A light year measure the distance a lightbeam can travel in one year one light year is equivalent to 9500000000000 km why do scientist use light years to measure distances in the universe.

Light years are the only practical unit for measuring the distance of space

Light years tell us about the temperature distance and age of their celestial bodies

Light years are the most accurate form of measurement for universal purposes

Light years measure the age of celestial bodies by determining how bright the light is

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.