I see a jet's vapor trail at a great distance, say 45 degrees from the horizon.?

Question

How many miles would I have to travel to be directly underneath the jet?

Answer 1

The answer in general is the altitude of the jet divided by the tangent of the angle of elevation (neglecting the curvature of the earth). If the jet is a commercial long-distance flight that has reached cruising altitude, its altitude might be around six miles. The tangent of 45° is 1, so the answer in that case would be six miles. Of course, by the time you got there, the vapor trail would have dissipated and the jet would be long gone.

Answer 2

For all i know you could be directly under the Jet now... all you mentioned was it's vapor trail...