From Lawrence D
Stephen Battersby reports a new definition of planets that would include our moon (21 November, p 9). I have long thought that Terralune is not a planet-plus-satellite, but a binary planet system. The sun’s pull on the moon exceeds that of Earth on the moon at every point in the latter’s orbit. So it is true to say that the moon orbits the sun, with its orbit being perturbed by the Earth, rather than the other way round. This is not true of any satellites of other planets in our solar system, as far as I’m aware.
Astronomers shy away from this description by pointing out that the centre of mass of the combined Earth-moon system lies within Earth, and therefore Earth dominates the moon somehow. But this has no physical significance. As the moon gets further and further from Earth, that centre of mass will eventually exit Earth’s interior, and there is no special energy barrier to prevent it doing so.
Hamilton, New Zealand
'Oliveiro
