Observer's Notebook for March, 2000

The "Grand Conjunction?"

by John Rummel

The western sky has been putting on a wonderful planetary show this winter and spring. Have you been watching? If you haven't heard yet, you probably will soon, that the spring of 2000 will bring an amazing planetary alignment. Some even suggest that this alignment may be the disaster that Y2K wasn't.

None of this is new. Every few years, some pundit points to a supposed planetary alignment and predicts disaster for the earth due to some gravitation or magnetic effect. This is completely without foundation scientifically, which is why astronomers pay little attention to such claims. Even if all 9 planets were lined up on the same side of the sun, like beads on a string, the combined gravitational effect on earth would be practically zero. Astronomer Chet Raymo, who has done the math, says you'd give the earth a bigger jolt if you tossed a 50 pound bag of sand out of a second story window of your house.

So no doomsday, but some very good doorstep astronomy - if you're so inclined.

First, to clarify things a bit, there is not really a conjunction, but a series of conjunctions, and secondly, there's nothing comically significant about it - conjunctions happen all the time.

A conjunction occurs when two celestial bodies share the same celestial longitude. More interesting to us casual sky-watchers are simple planetary groupings - loosely defined as two or more objects sharing the same patch of sky. The conjunction and the closest approach of the objects might not occur at the same time, so we'll stick to simple planetary groupings for this article.

Since about February 1st, Jupiter, Saturn, and Mars have been in the western sky in the evening. As the months pass, they're slowly moving closer and closer together. All three planets are actually drifting eastward against the starry background, but Jupiter is overtaking Saturn, and Mars is overtaking Jupiter. Jupiter and Saturn, being further away, move more slowly. Mars is the next planet out from earth, so it appears to move much more quickly. The following tables illustrate the motions of the planets relative to each other for the first few months of 2000:

Separations (in degrees)
Jupiter-Saturn Jupiter-Mars
 January 1 15 56
February 1 12 35
March 1 9 18
April 1 6 2

Movement (in degrees and arcminutes)
   January February March April
Jupiter  2 48'  4 47'  6 33'  7 0'
 Saturn  0 19'  1 48'  3 6'  3 40'
 Mars  23 52'  21 59'  23 0'  21 31'

So for instance, on January 1st, Jupiter and Saturn were 15 degrees apart, while Jupiter and Mars were 56 degrees apart (remember, a full circle around the sky and under your feet would be 360 degrees). By March 1st, Jupiter and Saturn have closed the gap to just 9 degrees, while Jupiter and Mars are now only 18 degrees apart. On April 1st, they are even closer, just 6 degrees and 2 degrees apart. The second table shows movement per month for each planet - clearly Mars gets the prize for haste.

The easiest way to mark the progress as these planets slowly close on one another is to let the moon be your guide. On March 8th, the razor thin crescent moon lies only 6 degrees to the left of Mars. On the 9th, it's to the left of and slightly closer to brighter Jupiter. And finally on the 10th, it's above and to the left of Saturn. The moon's crescent will get slightly fatter each night also. If you get clear skies on those nights, you'll be able to positively identify each planet. After that, the moon will continue to move off to the east, but you can now follow the progress of the planets as they slowly continue to move closer to one another.

By April 5th, Jupiter and Mars just 1 degree apart. On April 6th, Jupiter, Saturn, Mars and the crescent moon all in a box just 8 degrees wide. The best time to watch is about 7:30 Madison time. This grouping gets my vote for the best of the spring. See what you think.

April 7-13, all three planets fit into a circle just 5 degrees in diameter, but getting very close to the sun (Jupiter closest at just 17 deg from the sun). By the end of April, they're all getting too close to the sun to be easily visible during dusk.

Culmination of the "grand conjunction" is May 16-17, when Jupiter, Venus, and Saturn are a few degrees west of the sun, and Mars and Mercury are a few degrees east. All six solar system bodies are within about 19 degrees of each other - an unusual occurrence indeed, but virtually unobservable due to the fact that all are so close to the sun. Most of the accounts you will read point to May 5th as the actual "event" but this is only because the moon is then included in the grouping. This makes no difference as to the visual appreciation of the event since it's still almost completely unobservable. However, there is something remarkable about having all seven solar system objects (5 planets plus the sun and moon) in such tight formation. Astrologers will have a field day (for a few hours on May 4, all except Mars are in the constellation Aries - Mars is in Taurus). I prefer to focus on the tightest grouping of the planets themselves, bringing me back to my May 16-17 2000 event.

Early on the morning of May 17th, there will be a super close grouping of Venus and Jupiter. These two brilliant planets will actually come very close to touching each other (through line of sight only, they're obviously millions of miles apart). Sadly though, this event will take place only 6 degrees west of the sun so it will be almost completely unobservable. If you're very lucky that morning, just as the sun comes up, you may see a very bright star low in the east. The planets are less than 1/60th of a degree apart. They'll look like a single object to the unaided eye.

So May 2000 is somewhat anticlimactic since so few people will actually observe it. The real show is in March and April.

Next month, we'll take a look at some of the great planetary groupings of the past... and future.

For further reading, see the following excellent articles on the May 2000 conjunction:

Sky & Telescope (Jean Meeus)

"Bad Astronomy (Phil Plait)

Article by Truman Collins

Article by Brian Monson

Griffith Observatory - John Mosely

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