NavList:

Dear Antoine,

Here are my calculations for lower latitudes.

2018-10-20T11:00UT

Arcturus: SHA=145°3.4'; Dec=19°6'
Deneb: SHA=49°28.'; Dec=45°21.2'

11:00 (at 30°00'N 30°00'E) H_C-Arcturus=76°25'34"; H_C-Deneb=23°01'46"
11:10 (at 30°02'N 30°02'E) H_C-Arcturus=74°54'59"; H_C-Deneb=24°44'09"
11:20 (at 30°04'N 30°04'E) H_C-Arcturus=73°13'37"; H_C-Deneb=26°27'24"
11:30 ???????
11:40 (at 30°08'N 30°08'E) H_C-Arcturus=69°29'35"; H_C-Deneb=29°56'18"
11:50 (at 30°10'N 30°10'E) H_C-Arcturus=67°30'33"; H_C-Deneb=31°41'48"
12:00 (at 30°12'N 30°12'E) H_C-Arcturus=76°25'34"; H_C-Deneb=23°01'46"

QuadReg gives:
Arcturus: -1.851513153·x² + 31.56890186·x - 46.76559446
Deneb: 0.2234865581·x² 5.297575534·x - 62.28692828

So, at 11:30 interpolated HC-Arcturus=71°24'51"; HC-Deneb=28°11'29", which gives the QTH of 30°09'N 30°03'E.
Whereas the actual QTH was 30°06'N 30°06'E with HC-Arcturus=71°24'26"; HC-Deneb=28°11'29".
Those mere 0°0'25" (twenty-five arcseconds) resulted in a ~4nm position error.

This, as far as I can tell, proves your point - to use the interpolation with causion.

Thank you for your comments!

Warm regards,

Tony

60°N 30°E