Last Word is New Scientist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?
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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 from Earth鈥檚 true centre. The two bodies rotate around this barycentre, causing centripetal forces that depend on their distance from the common centre of gravity. This is one of the reasons why there are generally two ocean tides per day.
In the same way as water in the sea, the liquid in a spirit level is responding to the combined effects of Earth鈥檚 gravity and the centripetal forces caused by this Earth-and-moon dance. Of course, as with tidal forces, the effects are negligible for everyday purposes.
John Welch
Sevenoaks, Kent, UK
If the gravitational effects of nearby mountains and of underground strata with different densities can be ignored, the spirit level is indicating the plane that is perpendicular to a line radiating from the centre of Earth.
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