Download The Origins of Modern Astronomy Astronomy goes back to well

The Origins of Modern Astronomy
Astronomy goes back to well before anyone was writing about it. People, even ancient peoples, looked
up and paid attention to the sky. Why would they do that? Imagine that you are one of these people
from ancient times. Why would you look up?
There is the obvious answer: time keeping! Watching the sun rise and set or the stars cross the sky will
give you a sense of the time going by. Back then you couldn’t check your watch! It also allowed you to
observer the seasons passing. So, watching the skies was a good way of keeping track of time day-to-day
and through the year.
Watching the seasons was incredibly important! What is one of the most important tasks you must
complete as a person in ancient times? Getting food! There is no grocery store for you. You must hunt,
or gather, or grow your food. If you hunt, you will need to know seasonal changes for migration patterns
so that you are ready when those animals pass through your area. If you are planting, you need to know
when to plant and harvest. Plant too early and the crops will freeze in the ground, too late and they
won’t ripen before winter. If you are keeping your livestock, you must be aware of the seasons for the
breeding of your animals. Any way you look at it, keeping track of the seasons help you keep you and
your family fed!
If you are part of a nomadic group, you would look up for navigation purposes. There was no GPS back
then. Someone may show you a path or relate to you to turn left at the big eagle rock, but if you do not
have that, then you must look to the stars for guidance. And if you are part of a seafaring people,
turning left at the third big wave is never a reliable method of navigation. Use the stars!
And, of course, people looked to the skies as part of their religion or belief system. Astronomy is
connected to just about every religion humans have practiced through the millennium. You might have
looked to the skies to get a message from your god/goddess. Comets were often thought of as messages
from the Gods. You might keep watch on the seasons to prepare for the ceremonies at the solstice days
(the longest and shortest days of the year). Most religions have some sort of ceremony or celebration at
one or both solstice days. For example, there have been religions that believed that the gods made the
days get shorter and would continue to do so if they were not appeased. If you pay attention to the sky,
you will find that you notice the days getting longer again about 3 days after the winter solstice (which
falls approximately at Dec 22). Does your religion or belief system have a ceremony or celebration at the
solstice or about 3 days later?
As we move forward in time, even before people were writing they were leaving evidence of their
interest in the skies. Monuments that show astronomical alignments date back to about 3000 BC. These
monuments (including the stone rings, like Stonehenge, found on every continent except Antarctica)
were probably used as calendars and might even have predicted eclipses. Stone rings were an easy way
to keep track of time going by. Once you have the stones set, you can stand in one spot and, for
example, know that it is the solstice when the sun rises over a specific rock in the ring.
The first preserved written documents about ancient astronomy are from Greek philosophy. The Greeks
were an odd people who tried to understand things just because the objects were there. They didn’t
need a practical reason, like many civilizations that came before.
It is important to note that curiosity does not become a reason to look at the skies until survival is not as
much of an issue. If you were worried about starving tomorrow, chances are you wouldn’t be reading
this to satisfy some curiosity you have about astronomy. A portion of the Greek society had become rich
enough that others saw to their survival needs and they could actively pursue scholarly agendas.
With the rise of the Greeks, we also see the rise of mathematics. The Greeks, as a whole, tried to
understand the motions of the sky and describe them in term of mathematical models. Now, most of
their models were completely wrong, but that is almost beside the point. They were the first steps in a
long journey to our current understanding. No matter how ill-placed the first step, without it we
wouldn’t be here and we honor them for taking those steps.
The reason many of their models were incorrect is that they were based on things that were “obvious,”
but only to an ancient Greek! For example, the gods made Earth and all else is above or around it.
Therefore, Earth is at the center of the universe, and all things above it (where the gods live) are
completely perfect. (Why would the gods have anything less?)
Arguably the founder of Greek philosophy is Thales of Miletus. He tried to explain what the universe was
made of without resorting to supernatural explanations. This was unusual for the time (about 624-547
BCE). He suggested that the world is understandable and not just the result of incomprehensible
(possibly gods-driven) events. His model of the universe had Earth as a flat disk floating in the water
with a dome of air and water above. You can see why he came to this. The Earth feels flat to us small
humans living on it, and no matter the direction you walk you will eventually come to water. Air does
appear to make a dome above us, and water does sometimes fall out of the air. This model was not
widely accepted, however.
