The excellent list based website Listverse has an excellent post up about the Top 15 Common Astronomy Myths.
Here’s the first five:
1. Contrary to popular believe, Venusian is not the term to describe Venus and Venus related things. The correct term is Cytherean, which comes from Cytheria – the small island where Aphrodite emerged from a shell. Furthermore, Venusian is also not correct in that it doesn’t follow the pattern used for other planets – if you aren’t referring to Venus as the Cytherian planet you should call it the Venerean planet.
2. A very popular myth (which has even been repeated as fact by members of the Green party in the New Zealand parliament) states that crimes and accidents increase in number during a full moon. While it is almost impossible to debunk such a myth, there are no statistics relating to the incidence of crimes which supports this wacky theory.
3. Copernicus was not the first person to state that the Earth revolves around the sun. That theory was first taught by unknown ancient thinkers. While we don’t know their names, we do know for certain that, from as early as the seventh century BC, it was suggested in Sanskrit documents.
4. NASA did not spend millions of dollars trying to develop a pen to write in space when they could just have used pencils. First off, they did use pencils (like the cosmonauts) rather than trying to develop a pen, but when a smart man developed (at his own expense) a pressurized pen that not only would work in space but under the ocean as well, NASA purchased 400 of the pens at the cost of $6 per pen (they are now about $50 per pen and you can buy them online here). The Soviets also bought his pens. To this day, both nations still use the Fisher Space Pen (named after its inventor Paul Fisher).
5. I am a little loath to add this one as it has been mentioned on Listverse before, but for the sake of completion here it is: The Great wall of China is not visible from space with the naked eye even in a low Earth orbit. However, many other manmade objects are (such as bridges and dams).