Astronomers will celebrate a remarkable event on 11 July. It will be exactly one year since the planet Neptune was discovered. Readers should note a caveat, however. That year is a Neptunian one. The great icy world was first pinpointed 164.79 years ago – on 23 September 1846. And as Neptune takes 164.79 Earthly years to circle the sun, it is only now completing its first full orbit since its detection by humans. Hence those anniversary celebrations.