A student of Thales, Anaximader of Miletus (ca. 610-547 BCE), took up the study of the universe and
moved our ideas forward. He suggested that the heavens must form a complete sphere around Earth, to
explain the motion of the stars aound the north star. (If you watch the sky around the north star, you
will eventually notice that all of the stars nearby circle the it. This is called circumpolar motion.)
Anaximander considered that as you travel north or south, the sky changes, but it doesn’t change with
east-west travel. Therefore, he concluded that the Earth must be curved in one direction, but flat in the
other. He guessed that Earth might be cylindrical in shape and his model had the Sun, moon, and stars
going around the cylindrical Earth.
Pythagoras (ca 569-475 BCE) is more known for his mathematical influence on modern science (the
Pythagorean Theorem), however it is important to note that he taught that Earth was a sphere.
From here we take a side trip to Democritus of Abdera (ca. 460-370 BCE). Democritus (and his mentor
Leucippus) is the father of the atom. He believed that all matter was made of these little things called
atoms, which were eternal, invisible, indivisible, and incompressible. (REALITY CHECK: Atoms are not
eternal, invisible, or indivisible. We have found ways to annihilate, see, and split them, although the last
is very dangerous. We have not, as of yet, found a way to compress them, so that ancient belief still
stands valid.)
Democritus believed that the universe was made of an infinite number of atoms that came together
randomly. This led him to believe that there were other worlds out there, perhaps not unlike our own.
He claimed the moon had mountains and valleys, the Milky Way was a vast group of stars, and that
there were other worlds out there that might have more moons and Suns or none at all. However, his
theories never gave credit to a Creator(s), and therefore atomism became linked with atheism. This
meant that there were very few atomists. The link between the belief in atoms and atheism persisted
into the mid-1800s. (In 17th century France, you could be burned at the stake for believing in atoms.) In
the mid-1800s, chemists revived the idea as a way to explain some of their experiments. Many chemists
jumped on the atom bandwagon quite quickly. The rest of the world was slower to follow. Many people
did not believe in atoms until we had equipment that allowed us to view them in the early to mid-1900s.
So, Democritus was a man well ahead of his time… and completely underappreciated during his lifetime.
Plato (427-347 BC) never mentioned Democritus in his works, not believing that random motions of
atoms were responsible for the fate of the world or heavens. Plato had other ideas. Plato is, by many
accounts, one of the most influential authors in the history of philosophy. His written dialogues are
wide-ranging and thought provoking.
Pluto taught that the world cannot be known through the exclusive use of the senses. In his work The
Republic, he imagines a group of people who have lived chained in a cave all of their lives, facing a blank
wall. They watch shadows flickering due to a fire far behind them. Occasionally things pass in front of
that fire (behind the people) and shadows are projected on the cave wall. Plato explains that the
shadows are as close as the prisoners get to seeing reality. He says that a philosopher is a prisoner who
has been freed from the cave, one who can use pure rational thought to understand the true form of
the object or fire rather than the shadows. In this, he advocates the use of thought over observation.
This will cause some problems as time goes on and rational thought gives one explanation, while
observation gives another. For example, “pure thought” of the time brought about the conclusion that
the heavens are the home of the Gods. As such, the heavens would be perfect. To him, perfect meant
that all objects are perfect spheres and move in perfect circles. This belief that all things in the heavens
are perfect spheres moving in perfect circles around Earth became pervasive, beginning with the ancient
Greeks and perpetuated by the Roman Catholic Church through the dark ages and into the Renaissance.
The problem is that, although this works as a pure thought, it does not match observations of the
planets which periodically appear to go backward and then forward again in small loops. Philosophers
will try many times to explain this bizarre “retrograde” motion of the planets, none of them correctly.
