Why is knowing the true shape of the Milky Way challenging from Earth?

We are located deep inside the galactic disk, lacking an external vantage point.

Determining the Milky Way's true shape—its spiral structure and dimensions—is fundamentally difficult because our observational platform, the Solar System, is embedded deep within the galaxy's flattened disk. This internal perspective means astronomers cannot step outside to view the galaxy edge-on or face-on like other external galaxies. Looking toward the core results in an overly dense field of light obscured by dust, while looking outward provides a thin view. This situation is often compared to trying to describe the shape of a pancake while standing directly on top of it, meaning that any immediate visual observation is inherently biased by our location, specifically in the Orion Arm, roughly two-thirds of the distance from the center.