| Aspect | Axial Parallelism | Surface Parallelism | |--------|------------------|----------------------| | Controlled element | Axis of cylinder | Flat surface | | Tolerance zone shape | Cylinder | Two parallel planes | | Datum | Axis or plane | Plane only |
If you were to draw an arrow through the Earth’s axis and extend it into space, that arrow would always point toward the same spot in the sky (specifically, the North Star, Polaris), regardless of where the Earth is in its yearly journey. axial parallelism
To understand why this matters, let’s imagine a counter-example. What if axial parallelism didn't exist? What if the Earth’s axis wobbled or shifted to face the Sun directly as it orbited? | Aspect | Axial Parallelism | Surface Parallelism