Harrison BDP – Kepler’s Gaze [HAWS005]

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Kepler’s laws of planetary motion.


In astronomy, Kepler’s laws of planetary motion are three scientific laws describing the motion of planets around the Sun, published by Johannes Kepler between 1609 and 1619. Wikipedia

The Significance of Kepler’s Laws

Kepler’s laws describe the motion of planets around the Sun.
Kepler knew 6 planets: Earth, Venus, Mercury, Mars, Jupiter and Saturn.

 All these (also the Moon) move in nearly the same flat plane (section #2 in “Stargazers“). The solar system is flat like a pancake! The Earth is on the pancake, too, so we see the entire system edge-on–the entire pancake occupies one line (or maybe a narrow strip) cutting across the sky, known as the ecliptic. Every planet, the Moon and Sun too, move along or near the ecliptic. If you see a bunch of bright stars strung out in a line across the sky–with the line perhaps also including the Moon, (whose orbit is also close to that “pancake”), or the place on the horizon where the Sun had just set–you are probably seeing planets.

 A Polish clergyman named Nicholas Copernicus figured out by 1543 that those motions made sense if planets moved around the Sun, if the Earth was one of them, and if the more distant ones moved more slowly. The Earth then sometimes overtakes the slower planets more distant from the Sun, making their positions among stars move backwards (for a while). The orbits of Venus and Mercury are inside that of Earth, so they are never seen far from the Sun (e.g. at midnight).

 I hope that describing those features–the “pancake” of the ecliptic, the reversed (“retrograde”) motion, Venus always close to the Sun–will help students get a feeling for the appearance of planets in the sky, as bright stars moving along the same track as Sun and Moon.

The 12 constellations along that line are known as the zodiac, a name which should be familiar to those who follow astrology. Venus, the brightest planet, seems to bounce back and forth across the position of the Sun, and so does Mercury–but since it is much closer to the Sun, you may only see it whe it is most distant from the Sun, and then only shortly after sunset or before sunrise.

 Students will probably have heard or read that the pope and church fought the idea of Copernicus, because in one of the psalms (which are really prayer-poems) in the bible says that God “set up the Earth that it will not move” [that was one translation: a more correct one may be “will not collapse”]Galileo, an Italian contemporary of Kepler who supported the ideas of Copernicus, was tried by the church for disobedience and was sentenced to house arrest for the rest of his life.

 It was an age when people often followed ancient authors (like the Greek Aristoteles) rather than check-out with their own eyes what Nature was really doing. When people started getting involved, observing, experimenting and calculating results, that brought the era of the scientific revolution and of technology advances we live in today. Our modern technology is the cooperative result, and Kepler’s laws (together with Galileo’s work, and that of William Gilbert on magnetism) and are important, because they started the scientific revolution.


Author/Editor: Eugenio Zorrilla.

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