Naturally, there's a lot of uncertainty about the accuracy and validity of the Bible's interpretations about what happened over 2,000 years ago, but Thompson discovered a significant astronomical event that might explain what happened.
After scouring through historical records and using computer simulations of the positions of the planets and stars as they would have appeared, he thinks the Star of Bethlehem could be explained by the motion of Jupiter.
Jupiter's Retrograde Motion
Between Sept. 3 B.C. and May 2 B.C. there were three conjunctions (on Sept. 14, 3 B.C., Feb. 17, 2 B.C. and May 8, 2 B.C.) where Jupiter passed close to the star Regulus (the brightest star in the constellation Leo). This rare sequence of events would have looked very strange to those familiar with the night sky.
Thompson found that the gas giant passed Regulus in an easterly motion before appearing to reverse direction, passing the star again in a westerly direction. This change in direction is known as retrograde motion. Due to the near-circular orbits of Earth and Jupiter, as Earth has a faster orbital period than Jupiter, from our point of view we will appear to "overtake" the gas giant. The motion of Jupiter will therefore appear to change direction for several weeks before changing direction again continuing its easterly drift.