Virgin Galactic will take passengers aboard SpaceShipTwo as high as 65 miles above the surface of the earth. But from this altitude, passengers will only be able to see a certain segment of the curvature of the earth through windows as large as 17 inches in diameter.
How much further into space would SpaceShipTwo have to travel to give passengers a view of the entire sphere of earth through one of these windows?
Answer
Not completely clear what you are really asking with your question - it is obviously not possible to see the entire surface of the earth as more than half of it will be on the other side. However, if you have a 17 inch diameter window, you are half way to defining a view port - and it seems to me that you are asking how far away the space ship has to be so that the earth fits inside the view port. For this we need to make an assumption about the distance of the observer to the view port.
Diagram:
For a given height above the surface of the earth we can compute the angle $\theta$ from
$$\theta = \sin^{-1}\frac{R}{R+h}$$
And in order to see all that earth through a view port of diameter $d$, you need to be closer than distance $L$ from the outer part of the view port, so
$$L < \frac{d}{2\tan\theta}$$
It should be clear from this diagram that you never even see half the earth - and that being close to the view port is essential to see enough. If the need is to see all of earth within a 135° view angle ($2\theta$ in the diagram) then we can obtain $h$ from
$$(R+h) \sin(\frac{135}{2}) = R\\ h = R(\frac{1}{\sin 67.5} - 1) = 0.082 R = 525 km$$
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