The observation that allowed people to infer the connection between the moon and tides was the regular, predictable relationship between the phases of the moon and the height of the tides.

Here's a breakdown:

* Observing the moon: People noticed the moon's changing phases, from a crescent to a full moon and back again.

* Observing the tides: They also observed that the ocean levels rose and fell in a regular pattern, with higher tides occurring roughly twice a day.

* Connecting the dots: Over time, people started to realize that the highest tides (high tides) often coincided with the full and new moon phases, while lower tides (low tides) occurred near the quarter moon phases. This correlation suggested a connection between the moon's position in the sky and the height of the tides.

This observation, made over centuries, laid the foundation for understanding the gravitational pull of the moon as the primary force driving tides.