Joshua chapter 6 verses 1 - 4
shows that God knew the exact diameter of the moon:
"March around the city once
with all the armed men. Do this for six days
." - Joshua 6:3
6 days of marching around the city of Jericho x 360 degrees in a circle = 2160
2160 miles = "the exact diameter of moon" in miles (Technically, the exact diameter of the moon is 2159.14062 miles, which can be rounded off to 2160.)
NOTE: I am not the person who discovered this, it was someone on YouTube. I do not take any credit for discovering this interesting fact.