What was the critical analytical step Galileo took to establish the moons' orbital periods?

Answer

Meticulously recording their positions at the same time every night over several weeks

The critical step involved tracking their ingress, egress, and reappearance over several weeks, meticulously plotting their orbital periods by recording positions at the same time every night.

What was the critical analytical step Galileo took to establish the moons' orbital periods?
discoveryGalileoastronomyJupiterMoon