ju to be added manually, it seems good idea to write a short guide on how to find the Ju, on a map that we are already looking at. As that will allow to use any app that supports manually selecting Ju, no matter what system the author is using.
Lets say we look into a book, or video and the map that is there doesn't seem to reflect the map our apps or calculators are showing. That is usually based on the Ju, there could be other reasons and there are much more complex fixes there, but most of the time if we can select the Ju manually the card will align.
So how to know what Ju was used in creating the map in the book, if it isn't specifically said(and it often isn't)?
Its simple, just look where the Wu stem on the Earth Plate(lower row of stems in each palace) is.
The number of the Palace in later heaven sequence is the number of the Ju.
Meaning for example here are random maps from search of "qmdj map"in google images:
The Ju here is 8, as the Wu stem on the Earth plate is in Gen 8 Palace.
The Ju here is 1 as Wu on the Earth plate is in Kan 1 Palace on the
Its in Kan 1 here as well, so Ju is 1.
While I still can't recommend using the old app, at least in this way if someone wants to use it they can do so in wider range of sources. Of course, this is also useful for other calculators where Ju can be selected manually, as often in books different systems may be used. So if unsure why the maps are different, look where Wu Palace is. : )
