Just Checking - Calculating Angular Diameter of Moon

1. Mar 11, 2008

TFM

As part of an on-going project, I have to calculate the Angular Diameter of the moon. This is to be done using a digital SLR Camera. I have some pictures, as well as some calibration images (a ruler a fixed distance away). I am pretty sure I need to use the formula:

$$plate scale = \frac{s}{\theta}$$

where s is a fixed measurement on the image, and theta is the angle from the lens to either side of s of the actual object.

Therefore I need to work out the angle to the ruler (see diagram), and use that and the pixel size of the ruler to calculate the Plate Scale. then use plate scale and pixel size of moons diameter to calculate the anbgular diameter.

Does this seem right?

Also, does theta have to be in radians or arcminutes? (I think its arcminutes)

TFM

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Last edited: Mar 11, 2008
2. Mar 11, 2008

Staff: Mentor

I would use arcmin for your final answer. Your approach is correct. To help keep it in your head correctly, just draw yourself a long triangle. It's as simple as solving for the angles in it and you can use the small angle assumption on it (google that if you don't know what it is...).

3. Mar 11, 2008

TFM

I've tried it out and it does work using arcminutes - it gives me an answer of 29.0arcminutes which converts to 0.54 Degrees.

Thanks,

TFM