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This video shows Huygens probe descent to Titan moon from two different points of view:

https://sci.esa.int/web/cassini-huygens/-/39218-huygens-descent-to-titan-surface

The white dots on the upper part of the video represent West, North, East and South, while the white dot at center on lower part of the video represents the Nadir:

frame

I don't understand which projection has been used: I thought equirectangular or mercator, but in both cases the lowest edge would match with nadir, so that's not possible.

Additionally, I think that the East-side and West-side clips should match side by side, but they don't:

side by side

Once I understand the projection used, I could use FFMPEG to convert the video into equirectangular and view it into a panorama viewer or a VR viewer.

Can anybody help figuring out which projection is used in the video?

Equirectangular:

equirectangular

Mercator:

mercator

Or maybe it's the cropping of a stereographic projection, where nadir is visible?

stereographic

The conversion command would not be difficult:

 ffmpeg -i input.avi  -vf "v360:<unknown projection>:equirectangular:pitch=90"  -y  output.avi

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