Earth's equatorial radius, often denoted as "R," is the distance from the center of the Earth to the point on the Earth's surface that lies along the equator. It represents the Earth's average radius when measured from its center to its equatorial surface. The equatorial radius is greater than the polar radius because the Earth is slightly flattened at the poles and bulges at the equator due to its rotation.
The approximate value for Earth's equatorial radius is about 6,378.1 kilometers (or approximately 3,963.2 miles). This value may vary slightly depending on the reference ellipsoid used for modeling the Earth's shape, but the given value is a commonly used and accurate approximation for most purposes.
In contrast to the equatorial radius, Earth's polar radius (measured from the center to a point on the Earth's surface at the North or South Pole) is slightly shorter, approximately 6,357 kilometers (3,949 miles).
A "span" is a traditional unit of length used for measuring cloth or fabric. It was historically used in various countries, including England, and is based on the width of the human hand or the spread of the fingers. The actual length of a span could vary depending on the region and historical context.
In England, a span was typically equal to 9 inches or approximately 22.86 centimeters. However, it's important to note that the definition of a span could vary from place to place and over time.