Inner and Outer are QuickTime movies, about 400 KB in size each, of the retrograde motion of the inner (Mercury and Venus) or outer planets (all others beyond the Earth) against the background stars, as seen from the Earth.   The green blob and circle represent the Earth and its orbit around the Sun (the yellow blob in the center), whereas the blue ones represent either the inner or the outer planet, and its orbit. The outermost circle represents the celestial sphere, against which the motion of everything in the sky is observed.   Technically, the celestial sphere is supposed to be centered on the Earth, not on the Sun, but it's so large compared to the Earth's orbit that this small discrepancy is of no consequence. The blue blob on that outermost circle is the "image" of the planet as seen from the Earth.   Note that the line joining the planet, the Earth, and the Sun turns red when those three celestial bodies are aligned (that is, during conjunctions and oppositions). The Earth-Sun and Earth-planet lines also turn red when they form a 90 degree angle, a situation referred to as a quadrature.   Note also that, most of the time, the motion of the "image" is counterclockwise (which corresponds to eastward motion), but periodically it moves clockwise for a while (westward motion). That periodic and temporary westward motion is what's referred to as retrograde motion.   These animations were generated using Mathematica 3.0 on a Power Computing Power Center Pro 180 Power Macintosh running MacOS 8.1 with 48 MB of RAM, saving the frames as GIF files, then combining them into QuickTime movies using GraphicConverter 3.4.   You need QuickTime software installed in your computer to be able to see these animations. If you're unable to see them, go to Apple's QuickTime download page, download and install the latest version of QuickTime appropriate to your computer (a Macintosh, or a PC running Windows), then try looking at the animations again.         Inner.mov An animation of the apparent motion of an inner planet as it appears seen from the Earth. You may have to click on your browser's reload button if the animation doesn't show up the first time. Click here to go back to the top.         Outer.mov An animation of the apparent motion of an outer planet as it appears seen from the Earth. You may have to click on your browser's reload button if the animation doesn't show up the first time. Click here to go back to the top.         back to the main page       Last modified on October 22, 1998.   © '97-98 Wagner Truppel       Please send comments to wtruppel@uci.edu