The Star of Bethlehem
Imagine that you are an ancient astrologer, in the evening of late summer of 1BC, and you are watching the bright planet Mars in
the east as it turns retrograde to pass through Pisces. Pisces is regarded as the zodiacal sign where evidence of the messiah will appear. Mars will be at opposition in Pisces in mid-September, and at its brightest (magnitude -2.5). In that dark area of the sky, there are only a small number of stars brighter than sixth magnitude, and as Mars slowly backtracks through it, you notice that
there is one new star, about magnitude 5.7. It is not on your charts, and even more amazingly, it is moving, and also moving west! It continues to move west through the month of November, as Mars returns to prograde and passes by it less than a degree away. The new star continues its westward journey.
No planets are so dim, and it is clearly not a comet, and no other stars move. Your colleagues are convinced it is a sign and decide to follow the star west towards Jerusalem. You arrive in early December, and the star seems to have stopped, and stood still. For two weeks, it stays within an area of the sky of less than one arcminute in radius, after having spent the previous four months moving almost 240 arcminutes--about the width of the full moon every two weeks. After your conversation with the king, you
observe the star that evening. It is nearly directly south, and you follow it to Bethlehem.
In the next month, the star vanishes in the sunlight, and is not found again for centuries. Of course, it wasn't really a star, but a planet: Uranus.