Between 1908 and 1912, Henrietta Swan Leavitt published a simple idea which settled the debate about the scale of the visible universe. The intrinsic luminosity of a variable star - a star which waxes and wanes - is related to the period of its fluctuation; which is to say, the interval between its maximum and minimum apparent luminosity tells you its actual temperature. Knowing that then allows you to calculate its distance. This clever, elegant observation allowed astronomers at Harvard to establish that some of the nebulae observable in the night sky were beyond the bounds of our own galaxy, galaxies in their own right. It established beyond doubt that our galaxy is not alone, one among many. Ptolemy finally died. These variable stars are known as Cepheid Standard Candles. * My father bought a copy of Gould and Cornell's 'Poltergeists' within a year or so of its publication in 1979, and I am grateful that he did. Admittedly, I read it far too young, and it terrified