Malerapaso/Getty Images
Last Word is 51动漫鈥檚 long-running series in which readers give scientific answers to each other鈥檚 questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
When a bubble, or spirit, level is level, what does that mean relative to Earth?
Michael Paine
Sydney, Australia
The situation is slightly more complicated than simply referring to the centre of gravity of Earth. That is because our planet has a huge satellite, the moon, and the two have a common centre of gravity about 4600 kilometres…
