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Check for the Appulse

February 24, 1999
The planets dance their dance around the Sun, each following the rules of gravity as they see them. From our vantage point, the ellipses of their orbits are seen edge on, so they seem to roughly follow a line drawn across the sky. Since they move at different speeds, the planets pass each other in the sky, one overtaking the next. Because they don't quite orbit the Sun in the same plane, they deviate from a single path in the sky, and sometimes when they pass it's at a relatively large distance, and sometimes it's much closer. When two objects in the sky are as close as they can get, we say they have reached appulse. Venus and Jupiter appulsed on the evening of February 23, 1999, and it was a beautiful thing to see (or so I assume, it was cloudy here, dagnappit). Venus and Jupiter are the third and fourth brightest objects in the sky, and this was a particularly close appulse; they got within a quarter of a degree of each other. For comparison, the full Moon is a half degree across, so the two planets got close indeed. Such an event is very rare.

But not as rare as the ITN news network would have you believe. I happened to catch their report on the next day, February 24, 1999. The reporter started off saying (paraphrased) that ``after 2000 years, the Christmas star had returned''. She then went on to explain rather well what was going on with Venus and Jupiter, mentioning that such an appulse may have been what we now call the Star of Bethlehem. The video showed many dozens of people with binoculars and telescopes looking at the pair (which warmed my heart), and as it happens they scene was shot in Israel. The reporter said that people in Jerusalem and all over Israel were watching the event (as were people all over the world, of course).

So where's the Bad Astronomy in this? The reporter said that this event only happens every 2000 years, and the last time it occurred was in 1 A.D.

This is wrong. Such events as this occur more often than that! It is rare, but not as rare as she would have you believe. The last time such an appulse for Venus and Jupiter was visible this high in the sky was in January of 1702! Actually it happens even more often, but since Venus stays close to the Sun it is usually not very visible. Still, the appulse in 1702 was up high enough for people to see. The next time two bright planets appear so close together will be in 2016. I got this information from the Sky and Telescope website, which, incidentally, says that appulse thought by some to be the Star of Bethlehem was actually around 2 B.C., not 1 A.D.

Of course, sometimes the close encounters are closer than this one. In the 1970s I watched the crescent Moon pass directly in front of Venus. One second Venus was there, and blink! The next it was gone. Such an event is called an occultation, and the Moon occults stars and planets often enough that most amateur astronomers have seen one.

Remember: beautiful things happen in the sky all the time. You just need to pay attention to see them.

For more about planetary alignments, you can go to my web page about the non event in May of 2000, or you can read another account by Jan Meeus, the man who pioneered astronomical calculating on personal computers and handheld calculators.

©2008 Phil Plait. All Rights Reserved.

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