Eudoxus of Cnidus (408 or 410 – 355 or 347 BC) was a Greek mathematician, astronomer, scholar, and
student of Plato. His work in mathematics was ahead of its time and was not replaced until the algebraic
work of Descartes. De also developed the “Antiphon’s method of exhaustion,” which was a precursor to
integral calculus. His astronomical fame is due to his invention of the astronomical globe to explain
planetary motion. Eudoxus tried to answer a question supposedly posed by Plato. "By the assumption of
what uniform and orderly motions can the apparent motions of the planets be accounted for?" The idea
that the motion of the planets could be explained by orderly motions was a radical idea for the time.
Modern reconstructions of the Eudoxan model show 27 spheres rotating around Earth. The moon and
sun each had three spheres dedicated to their motion, while each of the five visible planets had four
spheres. Although an amazing model, able to explain many motions in the sky, it could not explain the
change in brightness of the planets. In his model, the planets were all at the same distance from Earth
and should’ve, therefore, had the same brightness. Still, Eudoxus’ accomplishment was very important
to Greek astronomy. He was the first to attempt a mathematical explanation of planetary motion.
Few other philosophers come close to Plato for depths and range, although one who might was his
student Aristotle (384 – 322 BC). Aristotle’s views shaped medieval scholarship and teachings. He is
credited with the earliest studies of formal logic, which remained the dominant form of the studies until
the 19th century. Aristotle was also known for his “natural philosophy,” whereby he studied the
phenomena of the natural world in an attempt to understand their “essences.”
Aristotle expanded on the astronomical globe created by Eudoxus. Instead of 27 spheres to explain the
motion of the sky, he had 55 spheres surrounding Earth. He did believe in the perfect heavens (all things
spheres and moving in circles) and the imperfect Earth, stationary at the center. He reasoned that if the
heavens were perfect, they must also be unchanging (other than the repetitive circular motion of
objects around Earth). He concluded that the moon was the closest heavenly sphere to Earth, and it
therefore marked the beginning of the heavens.
His “physics” stated that Earth was made of four basic elements: Earth, Air, Water, and Fire. These could
not, however, make up the perfect heavens, as they were not perfect. Therefore, the perfect element
was dubbed the quintessence and thought to make up the heavens.
A fundamental principle of Aristotelian physics is that sublunary material (closer to Earth than the
moon) seeks its natural state of rest. Natural motion is the motion of an object to its natural place of
rest. Fire was thought to have a natural place above us (it rises up), but well below the moon. Rocks and
water seek their natural place at the center of Earth, unless stopped by a surface. This gives a simple
explanation for falling rocks. Violent motion, on the other hand, is a motion forced on an object that is
not natural to it. For example, throw a rock upward. Eventually, however, the rock will revert to its
natural motion and fall down.
Aristotle used this explanation of natural motion to support his belief in a geocentric universe. If there
was more than one imperfect place (like Earth, not of the heavens), then when you dropped the rock
where would it go? How would it choose which imperfect place to fall to? Would it hover, spinning,
unsure? As rocks never act this way, he saw it as proof of only one imperfect place, one Earth at the
center.
Aristotle did base some of his beliefs in astronomy on his observations. For example, he viewed a lunar
eclipse, reasoned that the moon was in Earth’s shadow, and noted that the shadow was round. Ergo,
Earth is indeed round. This is also supported by the observations of different stars as people traveled
further south. He explained lunar phases with his observations, too. He said that as the moon revolves
around Earth, we see different portions of the illuminated side of the moon. You can try this by having
one light source in the room (for example, a flashlight) that represents the Sun. Then take a ball
(representing the moon) and have it travel around you (the Earth). If you start with it between you and
the Sun, you’ll notice that the part of the Moon that you see is dark. As it moves around you, you’ll see
more and more of it as bright. When you’ve gotten halfway around, it will either be in your shadow (a
lunar eclipse), or if you hold it up a bit higher you’ll see a full moon. Aristotle considered this a simple
way of explaining the phases of the moon… and he was correct!
