Which value is used for Earth’s radius R in the EF calculation example?

Which value is used for Earth’s radius R in the EF calculation example?

• A. 20,906,000'
• B. 20,900,000'
• C. 21,000,000'
• D. 20,910,000'

Answer: (A)

When performing EF calculations, you relate angular measurements to surface distances by using a single, representative Earth radius. The standard choice is Earth's mean radius, expressed in the same units as your distances. The mean radius is about 6,371 kilometers, which converts to roughly 20,900,000 feet. In the example, 20,906,000 feet is used because it’s a precise, commonly rounded value of the mean radius in feet that matches the problem’s unit system. Using this mean radius keeps curvature corrections consistent and avoids biases that would come from using the equatorial or polar radii. So the value used is the Earth's mean radius in feet, about 20,906,000 feet.

When performing EF calculations, you relate angular measurements to surface distances by using a single, representative Earth radius. The standard choice is Earth's mean radius, expressed in the same units as your distances. The mean radius is about 6,371 kilometers, which converts to roughly 20,900,000 feet. In the example, 20,906,000 feet is used because it’s a precise, commonly rounded value of the mean radius in feet that matches the problem’s unit system. Using this mean radius keeps curvature corrections consistent and avoids biases that would come from using the equatorial or polar radii. So the value used is the Earth's mean radius in feet, about 20,906,000 feet.