# How high would a mountain have to be?

I'll keep it nice and simple. On a world that does not rotate is it possible to have a mountain that is high enough that when you sit at the top, you can see the divide between daylight and night that spans the planet? If so, how high would it have to be and could a human survive at the top without a spacesuit?

• If it is at the divide it does not even need be a mountain, so the question is rather: Where do you want your mountain to be? Also: by not rotate, do you mean a) tidally locked to its sun? b) rotating at speed so that it does not seem to rotate when looked at from above (e.g. one day = one year) Oct 3, 2016 at 5:27
• I want the mountain to be at the divide so you can stand on top and look left and see light, then look right and see the dark side of the planet. Yes, tidally locked to the sun, so there is eternal day on one side and eternal night on the other. Oct 3, 2016 at 5:33
• What you're asking is really unclear. If you want to see the divide that spans the entire planet... Well, you can't, 'cause half of it is on the other side. To even see half of it (assuming a perfectly spherical planet), from simple geometry your mountain would have to be infinitely tall. And rotation is irrelevant. Oct 3, 2016 at 5:34
• I interpreted it to mean you can see some point on tbe terminator that are on opposite logitudes. That’s possible if not at the equator. I agree opposite sides is ambiguous so the question is badly worded; but I thought it obvious that anipodes could not be what was meant because that’s impossible. I found an interpretation that is meaningful. (Maybe I gave him too much credit?) Oct 3, 2016 at 5:39