Next we meet Aristarchus of Samos (c. 310 – 230 BC). Aristarchus was an astronomer mathematician
with access to the Great Library at Alexandria. He is most known for being the first person recorded to
propose a heliocentric (Sun-centered) model of the Solar System. In his model, only the Moon revolves
around Earth (he liked the explanation Aristotle gave for lunar phases). All other bodies, and Earth,
revolved around the Sun. Although not widely accepted, and much of his work was lost, the concept was
revived 1800 years later by Copernicus and was the foundation beneath the Copernican revolution.
Many others argued that his concept of a heliocentric solar system was flawed. They asked questions
such as, “If Earth is moving, why don’t we feel it?” and “If the Moon orbits Earth and then Earth moves,
why don’t we leave the Moon behind?” Although, possibly the most ingenious argument used against
him was, “If Earth is moving, why don’t we see any stellar parallax?”
Parallax is not a difficult concept. Most people know what it is, but do not know that it has a name. Take
your finger and place it in front of your nose. Now close one eye and look at the finger with your other
eye. Now switch eyes. Go back and forth. What did you see? You should’ve seen your finger appearing
to jump back and forth. Parallax is the apparent motion of a body due to the motion of the observer. As
your observer (your eye) moved, your finger appeared to move. Stellar parallax is the apparent motion
of a star, due to the motion of Earth. The concept is the same. As Earth moves, a nearby star will appear
to go back and forth in the sky over the course of a year. The problem is that we don’t see stars doing
this motion. Therefore, the argument used against Aristarchus was that if we didn’t see this motion, the
Earth must not be moving.
Reality check… Earth IS moving. So, why don’t we see stellar parallax?
Go back to your finger in front of your nose. Remind yourself what it looks like as you shift eyes. Now
move your finger outward and try again. Now move your finger even further out. What do you see? You
should see less and less of the apparent motion of your finger.
So, why don’t we see stellar parallax? The stars are REALLY far away. You won’t see stellar parallax with
your naked eye, even though it is there. We do have equipment that can now measure stellar parallax
for some of our closer neighbor stars.
This brilliant argument kept many from believing in a heliocentric model for a long time.
Aristarchus also attempted to find the size and distances of the Moon and Sun. Although he didn’t
manage to get the correct answers, his concept was good. He was just lacking information and
trigonometry (which didn’t exist yet).
Eratosthenes (276 BC - 195 BC) actually worked at the Great Library. He was an astronomer
mathematician, who is most known for determining the circumference of Earth. Being versed in
geometry, and knowing that Earth is round, he set out to mathematically determine the circumference
of Earth. He started with a circle. Then he placed himself at one point and a town by the name of Syene
at a small distance from him. It was said that on a specific day of the year at noon, there were no
shadows on the water at the bottom of the well in Syene. This signified that the Sun was directly over
the town on that day.
The sun was not overhead for him, but he could calculate the angle between overhead and the direction
to the Sun. He calculated this angle to be approximately 7º. This was then found (using geometry) to be
the angle between the two towns. This gave him a “pie piece” of Earth with a 7º point. Knowing that a
circle has 360º, you could then place identical pie pieces next to each other until you have a complete
circle. If you then know the distance between the towns, you can find the circumference of Earth.
So, did he get the correct answer? Well, that depends on how well he measured the distance between the
two towns. He couldn’t use GPS to determine, nor could he use an odometer, and it is a very long
distance to walk while measuring. He measured the distance between Syene and Alexandria to be about
5,000 stadia. It is hard to tell if this is correct, as we don’t exactly know how long a stadia is. If we use a
stadium as our basis for the length of a stadia, we get that he likely got the correct answer with less than a
14% error. Quite a good job! And the number was astonishing! It was a very large number. Larger than
anyone generally thought for the size of Earth.
Beyond that, Eratosthenes questioned why the Sun would only be overhead in Syene periodically. He
concluded that it could happen if Earth was tilted. Based on the height of the Sun in the sky at noon midwinter and at noon mid-summer, he found that Earth is tilted 23.5º.
