Merriam-Webster defines gnomon as the pin of sundial. A tall building such as the TransAmerica Pyramid can serve this purpose, provided there is a marker of some kind on the ground indicating the shadow measurement at a specific time of day. If the shadow aligns with the marker, the time of day should match the previous time of day when the marker was set. Because the TransAmerica Pyramid is so tall, the behavior of the “instrument” is particularly dynamic compared to an ordinary sundial from the vantage of the observer. All bets are off in foggy weather and during an earthquake.