A Complete History of Astronomy (Except for the Missing 99.99%)

Ancient Observations and Assumptions - "Dumb Ideas Revisited"

Once upon a time (probably before the first caveman went out looking for a "bud light"), human beings had no idea that the sun was the center of our solar system nor did they know that the earth made a complete trip around the sun once a year. Humans assumed that the earth was the center of the solar system, humans thought that the earth was stationary, and furthermore humans believed that the earth was flat.

Observations made from an earth that apparently stood still, led the early humans to believe that various "things" up in the sky (the planets, the moon and the sun for example) moved across a stationary backdrop of stars. Some early astronomers had noticed that the stars, themselves, move through the night sky but much more slowly than the moon or the planets.

One of the first things humans probably tried to understand was why there was a difference between day and night. Making the connection between the sun's presence or absence in the sky and the occurrence of day and night probably didn't take very much higher order thinking (these people were not "A.P." students by any means). Whenever the sun was "up" (the day), the early humans could hunt, gather and plant food and when the sun was "down" it was "dark" (night) and these early humans just had to hang out and wait for the light time. Explaining why the sun went up and down in the skies over a supposedly flat and motionless earth took some higher order thinking.

The scientific explanation of the occurrence of day and night had to wait for humans to discover that the earth was moving around the sun and that the earth spins on its own axis. Before these two discoveries were made, early astronomers had invented some wonderfully strange explanations for the occurrence of day and night.

Some cultures thought that the sun was carried across the sky during the day on a fiery chariot and that the sun was extinguished each evening when the chariot fell into the sea. According to this legend, it took all night to relight the fires of the sun deep below the sea and this explained the darkness. Interestingly, the chariot always reappeared in the east.

Other cultures claimed that the sun rode in a boat across the daytime skies and at night the sun would disappear into tunnels that ran under the earth.

A really disgusting and bizarre explanation for day and night was developed by the ancient Egyptians. They credited the motion of the sun across the desert sky to one of their deities - the goddess "Nut" (I'm serious!). The Egyptians thought that "Nut", the goddess of the sky, was responsible for the way the sun moved and therefore, day and night. Each night "Nut" supposedly swallowed the sun and each morning the sun was reborn after passing through Nut's body (presumably coming out the other end of Nut's digestive tract).

In addition to wondering what caused day and night, the ancients couldn't help thinking about the stars. Please remember that once upon a time there was no television (honest) and at night people actually looked at the sky for entertainment! (The original Star Search).

The fact that the objects they could see moving across the night sky all appeared to be moving across a non-moving backdrop of tiny little stars led the ancients to believe that the stars were somehow glued to the inside of a huge distant sphere that surrounded the earth.

The ball of stars that was supposedly surrounding the motionless earth was called the celestial sphere. As the earth turned, (the soap opera – "As the World Tums" - hadn't been invented yet), the ancient astronomers could see different parts of the celestial sphere at night and it appeared to them that the sun, moon and planets were always moving against this rotating backdrop or celestial sphere.

Now these crazy guys and gals spent a lot of time gazing at the stars (video recorders and video games like "Lame Boy" hadn't been invented). There wasn't much else to do at night except worry and make more people. They didn't know that the earth orbited around the sun or that the stars were very, very far away. The nocturnal star gazing led to some wild and crazy ideas. As the celestial sphere turned, the ancients saw different groups of stars show up at night during different times of the year. The ancients gave these different groups of stars names like Orion, Sagittarius or Capricorn, and this was the beginning of the zodiac concept which is one of the all time dumb ideas. Unfortunately, the zodiac concept persists today (as illustrated by the daily horoscopes in newspapers, in magazines and in the all time classic opening line- "What's your sign?").

Fortunately, ignorance and superstition regarding the zodiac didn't prevail everywhere and things became a bit more sophisticated as time passed. I know this is hard to believe based on the number of dummies who still believe in astrology and read the trash magazines and "news" papers located next to the check-out counter at the local supermarket.