Another astronomer-mathematician of great repute was Hipparchus of Rhodes (190 BC to 120 BC). He
may be best known as the father of trigonometry. He also invented latitude and longitude, measured the
length of a year to within 6 minutes, and attempted to explain retrograde motion using eccentrics. He
created the first large star catalog, including approximately 3000 stars. To help others find the stars he
discussed in his catalog, he labeled each by its constellation and magnitude. At the time, a constellation
was a grouping of recognizable stars that made a picture in the sky. Each culture had its own set of
constellations. Magnitude referred to the brightness of the star. He developed a scale from 1 to 6, where 1
was the brightest and 6 was the dimmest.
Hipparchus studied many star charts during his work. While studying old star charts, he noticed a
systematic movement of the placement of the stars in the sky over time. They were all very slowly
changing position (although, not relative to each other). He gets credit for discovering the cause of this,
the precession of the earth. Precession is a change in the orientation of the rotation axis of a rotating body.
Think of a child’s toy top. As it spins, it does not stand up straight. It spins at an angle, which changes,
tracing out a circle. Our Earth is tilted as spinning, and it wobbles around in a circle, too…. It precesses.
Whichever way our North Pole points, gives us our North Star. As we precess, our north star changes.
Currently, our north star is Polaris, but that will change and sometimes we will not have a north star. It
takes about 26,000 years to complete one full circle, one complete precession.
Jumping forward a couple hundred years, we find Claudius Ptolemy (or Ptolemaeus) (85 to 165). Born in
Egypt, under Roman rule, Ptolemy was an astronomer, mathematician, geographer and astrologer. Within
astronomy, he is most known for his written work The Almagest and for his Ptolemaic model of the
universe.
The Almagest was a combination textbook, encyclopedia, and astronomical almanac. It really was a
compilation of about 600 years of Greek astronomy, as well as his new work on planetary motion. The
star catalog within it was mostly the one completed by Hipparchus with several hundred new stars added.
The constellations listed were the 48 listed by Hipparchus covering only the portion of the sky that
Hipparchus could see.
The Ptolemaic model of the universe was not so very different from what others had said previously. He
believed in a geocentric system, with uniform circular motion. None of this was new. He liked how
Hipparchus had taken Earth out of the exact center of the universe, while still having everything orbit
Earth. Ptolemy did the same. In order to explain retrograde motion, Ptolemy placed Earth in the middle of
a circle called the deferent. He then placed a smaller circle, called an epicycle, on the deferent. The planet
then was placed on the epicycle. So, as the planet orbits on the epicycle, the epicycle orbits on the
deferent around Earth. This causes the planet to appear to do loops in the sky, sometimes going
backwards. This was considered a success! Backward motion of a planet using only perfect circles and
uniform circular motion! (Of course, planets don’t actually perform loops in space. He was completely
wrong, but it did fit his belief system.)
He also used Eratosthenes calculations of the size of Earth to determine the distance to the moon. Using a
right triangle and some trigonometry (thanks to Hipparchus), he found the moon to be about 60 times the
radius of Earth away from us.
Ptolemy’s view and teachings were picked up by the rising Roman Catholic church and were taught
through the middle ages to all within the Catholic reach. The library at Alexandria was burned. Much
knowledge was lost, however some was preserved by monk and some by the Islamic nations. The Islamic
world, from Spain to India, was doing well during the “dark ages” while Europe fragmented into warring
principalities simply interested in survival. Realistic astronomical models were not of interest to those few
who did study, as they were mostly of the Catholic Church and had full belief in the teachings of the
church. In the later middle ages, the teachings of Aristotle were raised again and added to the teachings of
the church.
By the time of the Renaissance (“rebirth”), the time was ripe for change. At this point, the Greeks were
almost lost to antiquity. They were studied by the learned monks and scholars and thought of as the wise
ancients. Then the New World was discovered and, among other things, people began to wonder if the
Greeks really were all-knowing. They didn’t know about the New World. Add to that the “rebirth” of
education (rising with the rise of the middle class, a group with some disposable income), and you have
people ready to question what was “known.” Problems with the Ptolemaic Model were now viewed with
greater seriousness.
There were four major players in Astronomical History in the Renaissance period: Nicolas Copernicus,
Tycho Brahe, Johannes Kepler, and Galileo Galilei. Copernicus was the first and began what became
known as the “Copernican Revolution.” The other three were contemporaries (they lived at the same
time). Brahe and Kepler actually worked together for a year.
