# Homework Help: Angular size and telescopes

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1. Mar 24, 2012

### monke

1. The problem statement, all variables and given/known data

a) what is the angular size of the moon as viewed from the earths surface?
b) The objective and eyepiece of a refracting telescope have focal lenghts 80cm and 2.0cm. What is the angular size of the moon as viewed though this telescope?

mean radius of the moon- 1.737x10^6m
mean distance from earth- 3.854x10^8m

2. Relevant equations

Im not sure what equations to use or why the answer from the book is in radians

3. The attempt at a solution

i tried using the equation m=near point/ object distace, with the near point being 25cm and object distace being the distance from the earth. HOwever the anwer from the book is in radians.

2. Mar 25, 2012

### cepheid

Staff Emeritus
Of course the answer is in radians. It's asking you for the angular size. The angular size is defined as the angle subtended (spanned) by the object as seen by the observer. In other words, how big does it look? How much of your field of view does it take up?

What is the equation for an angle in terms of of the radius and the arc length (which you can equate to physical size)?

3. Mar 26, 2012

### monke

would that be theata= h (size of object)/N ( near point =25cm)?

4. Mar 26, 2012

### cepheid

Staff Emeritus
No, I was just talking about the equation that gives you the definition of an angle (in the radian system)

5. Mar 26, 2012

### monke

i think i found it
theata = 2(radius)/disatance from the sun?

6. Mar 26, 2012

### monke

angle in terms of radians is
theta= s/r

7. Mar 26, 2012

### monke

how should i have applied that to this particular question?

8. Mar 26, 2012

### monke

for the second part of the question, if a refracting telescope functions like a compound microscope does that mean that you would use the same angular equation?

M= -L/foX N/Fe

9. Mar 26, 2012

### cepheid

Staff Emeritus
Well, how big is the object? How far away is it? Therefore, what angle does it span? This is its angular (or apparent) size. That will get you the answer to part a.