Daniel Pope

The night is come, but not too soon;

And sinking silently, All silently, the little moon

Drops down behind the sky.

There is no light in earth or heaven

But the cold light of stars;

And the first watch of night is given

To the red planet Mars.

- from “The Light of Stars” by Henry Wadsworth Longfellow

Mars is rising in the northeast not long after our Sun sets in the southwest.  Mars is in our sky all night long and sets in the northwest as our Sun rises in the southeast.  When our Moon does this we call it a full Moon. 

 How have the astronomers explained or accounted for the apparent retrograde motion by Mars that we have been observing?

Ptolemy (85-165) created a model of our solar system that consisted of multiple circles of various sizes with offset centers in motion about an Earth that did not move.  While the model did demonstrate the apparent retrograde motion of the planets it was cumbersome and complicated.  There were those who thought the actual movements of the planets could be explained it a more straightforward manner. 

Nicolaus Copernicus (1473-1543) published his model in 1543 and it was the first to challenge Ptolemy’s model in nearly 1,500 years.  Copernicus’ model had all the planets orbiting the Sun rather than the Earth.  It also had Earth exhibiting two motions: 1) rotating on its North-South axis daily, and 2) orbiting the Sun yearly. He assumed, as had all those before him, that the orbits were circles.  He chose the center of Earth’s orbit as the center for all the orbits rather than the Sun. His model explained the apparent retrograde motion more easily and more directly than Ptolemy’s.  By following the numbers in the graphic you can visualize what happens as Earth catches up to and passes Mars.  We will use the fixed background stars in Leo and Cancer to judge which way Mars appears to be moving.  As Earth gets close to Mars it will appear that Mars slows down, stops, and then starts to move backward.  Visualize running a race on a track with another person.  If you are running faster on an inside lane and about to pass the person, then to you the other person would first appear to be slowing down and then moving backward at the moment you pass them.  The speed and distance of each planet varies and by using circles and the center of Earth’s orbit Copernicus’ original model was not accurate in predicting the future positions of the planets.  To make accurate predictions he had to re-introduce one of Ptolemy’s constructs – the epicycle – which was not very satisfying.

Johannes Kepler (1571-1630) believed that Copernicus was correct about the two motions of the Earth and that all the planets orbited the Sun but he thought the Sun would be at the center.  Fortunately Kepler had access to data from the observations made by the most accurate naked eye astronomer in history, Tycho Brahe (1546-1601).  Using Tycho’s data Kepler worked on figuring out the orbit of Mars but like all those before him, assumed it would be a circle.  Kepler calculated and recalculated using different places for the center.  He came close to getting the observed data to fit but not quite.  He calculated where Mars should be at a particular time but found that Tycho’s observation showed it in a different position by just 8 minutes of arc (about the thickness of a penny held at arm’s length and looking at the edge). Kepler wrote, “The matter is obviously this: the planetary orbit is no circle.” Kepler had made the monumental leap from following the philosophy to following the data.  Since antiquity the philosophers had said the circle was the basis for astronomy.  Using Tycho’s data Kepler had shown this to be incorrect.  Perhaps we could say it was at this point that modern scientific research began.  Kepler had to do many more calculations but arrived at showing the orbits for the planets were ellipses with the Sun at one focal point.

On solving the Mars orbit problem, Kepler wrote:  “It was as if I awoke from sleep and saw a new light.”  “I write my book to be read, either by present-day or by future readers – what does it matter?  It may wait a hundred years for its reader, since God himself has been waiting 6,000 years for one who penetrated his work.”

Next week we will use Mars to help us locate perhaps the smallest and dimmest Zodiac constellation, Cancer.

Clear skies.