Pilot's Handbook of Aeronautical Knowledge Practice Test

Station numbering uses a fixed reference point called the datum. This datum serves as the origin for all longitudinal measurements along the aircraft, and moments are calculated from it. Station zero is that datum, so it marks the starting point from which every other station’s distance is measured.

In weight and balance, you multiply each weight by its moment arm from the datum to find the total moment about that reference. The nose, wing root, and tail cone are physical parts of the aircraft, but the reason station zero is identified as the datum is that it provides a consistent, fixed reference for all calculations, regardless of the aircraft’s shape or size.