Astronomy news for the week starting Friday, May 26, 2000.

The next Skylights will appear on Sunday, June 4. We begin this extended week with the Moon just passing its third quarter, on Friday, May 26. The remainder of the week sees the Moon in its waning crescent phase, and a week later, on Friday, June 2, it will pass new.

One of the great events of planetary alignment takes place this week, on Wednesday, May 31, when Jupiter passes Saturn, and the two are in their classic "Grand Conjunction." Jupiter takes 11.86 years to go around the Sun, while more distant and slower Saturn takes 29.42 years, very close to half again as long (which is probably no simple coincidence). Once Jupiter passes Saturn, because of Saturn's motion, Jupiter takes 19.9 years to catch it again, acting like a faster runner lapping a slower one on a giant outdoor track. The last Grand Conjunction occurred some twenty years ago, on July 30, 1981, when the two were beautifully placed in the evening sky. This one, unfortunately, is the reverse, as the two planets are not very well placed in morning twilight. You will have to wait until 2020 to see another. Nevertheless, have a look, making sure you have a good dawn horizon. The event, of course, means nothing physically, only that two planets are in the same direction as seen from Earth. They are at very different distances from us, Jupiter 6 astronomical units away, Saturn 10 AU, 70 percent farther. The conjunction has no effect on anything. The thin crescent Moon will be to the west of them the morning of their conjunction and to the east of them the morning of Thursday, June 1.

In the evening, Mercury is putting on a bit of a show, such as it is for the tiny planet (though it is working as hard as it can). It reaches greatest elongation next week and can be seen as a rather bright "star" low in the northwest not far above the horizon in evening twilight. The Moon passes the little planet the night of Saturday, June 3, and will be below it (and very hard to see) that evening, but well up and to the left of the planet the night of Sunday, June 4.

Compared to Pluto's, Mercury's show is spectacular. The most distant planet, visible only in substantial telescopes, will nevertheless be at its best, reaching opposition to the Sun on Thursday, June 1, within the confines of the constellation Ophiuchus, which is now climbing the eastern sky around real midnight (1 AM daylight time). Though this smallest of planets has been known since Clyde Tombaugh discovered it in 1930, we have only known anything of its nature in the last few years. Pluto is both the "last planet" and the largest body of the Kuiper Belt, a disk filled with small icy bodies that extends far beyond the planetary system. The Belt is the origin of the short-period comets, those that orbit in under 200 years. Pluto, a distinctly evolved body, is apparently an accumulation of them.
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