Now while all of this "zodiacing" was going on, the ancients were very interested in staying alive (The movie "Staying Alive" - the sequel to Saturday Night Fever still hadn't been filmed). Staying alive required food - not disco - and food production required successful agriculture. To have a successful agricultural system the ancients needed to know precisely when to plant their crops. The right time of year was always in the spring and the ability to know when spring was going to arrive was a very valuable bit of "insider" information (The movie Wall Street was not filmed yet).

Calendars were invented to tell the people just when to plant crops and their calendars were, and still are, based on the motions of objects in the heavens. The Egyptians developed a calendar based upon the star Sirius. They used the position of Sirius in the night sky to predict when the Nile River would flood. The flooding of the Nile was a very important time of the year in ancient Egypt because during the flooding of the Nile the silt in the Nile waters (water pollution is a good thing in moderation) would fertilize the soil for the next food crop they were going to plant. The Egyptian calendar was based upon the stars and their year had 365 days. Because the year is actually longer than 365 days, the early Egyptian calendar was off by one day every 4 years (leap years and Dick Clark New Year's Eve Party hadn't been invented yet) so the Egyptians had to wait a full 1,460 years for the calendar to correct itself. This correction led to the legend about the holy bird called the "Phoenix" who would die in flames and be reincarnated from ashes every 1,460 years (Phoenix, Arizona and Shirley McClaine hadn't been invented yet).

Other cultures came up with other ways to predict the arrival of the seasons. They developed calendars that were based on motions of the moon and the sun. Eventually, the occurrence of all four seasons was understood after the star gazers and farmers of the time (John Cougar Mellencamp wasn't born yet) noticed that the sun did some pretty strange things during the "year" and that these things occurred in regular cycles (Maytag washers hadn't been invented yet). The four changes in the height to which the "moving sun" climbed into the sky across the ancient skies corresponded to the four seasons.

At certain times of the year the sun rose, climbed very high in the sky and stayed up there for almost 15 hours. This always happened during the "summer" and the day was, and is, still called the summer solstice (around June 21). Crops had to be growing by the summer solstice if the ancients wanted to eat or heads would roll (Burger King and McDonald's hadn't been invented yet).

As the year progressed the sun climbed lower in the sky and the day length got shorter until on a special day there was exactly 12 hours of sunshine and 12 hours of darkness. This day was, and is, called the autumnal equinox (around September 21). This was the time of the year for them to go out and harvest the crops. (Halloween and various forms of school hadn't been invented yet).

As the year continued to move along, the sun climbed even lower into the sky each day until the shortest day of the year. This day had about 9 hours of light and 15 hours of darkness and was called the winter solstice (around December 21). This was the time of the year they ate the harvested crops and hung out (Christmas, Santa Claus and "the malls" hadn't been invented yet).

While they continued to watch the sun, it again began to climb higher into the sky and stay up there longer until the 12 hours of day and night returned on the day called the spring equinox (around March 21). This was the key day to plant the crops for the new season and heads would roll and stomachs would growl if they didn't heed this special day. (Spring recess, the Easter Bunny, and Ski vacations hadn't been invented yet).

The pattern continued until the year was completed on the day of the next summer solstice.

At Last - Some "Real" Science

One of the first great astronomical discoveries came around 600 B.C. The Greeks discovered that the earth was a sphere! Unfortunately, the "Flat-Earth Society" still hasn't heard this news. The Greeks had invented "geometry" which literally means "measuring the earth" and by using it, they became the first to realize that the earth was a spherical ball and not a flat pizza-shaped object (sadly, pizza hadn't been invented in those days). Knowing that the earth was a spherical ball led to the next big discovery in astronomy: the earth turned on its axis. The realization that the earth rotated finally provided a real explanation for why there was day and night.

Ptolemy's Geocentric Solar System

Our story gets interesting around 300 B.C. The Greeks were among the first people to try to describe their world using mental and mathematical models. The ancient Greek astronomers knew that there were some funky objects ("funky" was invented in the 1960's) roaming around in the ancient skies. They called these objects "planets" using the root word for "wander" since the planets seemed to wander across the night sky.

