And there are 16 peaks in Californa that exceed 4000 meters (13,123 feet) elevation and 46 peaks exceed 3000 meters (9843 feet) elevation.
Mount Whitney as seen from Mount Langley.
No rocket needed.

Why yes, but most of those don't have a "flat" ocean view.

Happily, the ancient Greeks already proved the earth to be (very nearly) spherical more than two thousand years ago. They didn't need rockets either... although Eratosthenes' experiment for the size of the earth requires a curvature, it doesn't exactly establish it.

All one needs is an old wooden sailing ship. Because water is a liquid it will always seek to be the same average (disregarding waves and tides) height everywhere, so the ocean will be the same height from the earth's centre (if spherical) or base (if flat.) Send a ship off in any direction from any point on Earth and it will slowly descend as it approaches the horizon and its mast will always disappear below the horizon at the same distance, to one part in three hundred. A smaller stationary boat may be required if the observer is not on shore, of course, but it is not essential.
So, this establishes that the Earth's surface has the same non-zero curvature everywhere (again to one part in three hundred for the sticklers) and is convex. There's only one shape that fulfills this.

As a bonus: the moon's eclipses are the Earth's shadow on the moon when it is between the moon and sun. The shadow is always the arc of a circle (again, to one part in three hundred) no matter the orientation of the earth. If the earth was anything but a very slightly ellipsoid sphere, the shadow would sometimes be a line, or an ellipse, or a lenticular shape, etc.

I'm sure the True Believers have plenty of counterarguments for this: the atmosphere refracts at distance faking curvature, Earth is always in the same orientation, the Cosmic Space Unicorn is pushing the ships down to uphold the Conspheracy... but unless any of these are proven first, they aren't allowed as rebuttals, sorry. :^p

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As a bonus: the moon's phases are the Earth's shadow on the moon when it is between the moon and sun. The shadow is always the arc of a circle (again, to one part in three hundred) no matter the orientation of the earth. If the earth was anything but a very slightly ellipsoid sphere, the shadow would sometimes be a line, or an ellipse, or a lenticular shape, etc.

Phase? NO. Eclipse, yes.

The phases are the moon's shadow on itself.

Thanks for catching that... hey it's a distracted Monday. :^) I edited it.

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