# 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

• Great question, and in this case you can replace the lens concept with a pinhole camera model and obtain the exact same result. As far as I can tell your thinking is correct, but my coffee hasn't kicked in so let's see what others say. Welcome to Astronomy!
– uhoh
Commented Mar 8, 2021 at 1:35
• Okay, you said the moon is 3 840 000 m in diameter, or 3 840 km. It's off by ~400 km. Not bad. Commented Mar 8, 2021 at 3:29