# Calculate size of comet

1. Nov 23, 2015

### Boxblax

1. The problem statement, all variables and given/known data
There is a picture, size 735x731px, that show the surface of comet that was taken from a probe distanced 3 kilometers from the comet. If the resolution of the camera that recorded the surface is 3.5 times lower then the resolution of Tycho Brache (that is 1 angular minute, 1/60 of a degree), what is the width of the comet? Use expresion for α (angle), arch (l) and distance (r)

2. Relevant equations
α=l/r
l=α*r

3. The attempt at a solution
l=(3.5/60)*3000meters
l=175meters
To me this seems to small, the comet is like 2,2km wide or more. Also I have no idea in what way to use the pixels to calculate the right size.
Any help would be nice

2. Nov 23, 2015

### Staff: Mentor

Hi Boxblax, Welcome to Physics Forums.

Wouldn't the resolution information imply that each image pixel represents angular size of 3.5 arc minutes? How many pixels across is the picture?

3. Nov 23, 2015

### Boxblax

It is 731px across. Could you please explain more how to solve it

4. Nov 23, 2015

### Staff: Mentor

If each pixel represents an angular size of 3.5 arc minutes, the what linear size does each pixel represent at 3 km distance?

5. Nov 23, 2015

### Boxblax

Yes, that is what i need to find out. I need to find out the real size of comet. How to find out the size of 1 pixel in meters?

6. Nov 23, 2015

### Staff: Mentor

Draw it out. You have an angle of 3.5 arc minutes and a range of 3 km. What's the length of the arc on the end of that radius? Look at your Relevant equations.

7. Nov 23, 2015

### Boxblax

175meters? I dont quite understand what you want to do. Could you solve it?

8. Nov 23, 2015

### Staff: Mentor

Helpers are not allowed to solve homework problems for you. We can only provide hints and suggestions, or point out errors. It's in the forum rules (which you read right? ).

175 meters is not correct. One pixel spans an angle of 3.5 arc minutes. That's 3.5 x (1/60) degrees. With a radius of 3 km, what is the length of the arc that is swept out by that angle? Show your calculations.

9. Nov 23, 2015

### Boxblax

my calculations are in a photo. still doesnt seem right to me, maybe i am wrong

#### Attached Files:

• ###### comet.jpg
File size:
47.1 KB
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10. Nov 23, 2015

### Staff: Mentor

Remember that geometrical calculations involving angles should use radians. Otherwise, you can approximate the figure with a right angle triangle since the radius is so large, and use the tangent function.