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Movement of the Moon and Planets - Part II

  • Jay Ryan
  • 2004 23 Jun
Movement of the Moon and Planets - Part II

As the Moon cycles through its phases, it passes by the stars, which are fixed objects in the celestial sky. The Moon appears to move among the constellations, since the Moon is in the same general direction as the stars, which in the background at a nearly infinite distance away.

In addition to the stars, the night sky also includes the planets, celestial objects that move in their own paths through the constellations, but much more slowly than the Moon moves around the Earth. Thus, as the Moon moves in its orbit throughout the month, it passes the same line-of-sight as the planets. The Moon thus appears to draw near the planets. These near-approaches of the Moon to the planets are called "conjunctions" because the Moon and the planets appear "conjoined" in the sky. And since the planets are very bright objects in the sky, conjunctions are always interesting sights.

Many people are unaware than the planets are the brightest "stars" in the sky. Though each planet can vary somewhat in brightness, most planets are often much brighter than even the brightest star. For example, the planet Jupiter is always twice as bright as the brightest star. The planet Venus is a whopping 15 times brighter than the brightest star. If you've ever looked at the night sky, odds are than you have looked right at one of the planets, but didn't know what it was. But if you were to learn the constellations and watch the planets over time, you could easily notice them moving around as compared with the background stars.

So each month, the Moon draws near to a planet over a period of nights, and is in conjunction with the planet on the night when it makes its closet approach. In the nights after, the Moon draws away from the planet. And the times of these conjunctions depend on where the Earth and the various planets are in their own orbits on any given night. In any event, it is always a beautiful sight to see a bright, dazzling planet in the same area with the Moon. Sometimes these conjunctions can be very close and other times the Moon may even cover the planet for a few hours. But that's another story.

You might wonder why the Moon always passes the general direction of the planets. In our public school lessons, we've all learned the astro-fact that all the planets move around orbits that are centered on the Sun. We've all seen pictures in textbooks of the solar system, with the orbits of the planets all lying in generally the same flat plane circling the Sun.

But the Earth is also within the plane of the solar system, so we see the entire solar system "edge on." For this reason, the Sun, Moon and the planets appear to lie along a straight-line path through the stars. And the plane of the solar system happens to coincide with a band of well-known constellations, through which the Sun, Moon and all the planets appear to move over a period of time. These constellations are collectively known as "The Zodiac."

Many Christians are troubled by the word "zodiac" since it is commonly associated with the superstition of astrology. However, this term rightly belongs to the legitimate study of astronomy. The term "zodiac" is derived from the Greek work "zoe" which means "life." The word "zoe" turns up many times in the Greek New Testament, such as this famous verse:

"For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish but have everlasting life." - John 3:16

Anyway, the zodiac is simply a "zoo" of twelve constellations, since nearly all the figures are of animals and people. And some of the zodiac constellations, such as "The Bull," "The Lion," and "The Scorpion," bear a remarkable resemblance to the animals after which they are named. Others... well, like many of the constellations, you might need a lot of imagination to see the intended figure! In one interesting twist, the star pattern of the ancient constellation "Sagittarius the Archer" bears a remarkable resemblance to a modern teapot! This pattern is one of my favorite sights in Summer evenings when it is best visible.

In any event, God nevertheless made the stars. And as we look at His celestial creation, we can agree that, "The heavens declare the glory of God." (Psalm 19:1) So as we look to sky we need not be troubled by the false superstitious relics of heathen antiquity that sadly persist into the world of today. But the traditional zodiac stars can provide a convenient "road map" to understanding the motions of the Sun, Moon and planets.

In the late Winter and early Spring for the next several years, the bright planets Jupiter and Saturn can be seen in the evening sky. When the Moon is in conjunction with these planets, we can get a sense of looking out into space, over the edge of the solar system. Also, such conjunctions can be a useful tool for helping us to spot the planets and learn the constellations of the zodiac. And learning the zodiac is an important step towards understanding and appreciating "the clockwork of the heavens," the remarkable celestial order that the LORD has ordained.

Anyway, these are things I never learned in school. And yet, these are things which had been understood by simple folk and almanack readers for centuries prior to our "modern times." And the celestial creation can still be learned and observed today, and taught to our homeschooled children.


Jay Ryan is the author of "The Classical Astronomy Update," a free e-mail newsletter for helping Christian homeschool families learn more about events in the starry sky. If you would like to receive the Update, please drop Jay an e-mail at  Visit the Classical Astronomy web site –