The moon is the smaller one. Its diameter is 2,159 miles, while the earth's diameter is 7,926 miles. If the earth were hollow, it would take 49 moons to fill it.

It is in some ways, but not in other ways. They both evolve around the same star and look blue from afar. However, Earth is a terrestrial planet and Neptune a Jovian one. Neptune also has more moons and is colder, while years take decades of earth-years.

That sounds like an excellent math experiment - see how many quarters it takes to fill one inch of the can - then multiply out how many inches tall the can is so you can know about how many quarters it will take to fill the whole can!