Nicholas Copernicus (1473-1543) was born in Poland, near Torun, the youngest of four children. He was
orphaned young and raised by his uncle, who was a Canon of the Catholic Church. Given an education at
the cathedral school of Wloclawek and then the University of Krakow, Copernicus studied Latin,
mathematics, astronomy, geography, and philosophy. He was then granted the position of Canon of the
Catholic Church. For a while he continued his studies, retaining the title but not duties. He visited Rome,
lectured on mathematics and astronomy, studied law and medicine, and became the private physician (and
secretary and helper) to his maternal uncle, the Bishop of Ermland. After the Bishop died, Copernicus had
more time on his hands and he started writing. One interesting little book he wrote (but didn’t put his
name on) is the Little Commentary, which sets out Copernicus’s theory of a universe with the Sun at its
center. It is believed that he wrote the Little Commentary in 1514 and began writing his major work, De
Revolutionibus Orbium Coelestium (The Revolutions of the Heavenly Spheres) the following year. It took
him a long time to get the book to a point he was willing to let it be read or printed by others. It went to
the printers in 1542, where the man overseeing the printing wrote his own letter to the reader and
substituted it for the original preface. The letter claimed that the results of the book were not intended as
the truth, but rather showed a simpler way to calculate the positions of the heavenly bodies. Some were
horrified by this act, but others think that this new preface was the reason that the work was not instantly
condemned.
Copernicus’ proposed system was heliocentric, with the Sun almost at the middle of the Universe.
Moving outward from the Sun, it was orbited by Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Note
that he had the correct order of the planets. He explained the often observed phenomenon of Mercury and
Venus being close to the Sun in the sky by saying they don’t just appear to be close, they ARE close to
the Sun. In fact, he gave us a calculation of how close they were to the Sun. Not knowing the distance
from Earth to the Sun, he defined it as being 1 Astronomical Unit (AU). Using that, he made a scale
model of the Solar System in Astronomical Units. (We still use Astronomical Units today.)
He stated that all planets moved about the Sun in perfect, uniform circular motion and that the only
Heavenly body to orbit Earth was the Moon. He explained that the day was caused by the rotation of
Earth on its axis and that the year was caused by the revolution of Earth around the Sun. (Note: he was
correct on both counts!) This system also allowed for a simple and elegant explanation for the vexing
retrograde motion. Copernicus believed that the apparent backward motion of a planet wasn’t backward
motion at all. It was what we saw when Earth passes a planet. For example, Earth will orbit the Sun more
quickly than Mars will. Therefore, at some point in time Earth will pass Mars. While we pass Mars it will
appear to go backward. This is similar to passing a car on the freeway. While you pass the other car, it
appears to be moving backward even if it is still moving forward.
It has been said that Copernicus received a copy of his printed work while lying on his death bed in 1543.
I sincerely hope he got the chance to see it. Although not everyone believed in his work, it was much
admired by his contemporaries and many who followed. Notably, Brahe, Galileo, and Kepler all thought
well of him, although Brahe did not accept his idea of a heliocentric universe.
Really, the concept of a heliocentric universe still had a way to go. There were still problems with
Copernicus’ model. For example, if Earth was moving why didn’t we all feel it? Why don’t we see stellar
parallax? Why don’t we leave the moon behind? And the Copernican model was no better at predicting
planetary positions than the Ptolemaic model. Why would anyone choose a model considered heretical
when it did no better than its predecessor that was accepted by the Church?
Going against the Catholic Church was still a dangerous proposition. Born 5 years after Copernicus died,
Giordano Bruno was an ordained priest with a penchant for thinking in new ways. His opinions evolved
to include those contrary to the Catholic Church and to include the plurality of worlds (that there are
more habitable worlds out there). He was eventually put on trial on allegations of blasphemy and
heresy. His trial was seven long years, during which he was imprisoned. He ended his imprisonment in
the lightless cells of the Tower of Nona when he was found guilty, declared a heretic, and turned over to
secular authorities to be burned at the stake.