For thousands of years, people observed that some of the stars in the sky moved relative to the others. The Greeks called them “planetes” (plan EE teez), meaning “wanderers,” from which we get our word “planets.” Over time, people learned how to predict those motions.The Earth does not feel to us like it is moving, so it was only natural for the earliest astronomers to assume that the Earth was the motionless centre of all things around which the planets (and Sun and Moon) revolve. They could even make a model, real or in their imagination, that would allow them to predict future planet motions.The problem was that the observed movements of the Sun, Moon, and planets were difficult to model with a system that had the Earth at the centre. Many people thought about it and came up with different ideas for how the system had to operate. Around 1514, a Polish astronomer we know today as “Copernicus” figured out a simpler model that did a much better job of predicting the motion of objects in the sky. The problem with Copernicus’ model was that the Sun, not the Earth, stood at the centre, and that was a very unsettling thought to people at that time.Once the telescope was invented, Galileo and other astronomers turned to the skies and saw that many things they thought ought to be true were not true. For one thing, Galileo saw that Venus had phases like the Moon, proving Copernicus right about the Sun being the centre of the solar system.On May 26, you will be able to see Mercury, Venus, and Jupiter all at once in the western sky soon after the Sun goes down. It just so happens that these three planets, plus Earth will be in nearly a straight line, with the Sun just a bit to the side. So when the Sun goes down, the three planets will soon come into view for a short time before they also set in the west. In the next few evenings, you will see the planets move around and swap positions as they each continue on their paths around the Sun. It will be very obvious why they were called “wanderers.”Play “Solar System Switch-a-Roo” and get better acquainted with the planets and Moons of the solar system. Go to http://spaceplace.nasa.gov/switch-a-roo.This article was written by Steve Williams and Diane K. Fisher. It is provided through the courtesy of the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.