The Earth's core has been acting weird for at least 70 years now. We're not sure why, but sometimes it's spinning faster than the surface, sometimes slower. Emily explains how we know what's happening down in the core, which is impressive enough ... but then she shows how astronomers do the same for very distant stars, and Chris's head explodes. Plus, it's been a big week for aurora fans, and there's even some bonus Einteinian weirdness at the end.Syzygy Merch! Get it at the store.Help us make Syzygy even better! Tell your friends and give us a review, or show your support on Patreon: patreon.com/syzygypodSyzygy is produced by Chris Stewart and co-hosted by Dr Emily Brunsden from the Department of Physics at the University of York.On the web: syzygy.fm | Twitter: @syzygypodThings we talk about in this episode:·      The Earth’s core rotation speed paper·      A good article about it·      Big week for aurora fans·      Changes in the Earth’s magnetic field over time·      Different definitions of a day·      Asteroseismology (SciShow Space video)·      The Doppler Effect for sound and light·      Relativity and GPS (MinutePhysics video)