Airliners over Arden: Calibration

HOW HIGH THE MOON?

With a camera, you can measure how high the moon is overhead. Eventually, I'll measure the height of airliners as they fly over Arden.

The basic principle is this: if you know how wide something is, it is easy to tell how far away it is using a camera. Here's a photo of the moon. It is 241 pixels in diameter in the original image. Can you tell me how far away it is if you know the moon is 2160 miles wide?

The first step, which I do on this page, is calibrate the camera - in this case, a 200mm lens on a Nikon D-100 digital body. First, on the Arden Green, I set out two stakes 10 feet apart that were 100 feet away. Here's how they look photographed.

The stakes in the resulting image were 2612 pixels apart.

In the second photo, I set the stakes 20 feet apart at 200 feet away. Since the stakes are twice as far apart but are twice as far away, I expect them to be the same number of pixels apart when photographed.

As you can see, the stakes in the resulting image were 2601 pixels apart, very close to the first measurement of 2612 pixels.

This is just common sense that the measurements would be the same. If you double the distance and double the width, the poles will be the same number of pixels apart. If I were to put the stakes 100,000 miles away and 10,000 miles apart, they would still be 2612 pixels apart or so.

Having calibrated my camera, I was ready to test it on the moon. My thinking is this. Here's a photo of the moon. Its diameter measures 241 pixels. We know the moon is 2160 miles (3479 km) in diameter, so we can measure how far away it is.

Click here to see the calculation.

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