If Earth is about 4.6 billion years old, how many precessional cycles have occurred? How is this calculated?
Nearly 180 thousand cycles.
Precession cycle period is 260 centuries.
So, the number cycles since the formation of the Earth 4.6 B years ago is 4600000000/26000 = 176923, nearly.
This has to be rounded to two significant digits (sd) as 180 thousands, on par with 2-sd approximation 4.6 B, for the age of the Earth.