If we have a 9 km mountain on an otherwise spherical planet (r=6371 km), the horizon would be 339 km away. With two mountains having a sightline between eachother the maximum theorethical distance doubles to 678 km.

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tombh 12 hours ago | parent [-]

Exactly.

And it could even be tweaked slightly with some favourable refraction.

	▲	stockresearcher 10 hours ago \| parent [-]
		Ah, well NASA left some mirrors on the surface of the moon so with some favorable reflection and infinite quality optics, you can theoretically see halfway around the world.