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Japan currently has two space launch facilities: Uchinoura Space Center (31°15′07″N 131°04′55″E) and Tanegashima Space Center (30°24′00″N 130°58′12″E).

Both of these are at the south end of the main archipelago, but Japan has access to the Okinawa islands (around 26° 19' 34.80" N) and the Yaeyama Island group (around 24° 21' 28.19" N).

Latitude elevation is an important factor when launching orbital boosters since the earth's rotation helps a lot, and since it reduces the need for maneuvers for geosynchronous orbits.
Six degrees closer to the equator sounds like a very large difference to me.

Why did Japan not build the space centers further south?

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    $\begingroup$ Perhaps 26 m/s is a small price to pay to avoid any evanescent components of the nine-dash line? Or maybe they just don't want their engineers taking days off when the surf's up? $\endgroup$ – uhoh Feb 23 at 16:57
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    $\begingroup$ $$\left(\cos(24°) - \cos(31°)\right) \frac{ 2 \pi \times 6378137 \ \text{meters}} {86164 \ \text{seconds}}$$ $\endgroup$ – uhoh Feb 23 at 17:30
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    $\begingroup$ Thanks for that, I’m too lazy to math today. $\endgroup$ – Russell Borogove Feb 23 at 19:42
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    $\begingroup$ Depending on the purpose of the launch, high inclination might also be a feature. They would not want to launch an Earth observation satellite that is unable to see all of Japan at a good angle. So they will either need a high enough altitude, which generally decreases resolution, or simply a high enough inclination. $\endgroup$ – mlk Feb 24 at 11:36
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    $\begingroup$ The US's main launching facility at Cape Canaveral is at latitude 28.5, while Miami is at 25.7, Hawaii goes as low as 19, and American Samoa is at -14. So clearly proximity to the equator isn't the only factor in choosing a launch site. $\endgroup$ – T.E.D. Feb 26 at 14:45
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Logistic concerns tend to outweigh small performance differences. Courtesy of Uhoh in the question comments, the 7° difference in latitude is worth $$\left(\cos(24°) - \cos(31°)\right) \frac{ 2 \pi \times 6378137 \ \text{meters}} {86164 \ \text{seconds}} = 26\ \text{meters/second}$$ difference in surface rotation speed, about one-quarter of one percent of the cost of ascending to Earth orbit; it may well be cheaper to construct, prepare, and launch a slightly more powerful rocket in the main archipelago, where hiring technical staff is easier, than a slightly less powerful rocket far from JAXA headquarters and the industrial centers of Japan.

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  • $\begingroup$ Unless the goal is to get to GSO, in which case it might be a bit more. But still, you are right. $\endgroup$ – PearsonArtPhoto Feb 24 at 19:38
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    $\begingroup$ So, basically the same reason that the US launches from Florida instead of Hawaii. $\endgroup$ – dan04 Feb 24 at 22:33
  • $\begingroup$ @dan04: 9 degree difference (19.5 °N vs. 28.5 °N). Florida's is approximately at the same latitude as Japan's. $\endgroup$ – Peter Mortensen Feb 25 at 17:22
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    $\begingroup$ That's the same reason why there is no space industry in Somalia. You could launch rockets right from the equator and drop your stages over the Indian ocean, but the lack of infrastructure and the political instability of the region outweigh the benefits. $\endgroup$ – Philipp Feb 25 at 18:00
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    $\begingroup$ @Philipp ... and there is a launch site in French Guiana. $\endgroup$ – RonJohn Feb 25 at 22:55
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That was the southernmost point in Japan (at the time)

The answer to your question has its roots in history more so than it does in science. Tanegashima was chosen in 1966 and the space center completed construction in 1969. This was before Okinawa (which included the Yaeyama Islands) was returned to Japan, in 1972. Another potential site, the Ogasawara Islands were only just returned to Japan in 1968, a few years too late.

As for why they haven't moved, one can only speculate, but I'm sure it has to do with the cost of moving something already well-established. Ogasawara Islands are now a UNESCO site, so I don't think moving there would be possible anyway. Also, Tanegashima is still close enough to the mainland to make it accessible, cheaper to maintain and transport parts.

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    $\begingroup$ And the return of Okinawa was by no means certain; the US had been using it as a bargaining chip. $\endgroup$ – Harper - Reinstate Monica Feb 25 at 15:59
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Probably due to population density further south.

enter image description here

The Southern islands are more densely (pictured in the top left corner) are more densely populated than Tanegashima (bottom left, oblong shaped island)

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    $\begingroup$ the question asks why not in the Okinawa islands or the Yaeyama Island group. What population density further south of those islands would be of concern? $\endgroup$ – uhoh Feb 23 at 17:01
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    $\begingroup$ But the northern point of the Ishigaki island (24°36'04.8"N 124°19'54.9"E) seems to be rather sparsely populated $\endgroup$ – Speedphoenix Feb 23 at 17:04
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    $\begingroup$ AFAIK Okinawa has a major shortage of available land; and if any major chunk can be freed up, it'll likely be used to relocate one or more military bases that're now surrounded by urban areas. OTOH they've been trying to do that for some time; and there's massive opposition to the land acquisition somewhere else on the island step. The locals are, understandably, resentful that military bases take up a disproportionately large chunk of their island. $\endgroup$ – Dan Is Fiddling By Firelight Feb 24 at 12:25
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    $\begingroup$ I thought your comment was funny, it got a down vote because you used an answer post to leave a comment. It's a little frustrating but as soon as you get 50 reputation points you'll be able to leave comments on other people's posts. In the mean time you might enjoy reading How (the heck) do you replace a space station window in orbit? ;-) $\endgroup$ – uhoh Feb 24 at 23:10
  • $\begingroup$ (comment was elsewhere, not here) $\endgroup$ – uhoh Feb 25 at 6:45

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