Answer: Since the Earth is a sphere the last thing that was observed was the tallest: the bridge.
Explanation:
As the Earth is a sphere, we are unable to see too far away because light travels in a straight line so cannot travel around the globe. Our vision is therefore restricted to the point where the straight line and the curve of Earth diverge.
The higher the altitude and structure, the longer the divergence will take to happen. This is why the higher you go, the further you will see and the more you will be visible. The San Francisco bridge was the tallest structure and so was the most visible.
