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Tilting the Planets

Doug Hamilton
Department of Astronomy
University of Maryland, College Park


 The 23.5 degree tilt of Earth's spin axis is responsible for the yearly cycle of the seasons: winter to spring to summer to fall. But why is Earth tilted at all, and why do the tilts of planetary spin axes vary so wildly in the Solar System?  Some planets are barely tilted (Mercury, Venus, Jupiter) while others are tipped over on their sides (Uranus). What can the tilts of the planets teach us about the formation of the Solar System, and about the processes that have sculpted it over 4.5 billion years of history? In this talk, I will discuss these issues and will focus on the giant planets Jupiter and Saturn, which have an interesting story to tell.  Solar System formation theories suggest that both planets should form with their rotation axes perpendicular to their orbital planes. Jupiter's 3 degree tilt is low as expected, but Saturn is tipped by almost 27 degrees. Despite a clean getaway billions of years ago, clues left at the scene implicate a surprise culprit - the planet Neptune!

Short Biographical Sketch
Doug Hamilton grew up in Alaska chasing moose and being chased by them. He left the state to attend two of the great southern universities Stanford and Cornell, worked overseas for a few years in Heidelberg Germany, and is now a professor at the University of Maryland.  Doug's scientific research focuses on planetary dynamics and the origin of the Solar System. He has studied the rings of Jupiter, Saturn, Uranus, and Neptune, the orbital histories of satellite systems, and the curious interactions of extra-solar planets. Doug has a strong interest in undergraduate teaching and has led a team of undergraduates in producing an interactive set of online tools called the Astronomy Workshop. Tools in the Astronomy Workshop animate planetary orbits, show what happens when an asteroid strikes the Earth, and generally allow users to explore the Solar System from the comfort of home. The Astronomy Workshop is available to the public at
Admission is free. Please RSVP online to reserve a spot.
Phone Contact: 301-286-2893/9690

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