Points of light were good enough for the ancients to name the planets and track their movements in precise detail. Points of light are good enough for astronomers detecting exoplanets today. You'll never convince me it's "not really seeing" something.
But it was bright sunshine over Nebraska at the time. I have a hard time seeing in the daytime a 747 flying at 35,000+ feet if it has no contrail to help point it out. I suspect the Enterprise was higher than that since the interceptor jet was climbing hard to get a good look.
You've just reiterated my point. An observer on the ground at night with their naked eye would be able to see "something" but could they identify it as a starship? I doubt it.
Well, that's my point -- sorry, I should've been clearer. I mean that the ship doesn't need to be invisible to the eye in order to avoid being identified as a starship from the future. It does no harm if it's visible as a point of light in the sky, because nobody will be able to figure out what the point of light is. So whatever shield adjustments they made to throw off radar don't require deflecting visible light as well.
That still sounds like a Prime Directive violation in the making. For instance, a primitive civilization could plausibly take religiously-motivated actions whenever an uncharted astronomical object is noticed, such as sacrificing people to their gods. If such action were taken upon the mere arrival of a starship to survey the planet, even without the natives knowing what the object is that has suddenly appeared in their night sky, then there would be blood on the Federation's hands. The arrival of the starship would have altered the course of their development. It would be better and more in keeping with the principle of non-interference to go completely invisibly.
a unknown satellite suddenly appears during the mid 1960's would quickly have pulled the attention of the military (east and west), and then they can't detect it on radar? but they do have telescope to follow up the naked eye.
From the teaser of "Assignment: Earth" [http://www.chakoteya.net/StarTrek/55.htm]: Captain's log. Using the lightspeed breakaway factor, the Enterprise has moved back through time to the twentieth century. We are now in extended orbit around Earth, using our ship's deflector shields to remain unobserved. Our mission, historical research. We are monitoring Earth communications to find out how our planet survived desperate problems in the year 1968. Not nominally a cloaking device, but good enough for practical invisibility while orbiting the Earth in 1986. Presumably that would cover the visible spectrum as well, wouldn't it, at least for the vantage points of ground observers (in other words, it wouldn't have to cover it in all directions)?
I think this is a key point. Mostly, one just needs to keep sunlight from reflecting off the hull and bounce it away from a planet. Surely, deflectors that can handle photon torpedo blasts and phaser strikes can deal with a little sunshine.
And if deflectors could make the starship invisible to ground-based radar, then that makes the case for having the capability of deflecting visible light only that much stronger. https://en.wikipedia.org/wiki/Project_Space_Track Note that Project Space Track, operational in 1961, also used observations in visible light in addition to radar observations.
Not necessarily - stealth aircraft are designed to not reflect radar waves (or at least minimize radar reflection), but they're clearly visible when you look at 'em.
Deflectors aren't material shields. Technobabble in TOS "Obsession" and in TNG "Deja Q" (sorry, I misremembered the exact details of the TNG reference) suggests that deflectors are related to gravitation and relativistic spacetime curvature. Cf. gravitational lensing, which functions at all wavelengths: https://en.wikipedia.org/wiki/Gravitational_lens On the other hand, in the case of the absorption and scattering properties of the Aztec plating on the refit Enterprise, you might have a point.
About the sunlight: I was actually just thinking about putting the ship in a shadow. As for the makeup of a deflector, there is this from TAS, The Survivor: SPOCK: It is not outside the realm of probability. If he could rearrange his own atoms to become an examination table, one would have to assume he could become a deflector shield.
Yeah, TAS fan that I am, I still had to head-canon it as to something like that. Deflector shield generator, maybe. The Vendorian's change of form effect was similar to a transporter sparkle, IIRC, so that suggests a broad range of capabilities. Topic drift, to be sure....