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ChaoticNeutralCzech@feddit.org ⁨1⁩ ⁨week⁩ ago

You actually only need to know the latitude for that… except the local terrain will play a larger role anyway, unless they started very close to a pole and follow rhumb lines (in this case a meridian/circle of latitude) as opposed to great circles, so better just ask for full coordinates.

We don’t have many data points in the question so let’s extrapolate into the past. There is the hint that they met 8 years earlier at the same spot, during which he’d have gone 1 262 304 000 ft or 384 750.2592 km, completing 9.617 polar circumferences of the Earth (40 007.863 km each).

Huh, that’s not a whole number. In some languages, “eight long years” might mean “a little over 8 years” so let’s assume he finished exactly 10 polar circumnavigations, which took 8 years and about 116 days. Her walked distance over that time is 5x smaller, 2 polar circumnavigations’ worth or 80 015.726 km. This is only exactly 2 great circles if they are polar, but we know that it’s impossible to go due east from either pole. Therefore, she must have gone around a circle of latitude. To end up in the same spot, she must have not-quite-circumnavigated-but-enough-for-Phileas-Fogg the Earth (aka crossed every meridian but not the equator) an integer number of times. After a simple conversion, we can construct a table of the options.

To calcuate latitude from circle-of-latitude circumference (colc), we’ll be using geodetic ↔ ECEF conversion equations (except those with the perverse prime vertical radius of curvature 𝑁 of course) and their notation (simplified with 𝑦 = 0, 𝜆 = 0, ℎ = 0 to ignore longitude and elevation) with values of the WGS-84 ellipsoid. The relationship we’re seeking is between colc/2𝛑 = 𝑝, circle-of-latitude radius, which is at zero longitude equal to ECEF 𝑋, and 𝜙 (latitude). See also Wikipedia on Earth radius by location but remember to skip anything with 𝑁, we’re not doing that.

The geocentric radius (𝑅) is related to 𝜙 (latitude) like this but we only need the distance to axis of rotation 𝑝.

(𝑍/𝑝)(cot 𝜙) = (1 − 𝑒²) → (𝑏²/𝑎²)(𝑍/𝑝) = 1/(cot 𝜙) = tan 𝜙 → 𝜙 = atan((𝑏²/𝑎²)(𝑍/𝑝))
(using 𝑒² = 1 −  𝑏²/𝑎²)

Since sin² 𝛂 = 1 − cos² 𝛂 and we can normalize 𝑍 and 𝑝 to the unit circle with ellipsoid radii 𝑏 and 𝑎 respectively:
𝑍²/𝑏² = (𝑍/𝑏)² = 1 − (𝑝/𝑎)² = 1 − 𝑝²/𝑎², therefore 𝑍 = √(𝑏²(1 − 𝑝²/𝑎²)).

All in all, 𝑝 → 𝜙 conversion is:
𝜙 = atan((𝑏²/𝑎²)(√(𝑏²(1 - 𝑝²/𝑎²))/𝑝))

(Presumably, this could be simpified further but since I can just put this into a calculator so idc)
Per WGS-84:
𝑎 = 6378.137 km
𝑏 = 6352.752 km

Here are the results. Finding appropriate meeting locations at some of the 25+ possible latitudes on either hemisphere is left as an exercise to the reader.

nqcbefPFs colc = 2𝛑𝑝 [km] Latitude [°N/°S]
1 too big N/A
2 6 367.449 3.277975
3 4 244.966 47.934779
4 3 183.724 59.758044
5 2 546.979 66.211738
6 2 122.483 70.346611
7 1 819.271 73.238734
8 1 591.862 75.380740
9 1 414.988 77.033209
10 1 273.489 78.347789
11 1 157.718 79.419029
12 1 061.241 80.309059
13 979.607 81.060439
14 909.635 81.703329
15 848.993 82.259706
16 795.931 82.745975
17 749.111 83.174629
18 707.494 83.555355
19 670.257 83.895779
20 636.744 84.201988
21 606.423 84.478901
22 578.859 84.730536
23 553.691 84.960206
24 530.620 85.170671
25 509.395 85.364245
26 489.803 85.542885

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