Treebeard's Stumper Answer 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?

Our word "planet" comes from the ancient Greek word for a wanderer. Everything in the sky revolves by day and year, but the stars keep their positions with each other. The ancients noticed that the planets (they included the sun and moon) wander across the background stars. It's not surprising that all the visible planets are arranged in a line, because the solar system is flat. Planets can be anywhere in their orbits, but we see them in the flat plane of the solar system, along the slice of the sky called the ecliptic and the constellations of the zodiac.

Notes:

I fell behind on my stumpers this week because of my Dunn Middle School Science Fair. (My project was about Secret Santa.) Actually I stole one of my own (ancient) stumpers about Pretty Planets all in a Row (05 Dec 97). The real stumper for me has been finding time to get outside to see all these planets. I live in a canyon with no view to the west, and I spend more time staring at my computer than the sky. Graybear came through with all the right answers!

• These five planets were named in ancient times, but how did those ancient astronomers realize that planets are different from the other stars?

  "Planet" comes from the Greek word for wanderer, as the ancients saw them wander across the sky compared to the stars. Early astronomers could also tell that they moved in a pattern.

• 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?

  Because they all orbit in generally the same plane around the sun, so the sun, planets, and moon appear to travel across the sky along a line we call the ecliptic. When they appear this close in the sky, we can see that they are not in exactly the same plane.

• 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?

  The superior planets (Mars, Jupiter, Saturn, etc.) moving the 'wrong way' is called retrograde, and can best be described by passing a truck on the highway. As you are catching up to it, it is obviously moving forward compared to the background scenery. As you overtake it, it appears to move backward, compared to the background, then forward again after you have passed it. The inferior planets (Venus and Mercury), can only get so far from the sun, as viewed from Earth.

• What would this planet dance look like from each of the other visible planets? Would the Earth be part of the line?

  I found a great website: http://liftoff.msfc.nasa.gov/academy/space/solarsystem/solarsystemjava.html. It has a 3D interative JAVA showing the positions of the planets and more. I don't know if each of the six planets involved are visible to the other five, but if they are, I deduce the following: Mercury would see Jupiter, Venus, Mars and Saturn at midnight, but not Earth. Venus would see Jupiter, Mars, and Saturn opposite the Sun, or Mercury and Earth on the other side. Mars is similar, it would see Jupiter and Saturn at night, and Mercury, Venus and Earth in the early morning.Jupiter would see Saturn at night, and maybe Earth in the early morning, but the others would probably be in front of the sun too much to be visible. Saturn has the only chance to see a similar show right now. In the early morning hours, it should see Mercury, Venus, and Mars almost juxtaposed near the horizon, with Earth nearby, and then Jupiter.

• 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?

  Jupiter transits Saturn every 20 (approximately) earth years, so they appear close in the sky from the inner planets. Once the two of them are in the same neighborhood, we wait until Mars, Venus, and Mercury are all on the same side of the Sun. You can find the number of years between transits if you know each planet's orbital period. Jupiter orbits the sun every 11.862615 earth years, while Saturn orbits every 29.47498 years. The formula is: 29.47498 x 11.862615 -------------------- = 19.852549 years 29.47498 - 11.862615

Thanks Graybear, that really did help.

For more information about who's who in this spring dance of the planets, check Sky & Telescope, NASA, and Abrams Planetarium (PDF). You can download fine star charts from Skymaps.com. The universe awaits if you Search the Web.

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