The Greeks had been keeping records of the motions of the planets for many years. The Greeks knew the paths of the planets and how long it took each planet to make its trip across the ancient skies. Planets were mysterious things to the ancients. Every once in a while the planets seemed to temporarily reverse the direction of their normal paths through the sky (this is called retrograde motion today). It wasn't until the 1600's that Kepler correctly provided an explanation for the strange reversal of the planets' motion, but the Greeks at least tried.

Most Greeks believed that the earth was the center of the solar system and most Greeks believed that all of the heavenly objects moved around the earth. Some Greek astronomers attempted to come up with a model of the solar system with the sun at the center and the earth moving around it (a heliocentric model) but it was never accepted.

A few hundred years B.C., the Greek astronomer Hipparchus, and others, came up with a geocentric model for the known universe. Using this earth-centered model they were able to predict eclipses of the moon and plot the accurate locations of many of the stars on ancient maps. In Hipparchus' model the planets moved in perfect circles around the earth. But the Greek astronomers, who believed that the earth was the center of the solar system, had a big problem - how could their earth-centered model explain the strange motion of the planets?

Almost 300 years B.C., Claudius PTOLEMY realized that Hipparchus' idea which had planets moving in perfect circles around the earth was bogus. He came up with a refined GEOCENTRIC MODEL of the solar system that attempted to explain how the planets and the sun could move around the earth in orbits that were not perfect circles. He also tried to explain why the planets seemed to stop moving in one direction and then temporarily reversed their paths (the retrograde motions). Ptolemy had to do this because the carefully maintained records of the locations of the zodiac constellations and the planets made the existence of simple circular orbits impossible.

Ptolemy came up with a new idea that had the sun and the planets moving around the earth following paths he called epicycles. According to Ptolemy, each planet moved in many smaller circles (the epicycles) while simultaneously moving around the earth in a larger circular orbit.

To understand an epicycle, try to imagine a giant ferris wheel with the earth at its center. A rider on this ferris wheel would go around the wheel like a planet around Ptolemy's earth. While the rider rides the big circle all the way around the wheel he or she would also be making smaller circles as the seat rotated around in the same plane.

This complicated combination of circles on circles moving in circles worked pretty well and explained most of the motions in the heavens. Because the Greeks like the idea of an earth centered universe (the geocentric model) and because they thought that the circle was a perfect shape, Ptolemy's erroneous model of the solar system lasted for almost 1700 years!! (Sadly, 28% of adult Americans still think the earth is the center of the solar system).

Copernicus' Heliocentric Theory

The earth remained at the center of Ptolemy's geocentric solar system for almost 1700 years. Now 1700 years is a very long time - even longer than the average faculty meeting. The geocentric model was supported by the Greek belief that circles were perfect objects and by the fervent desire of the Catholic Church to keep the earth at the center of the universe since mankind and the earth were created by god. Maybe the concept should have been called the "ego"-centric theory?

In the early 1500's, a Polish astronomer Nicolaus Copernicus tried to create a model of the solar system that could correct the factual errors in Ptolemy's geocentric model. (There is no truth to the rumor that Nicolaus Copernicus' family invented tubular hotdogs called Polish sausages to support his astronomy habit). No matter how hard Copernicus tried, he couldn't fit the known astronomical facts about motions in the heavens into Ptolemy's geocentric model. Copernicus tried another revolutionary model in his attempt to explain the motions of the stars, sun and planets.

To account for the known facts, Copernicus resurrected the heliocentric theory. In Copernicus' model of the solar system, the sun was at the center of the solar system and the planets moved around the sun. This model is called the heliocentric theory ("helio" = sun). The strength of Copernicus' model is that it almost perfectly explained the retrograde motions of the planets. Unfortunately Copernicus, like Ptolemy, used circular orbits in his model. Because the orbits of the planets are not perfectly circular there were minor, but important, problems with Copernicus' model and the world was not ready to accept its veracity.

