Polar opposites - double FDOI IGY covers

I was going to post next on the design of the U.S. IGY stamp, but I decided to delay that to again take my prompt from today's New York Times crossword. The revealer at the center of the puzzle was "polar opposite", as clued by "One totally unlike another ... or what each answer on the edge of this puzzle has".

This reminded me of three first day covers I have that involve polar opposites, if you'll give me some liberty with that term. 

The first cover comprises two first day cancellations of the IGY stamps from two countries, the U.S. and Canada. The stamps and their cancellations are in "polar opposite" catty-corner positions of the cover. Both countries do have geographic areas north of the Arctic Circle. So, polar opposites? I'll return to the Canada stamp at some point. Its issue date of March 5, 1958, predates the U.S. stamp by almost three months. I like the bilingual FDOI slogan.  If you look at the back of the envelope, Canada is right-side up.

Two FDOI cancellations on the same cover, for U.S. and Canada IGY Stamps

The second cover has two U.S. stamps, the IGY stamp from 1958 (with a cachet) and the Antarctic Treaty stamp from 1961. The Antarctic Treaty partly grew out of cooperation that occurred on that continent during the IGY, another topic for the future. Two main areas of scientific interest during the IGY were solar physics and polar science. Hot and cold, up above and down under - polar opposites?

Two FDOI cancellations of the U.S. IGY and Antarctic treaty stamps

The third cover again contains the 1958 U.S. IGY stamp, but also two stamps issued during  the 2007-08  Fourth International Polar Year, 50 years after the IGY. The IGY itself was also the Third International Polar Year. Not only do the stamps from the two eras show the solar and ionospheric environments, respectively, but the two IPY auroral stamps are for both the Northern (boreal) and Southern (austral) lights, polar opposites you might (?) say. The cancellations are much clearer than for some of my covers. The logos for the IGY and IPY are the de facto cachets on this cover.

A cover showing FDOI cancellations for the U.S. 1958 IGY stamp and two 2007 84¢ IPY stamps

Back of cover, showing remainder of souvenir sheet for the IPY stamps

The excellent book, Topical Adventures: A Guide to Topical and Thematic Stamp Collecting (Jack R. Congrove, Dawn R. Hammer, and Martin Kent Miller, eds.; American Topical Association, Handbook #168, 2020, 190 p.), says on p. 72:

Covers, mostly philatelically contrived but sometimes not, used on a stamp's first day of issue are highly collectible, especially when accompanied by a colorful cachet.

From a philatelic standpoint, you could say these covers with double FDOI cancellations are indeed contrived, or at least my interpretations of them are, but I like them!