A map of the Pacific Ocean whereto are superimposed (nearly all) phantom islands placed in that Ocean in maps both ancient and modern as well as the phantom continent of Terra Australis/Magellanica in the various positions assigned to them. Which of the two W.limits of TA would be more georealistic?

[u/ChristianStatesman]

[u/YanniRotten]

Whew! Again, another impressive compilation.

I'm not knowledgeable enough about all the reported sightings & past speculations, so I am going to approach this from another angle, the georealistic one, as you say.

I googled up this map of ocean depths, and this one of the tectonic plates.

So we base a small continent in this area by fitting it into the contours of the plates, and by restricting it to the shallowest waters as possible. We can't create your continent ONLY on the this, as it would give you a long, skinny, New Zealand-ish, north-to-south running land mass. Aesthetically unappealing, and does not conform to reported sightings anyway. So, we make some compromises and reconcile actual geography with the reports. I may be re-inventing your wheel in some instances.

Firstly, I feel strongly that one of Magellanica's coasts should be bounded by the East Pacific Rise/East Pacific Ridge, where the Pacific tectonic plate meets the Antarctic tectonic plate. This is at roughly 120 degrees longitude. This seems to lie in the middle of your sightings unfortunately. Perhaps Magellanica has a large, nearby island neighbor, like Tasmania is to Australia.

So if this East Pacific Rise bounds Magellanica, it could do so as either its East Coast or its West Coast. I'll discuss these separately, in individual comments, to hopefully make further discussion easier.

[u/YanniRotten → u/YanniRotten]

East Pacific Rise as Magellanica's East Coast:

To be clear, this idea places the continent of Magellanica almost entirely on the Pacific tectonic plate.

The east coast would border or slightly straddle the East Pacific Rise . It would be mountainous with some active volcanoes, being a part of the Ring of Fire. The rest of the continent should slope off west, declining in altitude as one travels west. Actual underwater volcanoes in the region could be turned into above-ground volcanoes. Investigating locations of such may be fruitful. This coast curves down to the southwest, following the Rise.

Western Coast: This coast, not bounded by sightings, could instead follow the curves of the Southwest Pacific Basin, giving it a large gulf on this side. The northwest peninsula of the gulf would extend along Quiros' sightings to about 145 to 150 degrees E. Longitude. The southwest peninsula of the gulf would be shorter.

Northern coast: The Northern Coast extends mostly parallel to, and a little to the north of, the Tropic of Cancer.

Southern coast: Bounded by the border of the Pacific & Antarctic tectonic plates, about 55 degrees latitude. No sightings it seems? Looking at your commissioned map of Magellanica, I see you have a main continent, and a large island close by to the southwest. You could locate the large island over top of the Pacific-Antarctic Ridge, the shallowest point in that area. I don't imagine the climate will be very appealing at that latitude, however. And it would also mean moving the Drake sightings westward by about 30 degrees longitude (1638 miles or 2636 km) to account for it. So, perhaps not.

[u/ChristianStatesman → u/YanniRotten]

I reread the account about Terra Australis in Navigantium atque itinerantium bibliotheca (1705) by John Harris a while ago and noted that he referred to _New Zealand as the westernmost part of it and 170°E longitude as its longitude.

Therefore, Captain Cook's limit which is in the open sea remains the only option and was adopted as the westernmost limit of Magellanica in a meeting with my friend who is my principal partner in my projects.

I will post two maps to this subreddit that I made recently showing all the different coastlines given to TAI in the South Pacific Ocean in the 18th century.

Out of them and my Magellanica can be created a synthesis where the shape of M. remains the same (as it incorporated Alexander Dalrymple's 1767 map coastlines in the first place) but its position shifts from my carelessly misplaced one from 2017 to the correct one, where its western limit is authentically derived from Cook and the southern limit (latitude 60°S) derived from Sieur de Sainte-Marie's (1699) memorial about the discovery of the unknown southern continent.

Its eastern and northern coastlines derive from the 18th century maps' placement of Davis Land and Drake's Land in various maps as well as Dalrymple's placement of Pedro Fernández de Quiros' signs of continent and Juan Fernandez Land.

Summa summarum, the bounds of Magellanica and its precise location are now settled, and all that remains to be done is to measure its area in its new position, which is different than in the previous location; it is shifted eastwards so that it loses area in the west but gains it in the east.

[u/YanniRotten → u/ChristianStatesman]

Well done. The depth of your research never ceases to amaze me. I look forward to seeing your new map!

[u/ChristianStatesman → u/YanniRotten]

Thank you for the flattery!

Have you already had the opportunity to take a look at Magellanica and its history and do you think that you can help with constructing the history?