10. Solar Eclipses

If you've ever witnessed an eclipse, you understand what a wondrous sight it can be. Now imagine how they appeared to people thousands of years ago, before it was understood exactly what was happening when the moon passes in front of the sun. No wonder eclipses have long been given religious significance.

However, exactly when the first solar eclipse occurred is a matter of dispute -- aligning sometimes vague historical records with the exact date of an eclipse based on what we know of their cycle can be difficult. One Chinese story recounts the tale of Ho and Hsi, astronomers who got drunk and failed to predict an eclipse around 2134 B.C. Other Chinese astrological texts mention eclipses as far back as 2800 B.C. An Irish archaeologist and astronomer named Paul Jordan believes he has found evidence in Neolithic stone carvings of an eclipse occurring even earlier -- 3340 B.C. -- although this hasn't yet been substantiated.

Through repeated observation, ancient astronomers learned how to predict eclipses and drew conclusions about the relationship between the sun, Earth and moon. Historical records of solar eclipses are invaluable. Since we can now predict when they will occur down to the second, historians can use records of past eclipses to date other events. Astronomers can also use them to learn how the Earth has changed over time.