How far can you see on a perfectly clear day? For instance, do you think you are seeing up to 200 miles out, 300? Maybe less?
Well done.
Squink’s site might not take account of atmospheric refraction. My program gives me just under 245 miles. It is common in determining radar ranges to use an earth with a diameter of 4/3 of the actual diameter to account for refraction and that what the program I used does.
So take your pick since both answers are pretty close, differing by only about 5%.
A little further checking reveals that Squink’s cite does take into account refraction. It’s just that the formula I use apparently has a stronger refraction effect.
The geometric distance without regard to refraction is 212 miles.
So anywhere around 230 to 250 depending on the atmospheric refraction on your particular day ought to come close to covering the answer.
When you are about 30,000 feet in a plane
Maybe more?
93 million miles give or take. 
That first link owns. Thanks.
He didn’t ask for the distance to the horizon – he asked how far can you see, from 30000 ft. No doubt Mt Shasta would be plainly visible on a clear winter day from an airliner 300+ miles to the south – over Monterey, say. For a passenger to get that view out the window you’d have to be on a flight to/from Hawaii, but I bet it’s not a rare sight for pilots on northward flights before they start the descent into the Bay Area.
The OP of this thread was banned 17 years ago. I doubt he is seeing your reply.
While you are “technically correct”, I think we can safely infer that the question from the suspended poster was about something near sea level. Otherwise, I’m pretty sure that the moon, the sun, and at least a handful of the stars are over 300 miles away and we can see them from 30,000 feet just fine.
Therefore the technically correct answer is “as many light-years as the age of the universe.” (Not, technically correctly, “infinity”, because light has finite velocity.)
Certainly, an extremely large high-luminosity zombie would be visible at 19 light-years away.
He could on a Flat Earth.
Until you look at the title to his question.
Nonsense. In the context of the question as asked (and as the OP later more or less confirmed later in the thread before it was revived nearly 2 decades later), the question was indeed about the distance to the horizon
“When you are about 30,000 feet in a plane, How far can you see on a perfectly clear day?”
All agreed he means how far, on Earth. People who misread the question told him about the horizon, and he trusted them, forgetting that nothing prevents seeing past the horizon.
And apparently that included the OP, who thanked posters for the link to a site specifically listing the horizon (it’s even in the site address).
I’m all for being “technically correct”. Congratulations!
But when that includes telling the person who asked the question in the first place that they ‘actually’ meant something other than what they meant, it goes too far.
I don’t recall coming to that agreement. I think we all agree, minus you, that the intended meaning was at the horizon AND on the Earth. If the horizon is not the case, the Earth filter falls away as well for myself. So, aside from you, I believe we’re all in agreement that the answer is either around 230 miles or an extremely large number of light years, with a heavy lean towards the former. I think you’re all alone in the 300+ mile club.
So you figure if you can see Mt Shasta from 300+ miles, that’s not the answer to his question?
Would the answer be then how far away at 30K feet can you see Mt. Everest?
So you figure if you can see the ISS from 1400ish miles, that’s not the answer to his question?
So you figure if you happen to have a pair of binoculars and conditions are excellent enough that you are able to see a satellite in the GEO belt at 22,000+ miles, that’s not the answer to his question?
Sure, you can pick any arbitrary object you want and claim that that is what was meant, but the suspended OP was looking for the answer at the horizon of the Earth, as they implied in post #7. If you want to get “technically correct” and maximize the answer, then the answer is the light years response. You’re welcome to assign values for every mountain, elevation, or even signal tower if you’d like, but I’m guessing that no one else is going to agree that that any of those are the correct answer.