What did God use to guide the Israelites by day?

Answer: A pillar of cloud

By day the LORD went ahead of them in a pillar of cloud, and by night in a pillar of fire to give them light.

“And the LORD went before them in a pillar of cloud to guide their way by day, and in a pillar of fire to give them light by night, so that they could travel by day or night.”— Exodus 13:21 (BSB)