Highlights of Astronomy from an Historical Viewpoint


NOTE: The following talk was delivered last week by Ed Dennison, a retired astronomical engineer. Ed and his wife Alice own the cabin on upper Hamlin Lake, in Ludington, Michigan, where Connie and I have been staying for the last month. They invited us to stay here last year as well. Their generosity has been (and continues to be) an enormous blessing to us!

Tonight I am going to talk about one of many astronomical discoveries – the reality that the Earth revolves around the Sun.  Like many astronomical discoveries, the first observations occurred at least 10,000 years ago but the definitive interpretation did not occur until very recently, 1838 to be exact.

Before I continue, I would like to remind you about the standard protocol of science.  New ideas often start with an intuitive bit of insight or hunch.  This initial concept starts in the human brain from one or more previous ideas.  The next stage is to make an observation of a fact.  A fact is statement that is generally accepted by many people who are knowledgeable about the topic.  Following the statement of fact is one or more interpretations.  These interpretations are generally not agreed upon initially.  These different interpretations often exist for many years or centuries.  Interpretations can change as new facts or analytical tools become available.

I point this out to remind you that anyone who says that they know the “Truth” about any subject should be viewed with skepticism.  This statement applies to science, history and particularly to religion.

The astronomical subject I will be talking about is the relationship of the Earth to the Sun.  The geocentric (the Sun revolves around the Earth) and the heliocentric (the Earth revolves around the Sun) views.

Let us start with some observed facts.  10,000 years ago people looked up at the sky and saw it the same as we do today.  The stars have different colors and brightnesses.  They are in fixed patterns and different peoples have given these groupings or constellations names and created stories or myths about them.  If we go out tonight we will see these same stellar images that people saw many thousand years ago.  The existence and nature of these stars is a fact.

There are some exceptions.  Seven of these celestial objects move with respect to the background pattern of stars.  These are the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn.  I believe that the number of these wandering objects is the reason we have seven days in the week.  I have been told that the number seven appears in modern and ancient writings from around the world.  The number seven has a mystical power for all cultures.

The ancient peoples recorded the positions of these wandering objects.  Accurate tables of solar and lunar eclipses were created thousands of years before the actual geometry of our solar system was understood.  Ancient wise men used this knowledge to claim mystic powers.

In the Western world, a Greek astronomer Aristarchus (310 BC – 250 BC), from the island of Samos, was perhaps the first to point out that instead of the Sun going around the Earth, the Earth goes around the Sun.  He did not dispute the basic facts of the motion of the planets, but he did offer an alternate interpretation.  He made some observations and interpretations that led him to believe that the Sun was much larger and heavier than the Earth.  From this he reasoned that the Earth must revolve around the Sun.  His idea was not generally accepted although it was referred to by Copernicus many centuries later.

Claudius Ptolemy (85-165) was a strong advocate of the idea that the Sun and the planets revolve around the Earth and that the Earth is stationary.  Studying the ancient observations he noticed that while the Sun moved in a smooth circular path around the Earth that the planets moved at an uneven rate and Mars, for example, appeared to move backwards for part of its orbit.  This apparent backward motion is called retrograde motion.

To visualize this, imagine sitting on the outer edge of a slow moving carousel.  Now add, in your imagination, a bicyclists riding around the carousel at a rate that is slower than that of the carousel but in the same direction.  As you approach the rear of the bicycle, it will appear to slow down and then go backward with respect to your position on the carousel.  When the bicyclist is on the opposite side of the carousel it will appear to move at a rapid uniform rate.

This can only be explained by adding an additional circular orbit to the basic circular orbit.  Further uneven rates were explained by adding additional small circular orbits.  These additional circular orbits are known at epicycles.

Nicolaus Copernicus (1473-1543) introduced two new concepts.  The first was that the Earth and all the other planets revolved around the Sun and second that the daily motion of the Sun and planets was the result of the 24 hour rotation of the Earth around its own axis.  His concept had the advantage of being simpler but he had no observational facts to support his idea.

From today’s prospective, his work seems to be primitive and trivial but we must imagine what the universe looked like through 500 years ago.  Copernicus did not have a telescope or a mathematical concept of gravity.  Eyeglasses had been invented but no one thought to use two lenses to make a telescope.  Images of the universe were carried in people’s minds.  Everything that happened appeared to be caused by an invisible force.

The team of Tycho Brahe (1546-1601) and Johannes Kepler (1571-1630) provided the observations and interpretation that led to an accurate mathematical formulation of the motion of the planets.  They did not like each other but Kepler, the theorist, did trust the data of Brahe, the observer.  Their work was the basis of the laws of gravity that were derived by Isaac Newton.

Galileo Galilei’s (1564-1642) primary contribution was to develop and use a refracting (lens based) telescope to observe celestial objects.  His main contribution to the geocentric/heliocentric debate was the observation that there were four moons circulating around Jupiter.  This confirmed the idea that small objects can orbit around large heavy objects.  He was completely convinced that the Copernicus was correct.  Even though he made many other discoveries and scientific contributions, this one observation ultimately had a major contribution to our understanding of the solar system.  He also realized that a telescope could be used to demonstrate stellar parallax.

Stellar parallax refers to the idea that as the Earth moves around the Sun, the nearby stars will shift with respect to the more distant stars.  To demonstrate this effect, hold your thumb a few inches from your nose (centered opposite your nose).  Look first with your right eye only and notice the position of your thumb against the objects on the wall.  Then close your right eye and open your left eye and notice the position of your thumb against the objects on the same wall.  Alternately open your right and left eyes.  Now notice the shift in position of your thumb against the wall.  Repeat this several times to memorize the apparent motion of your thumb relative to objects on the wall.

Now repeat this experiment with your arm fully extended.  You will notice that the motion of your thumb against the wall will be smaller with your arm extended.  (If you have trouble closing your eyes alternately cover your eyes with a small piece of paper.)  If you measure the distance between your eyes and the amount of apparent shift of your thumb on the wall, you can calculate the distance between your eyes and your thumb.

By analogy, the distance between your eyes corresponds to the diameter of the Earth’s orbit around the Sun and your thumb represents a star.  The wall represents the background of distant stars.

If the distance between your eyes represents the diameter of the Earth’s orbit the around the Sun and your arm was 6.4 miles long your thumb would correspond to the nearest star (Alpha Centauri).   This is the brightest star in the constellation Centaurus which is visible from the southern hemisphere.  This is the nearest star to our solar system.

Back to our story about the Sun and the Earth, if a nearby star could be found with an annual motion resulting from the motion of the Earth around the Sun then we would have positive proof that the Earth revolves around the Sun.  As you can see from our demonstration, this was a colossal challenge.

Over the years many new telescopes were invented and many observations were made by skilled astronomers like Hooke, Bradley, Struve, Herschel, and many others.  Finally Friedrich Bessel measured the parallactic motion of 61 Gygni in 1838. This is a star in the constellation Cygnus.

Following this many other stellar parallaxes were measured.  It was finally proved that the Earth does indeed revolve around the Sun.

This has been a long journey that demonstrates that simple facts like the motion of the planets can take many years or centuries to correctly interpret.

This is science at its best.