Posted by: T. Boyd | April 2, 2009

The Dance of the Planets

Jupiter, Venus, and the Moon over Oahus Koolau mountains.

Jupiter, Venus, and the Moon over Oahu's Ko'olau mountains.

Let the sea roar, and all that fills it;
the world and those who dwell in it!
Let the rivers clap their hands;
Let the hills sing for joy together…
(Ps. 98:7-8 ESV)

Our solar system moves with wondrous complexity. All of the planets revolve around the Sun at different rates, all of the bodies rotate on their axes, the moons revolve around their respective planets. Some call this motion the dance of the planets.

But what is in our solar system? The sun (named Sol), of course, the 8 or 9 planets (depending on what you call Pluto), the many moons of the planets, the asteroids, the meteoroids, the periodic comets, an abundance of debris and dust – mostly natural, but some man-made – all of these are captivated by Sol’s gravitational pull.

An aside: my granddaughter’s third grade class voted to reinstate Pluto to the planet status. Pluto’s demotion to a dwarf planet due to its small size ( done by an international committee of astronomers) apparently has not gone down well with some members of the elementary school set. I think I agree, even though, I have never been able to find it in the sky. I will have to get back into that endeavor … maybe after getting my garden started for the spring.

Here is an amazing fact: with only a very few exceptions, the orbital motions – the revolutions of the planets and moons around their “parents”, and the rotations of these bodies on their axes – are all in a counter-clockwise direction as seen from the north star. In other words, if you could fly out to Polaris (our north star) and look “down” on the solar system, you would see practically all of the rotational motion moving in the same counter-clockwise direction, rather like a ballroom full of dancers all twirling in the same direction.

This orderly motion makes it possible to predict where everything will be in the future unless some outside object intrudes into the system. What are some of the easier predictions?

The most obvious motion is the daily rotation of the earth on its axis, which makes the sun and moon move across the sky from east to west, but likewise the stars and planets move at about the same rate across our sky, also from east to west. But because we are also traveling around the sun once per year, the stars and planets rise (and set) about 4 minutes earlier each day. It is this motion that causes different stars to appear overhead at different times in the year.

At the same time, the moon is revolving around the earth at a rate of about 28 days, so that it rises and sets about an hour later each day. An intricate addition to this orbital dance is the motion of the planets, each of which gradually moves among the stars westerly across our sky. It takes, for example, Jupiter about 12 years to orbit the sun, and since there are 12 constellations around the sky’s equator, it moves about one constellation per year in its orbit.

At present Jupiter is in the constellation Capricorn. Next year at this time, it should be in the constellation Aquarius (I am having a difficult time confirming this away from home at this time, because most of the internet sources talk about astrology instead of astronomy – we need another article to talk about that!)

Anyway, all of this makes it exciting to look up after the weeks of cloudiness and rain with which we have been blessed in central Virginia and see an unexpected bright light among the “fixed” stars, and wonder what it is. Is it Jupiter or Mars or Saturn, or maybe something else?

This article supposed to appear in the Caroline Progress on April 2, 2009, and will be copyrighted by the newspaper.


  1. Thanks, Laurel, for the comments. I corrected the posting to call Pluto a dwarf planet instead of an asteroid. I did not really check up on the facts.


  2. Pluto’s controversial demotion by only four percent of the IAU, most of whom are not planetary scientists, has not gone over well with many professional astronomers either. That is why hundreds of professional astronomers rejected it in a petition led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto.

    Pluto is not an asteroid because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity, a state known as hydrostatic equilibrium. This state is characteristic of planets and not of shapeless asteroids. Additionally, the IAU definition makes no sense by saying that dwarf planets are not planets at all and by classifying objects solely by where they are while ignoring what they are.

    Kudos to your granddaughter’s third grade class for having more sense than 424 members of the IAU. These kids are ahead of their time and will be happy to know that there are many scientists and lay people working behind the scenes to overturn Pluto’s demotion.

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