Discovery - will it transit the Moon?

Posted by Rich Talcott
on Thursday, December 07, 2006

NASA hopes to launch the space shuttle Discovery this evening. If it blasts off on schedule, observers in northeast Florida may witness a rare, perhaps unprecedented event: a shuttle's passage across the face of a nearly Full Moon.

Discovery looks like it will be ready. By yesterday afternoon, engineers had resolved two last-minute technical issues, and NASA cleared the shuttle for flight. Now, all we need is cooperative weather. As of this morning, there appeared to be a 60-percent chance of low clouds, which would scuttle the launch.

Let's be optimistic, however, and look at what will happen if the shuttle goes up on time. Thursday's launch window runs from 9:31 to 9:41 P.M., with the target liftoff time at 9:35:45 P.M. At that time, the 88-percent-lit Moon lies approximately 17° high and 20° north of due east. (It appears in line with Gemini's two brightest stars, Castor and Pollux.) Discovery will be heading northeast on its way toward the International Space Station.

Some lucky people along Florida's coast and a bit inland should see the shuttle cross the Moon as the spacecraft moves almost horizontally from right to left. The problem is, no one knows for sure where the prime spots will be. As Astronomy columnist Jim Oberg told me: "It may be impossible to predict the precise spot in northeast Florida where this orientation will occur." The problem comes in predicting the shuttle's precise altitude and azimuth for each observing location and matching those to the Moon's exact location. And remember, Earth's rotation changes the Moon's altitude and azimuth noticeably in just a minute, so even a minute's delay in the launch would change the Moon's position.

If the shuttle doesn't go up tonight, there won't be another chance. The launch window gets earlier each evening and the Moon rises later, so the Moon will be below the horizon during subsequent launch opportunities. My best advice: Head out tonight and take a look. And if you get lucky, be sure to let us know.

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