Treebeard's Stumper 19 April 2002

Line Dance of the Planets

Over the next few weeks, we can witness a rare and beautiful dance of the planets. All five visible planets will be in the sky at once -- Jupiter, Saturn, Mars, Venus, and Mercury. Look west after sunset, and keep looking night after night because the planets will change positions in an intricate line dance. These five planets were named in ancient times, but how did those ancient astronomers realize that planets are different from the other stars? It's unusual that all these planets are in the sky at once, but it's not surprising that they are all in a line. Why?

These handsome horizon charts illustrate some of the moves in this line dance of the planets. They are from Alan M. MacRobert's article on A Rare Dance of Planets at the Sky And Telescope web site. Click on any image to go to the original which gives a detailed week by week description.

Star charts adapted from Alan M. MacRobert's A Rare Dance of Planets at Sky And Telescope. You can create your own custom maps using the nifty Sky And Telescope Interactive Sky Chart.

Note the different motions in this line dance. Jupiter and Saturn are moving down from the upper left. Brilliant Venus is rising at the same time, and finally overtakes Jupiter on June 3. Mercury rises and sinks. Mars and Saturn swap positions, and then they both sink. It's not quite an "allignment," though all five visible planets (plus the crescent Moon) will be clustered within 33° in sky on the evening of May 14th.

Here are a few more related planetary stumpers:

• The planets all orbit in the same direction around the Sun, so why is their dance so complicated with planets moving both up and down in the sky?
• What would this planet dance look like from each of the other visible planets? Would the Earth be part of the line?
• I've read that such gatherings of the visible planets happen every 20 (or 40?) years or so, though the planets may be hidden in the sun's glare. Where does that number come from?

Answer

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