Which tool should be used to measure the mass of an object?

a balance

Explanation:

For the majority of everyday objects, a balance is used to determine an object's mass. The balance compares an object with a known mass to the object in question. Different types of balances include digital scientific balances and beam balances, such as a triple beam balance.

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.