Indian vedic sky culture: Show longitude for the 27 asterisms

[alex-w]

Original report by vishvAs vAsuki: https://bugs.launchpad.net/stellarium/+bug/1710443

Indian vedic sky culture divides the sky into 27 strips (full list, with western equivalents here: https://goo.gl/2PVi28 ), each approximately 360/27 ~ 13 degrees. This division is critical for several purposes including timing rituals and determining the birth lunar mansion/ nakShatra. As it is, users are forced to enable the ecliptic grid and then imagine parallel grid lines running through asterisms in the set of 27. Example pic: https://i.imgur.com/gRcwqcD.jpg . This is clumsy and inaccurate. So, the vedic indian sky culture should show the required 27 grid lines by default.

PS: Provided guidance, I will be glad to assist if needed - I am an active in the sanskrit programming community ( groups.google.com/forum/#!forum/sanskrit-programmers ) .

[vvasuki]

Sorry for the confusion - https://github.com/Stellarium/stellarium/pull/930 does not actually solve this issue (which is essentially about being able to show the hindu ecliptic grids (at either 360/27 degrees for lunar mansions or 360/12 degrees separation for raashi-s starting at a given reference point)).

Could you please reopen this?

[alex-w]

This is a good task for the community to participate in the contribution into Stellarium. Who wants to help us?

[vvasuki]

By help, you mean just info? The most common division (there are other traditions) would be as follows:

• Offset eclipltic longitudes such that Spica would lie exactly at 180 degrees.
• Starting at 0 degrees, every 360/27= 13.33 degrees, show an ecliptic longitude.

[gzotti]

Basically it is an ecliptic grid with 27 partitions, rotated so that zero goes through Spica. But I have read this Zero point may (in other traditins) also go elsewhere, so this should be settable to "Spica" or "other longitude" (or other stars?). Here I don't know what's better, offset from current equinox or offset from J2000 equinox? Also, while we can (moderately) easily link the grid to Spica's position, traditional positions for Spica may differ from the modern computation.

This should go into a "cultural astronomy" plugin (name to be coined) where this is one of several more grid systems.

[vvasuki]

I forgot to mention - would be very useful to show longitudes (in a different color) for a 12-fold 30 deg "rāśi" partition (starting from the indicated zero) as well. That's another important way in which Hindus and their texts describe the skies.

Basically it is an ecliptic grid with 27 partitions, rotated so that zero goes through Spica. But I have read this Zero point may (in other traditions) also go elsewhere, so this should be settable to "Spica" or "other longitude" (or other stars?).

There are less popular traditions which deviate slightly, some listed at the linked point in this video https://youtu.be/DyUk1iDZOQs?t=859 and a fuller list is here. Of these, it might (if time permits) be worth to also add the "puShya at 16 deg Cancer" offset (described in this slide - https://youtu.be/DyUk1iDZOQs?t=1279 ).

Here I don't know what's better, offset from current equinox or offset from J2000 equinox?

This turns the ordinarily ecliptic coordinate system to a purely sidereal one - so it shouldn't matter - in either case, you offset sufficiently to make the 0-180 degrees longitude pass through Spica.

PS: I very much hope that you will use the exact position of Spica on a given date ie account for true motion of the star, changes in ecliptic plane orientation.

Also, while we can (moderately) easily link the grid to Spica's position, traditional positions for Spica may differ from the modern computation.

As far as stellarium is concerned, it should by default go with whatever they (ie science and astronomy) think is the best estimate of Spica's position (this is indeed the clear intent of ancient texts). As you say below, for those who want something else (be it something fixed in 1955 by the Indian Calendar Reform Committee, some traditional estimate etc..) may rely on non-default options.

This should go into a "cultural astronomy" plugin (name to be coined) where this is one of several more grid systems.

[github-actions[bot]]

Hello @alex-w! Thank you for this suggestion.

[gzotti]

@vvasuki we will not do this next week, but for clarity: The 12*30 ra´si are just a complete (rotated) ecliptical grid, but are the 27 mansions also partitioned in smaller units (1/2, 1/3, ...), or are these just 27 "lunes" (slices of the sphere)?

[vvasuki]

@vvasuki we will not do this next week, but for clarity: The 12*30 ra´si are just a complete (rotated) ecliptical grid, but are the 27 mansions also partitioned in smaller units (1/2, 1/3, ...), or are these just 27 "lunes" (slices of the sphere)?

Each of the 27 mansions is indeed partitioned into 4 quarters (pāda-s) each. If the corresponding longitudes can be shown with lighter/ dashed arcs, that would be great. Note that 13.3333333*2.25 = 30 deg, so that some quarter-mansion longitudes will be hidden by rāśi lines.