Now our friend Copernicus was no John Rambo and he sure wasn't an idiot. El senor Copernicus knew that his idea was too revolutionary to be accepted by the church and he feared the consequences of its implications. He didn't have his work published until one year after his death because he knew that, if he was right, the Bible was wrong about the earth being the center of the solar system. Copernicus wasn't foolish in fearing for his life. Fifty seven years after Copernicus died, an Italian astronomer named Giordano Bruno was burned at the stake in Rome for claiming that the heliocentric theory was true.

Tycho Brahe's Contributions

Now the story gets stranger and stranger. Three years after Copernicus died, with his heliocentric theory getting little or no acceptance, the stork brought Mr. and Mrs. Brahe of Denmark a little son they named Tycho (sex education hadn't been thought of yet).

Tycho Brahe was a strange fellow (he is one of my all time heroes). He went to school in Germany and lost his nose in a sword fighting duel for starters. His rich uncle convinced the Danish King to give Tycho an island and Tycho (sans his nose) went off to live on his island in a cute little starter castle equipped with a dungeon for the peasants. Poor little Tycho didn't have a telescope because they hadn't been invented yet but he did have an overwhelming passion for astronomy.

Tycho Brahe didn't like Ptolemy's geocentric model or Copernicus' heliocentric model of the universe. He didn't like the peasants much either (see the previous paragraph about the dungeon). Tycho decided to create a new model of the solar system. Tycho's model had features of both the geocentric and heliocentric models. Tycho's model had the sun and the moon going around the earth but it had the planets orbiting the sun. Good grief! Tycho's model was so complicated that I'm not sure he understood it himself, but what can you expect from a guy living on an island trying to breathe through a metal nose?

Now before I jump all over Tycho, let me give him credit for two very important achievements. First, Tycho kept careful records of his observations of the motions of the planets for over 30 years while partying on his island and second Tycho had the good sense to hire a young assistant named Johannes Kepler. Tycho Brahe's records were the raw data that Kepler finally used to develop the current model of our solar system and Brahe's data enabled Kepler to derive his three laws of planetary motion.

Kepler's Genius and the Three Laws of Planetary Motion

In 1601 Tycho Brahe died after overeating at a party. Immediately after Tycho Brahe’s death, Kepler had to fight with Tycho Brahe's family to get Tycho's records. It is rumored that Tycho was so jealous of Kepler's genius that he hid much of his raw data from Kepler and it is quite possible that Kepler had to steal the data so he could work with it (so much for "pure" science).

Kepler worked with Tycho's data for years trying to find a model of the solar system that would work and be able to explain the 30 years of accurate observations that had been made by Tycho. In 1618, Kepler published his three laws of planetary motion in The Harmony of the Universe.

Kepler's first law (The Law of Ellipses) states that the planets orbit the sun in elliptical orbits instead of the circular orbits as Copernicus had predicted.

Kepler's second law (The Law of Equal Areas) states that the velocity of each planet changes as each planet travels along its elliptical path around the sun. A planet moves faster when it is nearer the sun and slower when it is farther from the sun. These changes in each planet's speed can be seen by observing that each planet sweeps out equal areas of an imaginary ellipse during equal amounts of time anywhere during its orbit.

Ten years later, in 1619, Kepler published his third law (The Law of Harmony). Kepler noted that there was a relationship between the time it takes each planet to complete one of its orbits and the planet's distance from the sun:

Kepler's third law states that if you take the average distance of each planet from the sun and cube this distance (D³) and then divide this number by the square of each planet’s period of revolution around the sun in years (T²) you will always get a constant (K). The harmony of the universe is that the value of "K" is a constant and "K" is the same value for every planet in our solar system!

With the publication of Kepler's three laws, the basic understanding of the spacial configuration of the planet's and the sun and the motion of the planets within our solar system was in place.

It was a long intellectual journey and many men made major contributions (women were still not into astronomy during those sexist days). Today we know that the earth is an almost spherical ball orbiting an insignificant star (our sun) along with other balls (the planets). We know that our path follows a slightly elliptical orbit around the sun and that our speed varies as we get closer or farther from the sun. We also know that the mind of the human species is an extremely powerful tool because most of this work was done without the aide of a telescope! Thanks guys !!

© J.R.Neri, 1993-94 Physics is Everywhere Chapter 11