If Christmas this year offers a midnight clear, stargazers could catch a view of a gorgeous close pass of the planet Jupiter and the glowing moon. Here are some commonly asked questions about the Christmas night sky:
What's that bright star beside the moon?
Astronomers often get asked that question when the moon passes close to a bright star or planet. On Christmas night, Dec. 25, the answer will be: the planet Jupiter.
Astronomers call this a conjunction. With the moon rolling through all of the zodiac constellations every month, it gets "up close" with all of the planets and several bright stars at various times, though few conjunctions are as close and bright as this one will be.
Don't wait until Christmas night to look for this. If you look the night before, on Christmas Eve (Dec. 24), Jupiter will be in almost the same place, but the moon will be well off toward the west.
If you look the night after, Dec. 26, the moon will be much closer to the horizon than on Dec. 25. This is an excellent demonstration of how far the moon moves in just one night. Without a bright star or planet as a reference point, most people are totally unaware of the moon's rapid movement. [Space Christmas: Festive Photos of Cosmic Beauty]
Is there always a star next to the moon?
No, but many people make this incorrect observation, which comes about for two different reasons. First of all, it's physically impossible.
As we just saw, the moon is never in the same place two nights running. However the moon is close to bright stars and planets fairly often, and people see and remember that. People tend to forget about the nights when there's nothing close to the moon, and remember only nights when there was something bright near the moon, so selective memory is also involved.
There is currently an interesting situation that could cause some people to conclude that there is always a bright planet in the east.
Look to the east just after sunset, and you'll see Jupiter there. Twelve hours later, just before sunrise, look to the east again and you'll still see a bright planet, but in this case, it's Venus.
If you forget about the fact that Earth has completed half a rotation in that time, bringing a whole new "sky" into view, you'd conclude, incorrectly, that there is always a bright planet in the east.
Is that the Star of Bethlehem?
At this time of year, many people are interested in the Christmas story, and astronomers are often asked about the Star of Bethlehem mentioned in the Bible.
Most modern biblical scholars believe that the Star of Bethlehem was added to the Bible a couple of centuries after the events took place. But there remains a lot of interest in what the wise men might have seen in the sky.
First of all, it was not a nova or a supernova. We know this because astronomers in China were keeping close tabs on the sky, and noted many novas and supernovas, but none around the time of Christ's birth.
An unusual conjunction, like the one we will see on Christmas night, is a much more likely candidate. The "wise men" were probably astrologers, and kept close watch on conjunctions between the moon and stars and planets. There were several striking conjunctions around that time which might have led the wise men to conclude something important was happening. A close conjunction in the east would have even led them in the right direction.
So, enjoy this pretty sight for itself on Christmas night, but also give a thought to what a similar sight might have meant 2,000 years ago.
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