<p>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.</p>

CC BY-SA 4.0
2021-03-09T15:24:03.863Z
2021-03-09T15:24:03.863Z
stretch
<p>I'm reading a chapter on convex lenses and it has this diagram:
<a href="https://i.sstatic.net/m2kMM.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/m2kMM.png" alt="enter image description here" /></a></p>
<p>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</p>
<ul>
<li>if I look at the moon in the telescope that is 1 m long,</li>
<li>and the moon appears to be (I will make up a number) 1 cm in diameter,</li>
<li>knowing that the distance between moon and Earth is 384,000 km,</li>
</ul>
<p>I can conclude that the moon is 3,840,000 m in diameter?</p>
<p>This is coming from <code>384,000 km / 1 m = X / 1 cm</code> or, when converted to meters, <code>384,000,000 / 1 = X / 0.01</code></p>

CC BY-SA 4.0
2021-03-08T01:35:27.343Z
Great question, and in this case you can replace the lens concept with a [pinhole camera model](https://en.wikipedia.org/wiki/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
CC BY-SA 4.0
2021-03-08T03:29:55.157Z
Okay, you said the moon is 3 840 000 m in diameter, or 3 840 km. It's off by ~400 km. Not bad.
slowerthanstopped
<telescope><telescope-lens>
Diameter of the moon from the diameter seen in the telescope
user3349993
# Diameter of the moon from the diameter seen in the telescope

(Asked by: username_0 on 2021-03-08)

I'm reading a chapter on convex lenses and it has this diagram:
[![enter image description here](https://i.sstatic.net/m2kMM.png)](https://i.sstatic.net/m2kMM.png)

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`

**Tags:** telescope, telescope-lens
> **Comments:**
> **username_1**: Great question, and in this case you can replace the lens concept with a [pinhole camera model](https://en.wikipedia.org/wiki/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!*  
> **username_2**: Okay, you said the moon is 3 840 000 m in diameter, or 3 840 km. It's off by ~400 km. Not bad.  

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## 1 answers

### Answer by username_3 on 2021-03-09. Score: 1

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.


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