ILLUSIONS IN THE SKY

         Observing the skies at night could be a way of enjoying one's self. Trying to find the constellations or to simply gaze at the stars can be relaxing. And if you've been observing for days, it would seem that the universe is in order. Then one night, you see a star or two moving differently from the rest of the stars. And you happen to ask yourself, " Are my eyes deceiving me or is it the skies?"

         This motion in the heavens have been observed through time by people all over the world. Some stars would move opposite the rest of the stars then after a short period of time, move again in sync with the rest of the stars. Ancient astronomers worked hand in hand to put a logical explanation to this startling phenomena. Without telescopes, they looked up at the heavens and they could only see the "stars" in addition to the sun and moon. These "stars" were classified either as fixed or wandering. The fixed stars were those that maintained their positions year after year and were only seen to be in one direction of motion, east to west. The wandering stars were those that at one time or another, would contradict the motion of the fixed stars. The ancient astronomers also observed that the five largest stars in the heavens exhibit this extraordinary motion. These stars were later known as the planets Mercury, Venus, Mars, Jupiter, and Saturn.

         With the help of Ptolemy and Copernicus, modern astronomers have given us a more logical explanation to describe this motion.They called it "Retrograde Motion", to describe the way the five planets moved at certain times. So who are Ptolemy and Copernicus? Both were astronomers in the past who contributed a lot to the field of cosmology. Cladius Ptolemy lived in the 2nd century A.D. He proposed a model that had the earth as the center of the universe and all the heavenly bodies surrounding the earth revolved in a second spherical circle ( orbit ) called an epicycle. Sometime later in the 16th century, Nicolaus Copernicus replaced this model with a newer model which was more concise. The new model had the sun as the center of our solar system and not the earth. The Church did not appreciate Copernicus's action although it provided a simpler and more precise explanation of the planets' motion. They had him publicly executed for having the sun at the center of the solar systen and not the earth which God had created.

         With the newer and more concise models of our solar system, retrograde motion can be explained more clearly to us. This motion is just the seemingly backward ( west to east ) motion of a planet as the earth overtakes it in orbit around the sun. A bit lost now? Let's take it step by step. Since the planets and stars have their own orbits, it is now the size of the orbits that matters. If planet_1 has a smaller orbit than planet_2, what you see from planet_1 at the time that it overtakes planet_2, is planet_2 going on the opposite direction ( west to east ). Still not clear to you? Let's take Mars' retrograde motion for our example.