Diameter of the moon from the diameter seen in the telescope I'm reading a chapter on convex lenses and it has this diagram:

The chapter explains that triangles ABC and ADE are similar. Thus, ratio of CB to DE is equal to the ratio of BA to AD.
Does this mean that

if I look at the moon in the telescope that is 1 m long,
and the moon appears to be (I will make up a number) 1 cm in diameter,
knowing that the distance between moon and Earth is 384,000 km,

I can conclude that the moon is 3,840,000 m in diameter?
This is coming from 384,000 km / 1 m = X / 1 cm or, when converted to meters, 384,000,000 / 1 = X / 0.01
A telescope wouldn't have a single lens, I'm sure you know. If you were to use the setup in your diagram, focusing the image of the moon on a flat surface, you could find its diameter with the method provided. The distances f in the diagram are the focal length of the lens, where a very distant object will be in sharp focus, and that's where you'd end up placing the flat surface.