Question

Why do people use Earth’s motions to determine units of time?

Answer 1

Because before we had clocks, everybody's time system was related
to the cycle of light, dark, and the motions of the sun during the day.

Even now that we do have machines to keep time with, our bodies still
operate on the cycle of light, dark, and the motions of the sun during
the day.

Those people who are technically inclined understand that these cycles
are actually caused by motions of the Earth.

Answer 2

• In order to measure any entity a more unique standard is taken, and for that reason a number of steps are taken by a number of professional to find a standard to calculate the whole scale for comparing all the rest of the quantities and different entities around the space and on the different celestial bodies.

• So, the planet Earth is all analyzed and then a valid scale is generated, so, that there is a standard for comparing the different entities inside the universe because we take our self as the point of reference.