Is there a way to measure the size of a distant object, like a tree, a geological formation or a construction?  Can we use the Moon as a measurement unit? 
The answers: Yes and yes! The trick is to take a picture of the Moon and another one of our object at the same focal length or combine them in one shot. Then, you measure the size of the object compared with the Moon's diameter in our photo. There are a number of ways to do that. I have used a desktop screen ruler. What we also need is the distance between the photographer and the object. Google Earth is the tool for the job. Finally, a simple trigonometry formula does the rest. In this photo we will find the diameter of the observatory dome next to the Moon.

What we know:

1) The distance between the photographer and the dome is 1440 meters. See attached photo here.

2) The dome's angular size based on the photo measurements.
is approximately 0.644 degrees. See attached photo here. 

3) Moon's angular size at November 12. 2008: 0.55 degrees

4) The tangent of 1 degree (from the old days at school...) is 0.017455
The tangent of 0.6448275862 degrees = 0.017455 * 0.6448275862 = 0.0112554655

Now we can calculate the dome's diameter:

Dome angular size = Dome actual size / photographer to dome distance -->
0.0112554655 = Dome actual size / 1440 -->
Dome actual size = 1440 * 0.0112554655 = 16.2

We found that the dome's diameter is approximately 16.2 meters (or almost 53')

Have fun measuring!