You've probably read about the Great Conjunction of Jupiter and Saturn which is due to happen tomorrow, when the two planets are so close together (at least from the persoective of Earth) that they appear almost as one very bright star in the night sky. You've probably also read that this is almost certainly the origin of the "Christmas Star" mentioned in the Bible, you know, the one that the three Magi followed to the birthplace of Christ.
But I got to thinking, hold on, if this Great Conjunction only happens every 400 years (or every 800 years, depending on what you read), then that would not coincide with the "Year Zero", not even close. So, why is this "fact" being bandied about in the less scientific media?
If you look at a more reliable source (like Space.com, for example), it turns out that there is all sorts of misinformation being propagated about the Great Conjunction.
For one thing, the two planets will actually be about 6 arc minutes apart (about one-fifth of the apparent diameter of the moon), and so they will not actually be close enough to appear as a single star unless you really squint. They will, though, be closer than they have been for nearly 800 years.
Secondly, different reports say this kind of convergence only happens every 400 years (or, according to some sources, every 800 years). The last conjunction of the two planets was indeed on July 16th 1623, about 397 years ago, when they were about 5 arc minutes apart, although even this was only visible to the naked eye from tropical latitudes, where it was further distanced from the glare of the sun. The last time most of the world's population was able to view the conjunction was March 5th 1226, i.e. 794 years ago (which is presumably where the commonly-quoted 800 years metric comes from). This one had the two planets just 2 arc minutes apart, which would in fact have looked like one very bright star to the naked eye.
Previous occurrences would have occurred on July 3rd 769 (457 years before 1226, 4 arc minutes apart, but not easily visible because of the morning sun), December 31st 431 (336 years earlier, or 793 years before the 1226 occurrence, 6 arc minutes apart and easily visible), and March 6th 372 (just 59 years earlier, 2 arc minutes apart and visible). Future predictions of the event indicate that it can be expected in 60 years from now, then after another 337 years, then another 60 years, then 397 years more. So, this is not exactly Old Faithful!
What about the Great Conjunction as the "Christmas Star" that the Magi folowed to find the Christ child in his manger. Well, the best we can do is a rare triple conjunction in the year 7 BC (or 7 BCE, if you prefer). In that year, there was one conjunction on May 29th, another on September 30th, and a third on December 5th (all with varying degrees of proximity). This was certainly a very unusual occurrence, and may well have encouraged some ancient viewers to "follow" it. It was probably Johannes Kepler's calculation of this sequence of events in 1603 that probably started the whole Christmas connection.
At any rate, it would not have occurred close to the anecdotal birth of Christ ("Year Zero"), although it may have lined up quite well with the actual date of Jesus' birth, which most scholars put between 6 BCE and 4 BCE (although who knows how they would have used a star to find a cowshed near Bethlehem?) One last point on that: there actually is no "Year Zero"; the year 1 BCE is followed by the year 1 CE.
So, get out and see the Great Conjunction tomorrow, weather permitting. It's a cool thing. Just don't expect it to look like the Star of Bethlehem on your Christmas card!
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