College Level Astronomy - Audio Study Guide

Astronomy

Key Takeaways

Jupiter’s Characteristics............................................................................................114

Atmosphere of Jupiter...............................................................................................115

Jupiter’s Magnetosphere...........................................................................................116

Jupiter’s Orbit and Rotation .....................................................................................117

Jupiter Exploration ...................................................................................................117

Jupiter’s Moons.........................................................................................................118

Saturn............................................................................................................................119

Physical Characteristics of Saturn.............................................................................121

Saturn’s Atmosphere.................................................................................................121

Saturn’s Magnetosphere........................................................................................... 122

Saturn’s Orbit and Rotation..................................................................................... 123

Saturn’s Natural Satellites or Moons ....................................................................... 123

Exploration of Saturn............................................................................................... 125 Uranus.......................................................................................................................... 125

The Orbits and Rotation of Uranus.......................................................................... 126

Physical Characteristics of Uranus............................................................................127

The Atmosphere on Uranus .....................................................................................128

Climate on Uranus....................................................................................................128 Moons of Uranus ...................................................................................................... 129 Neptune........................................................................................................................ 129

Physical Characteristics of Neptune.........................................................................130

The Atmosphere of Neptune .....................................................................................131

Neptune’s Magnetosphere

Chapter One.................................................................................................................282

Chapter Two.................................................................................................................283

Chapter Three ..............................................................................................................284

Chapter Four................................................................................................................285

Chapter Five.................................................................................................................286

Chapter Six...................................................................................................................287

Chapter Seven ..............................................................................................................288 Chapter Eight...............................................................................................................289

Chapter Nine................................................................................................................290

Chapter Ten.................................................................................................................. 291 Chapter Eleven.............................................................................................................292 Chapter Twelve ............................................................................................................293

Course Questions.........................................................................................................294

PREFACE

This course is designed to teach you what you need to know as part of a college-level astronomy course. The study of astronomy has made a great many strides in the last several centuries but certainly, its forward progress is still occurring today. While mankind has always questioned what exists in the skies, we are now exploring these areas in ways that tell us a lot about how the universe was created, what it is made of, and where it is going.

We will talk about the aspects of astronomy that are close to home, such as that of Earth and its single moon. Other planets in the solar system have been extensively studied and are covered in detail in the different chapters of this course. The moons, asteroid belts, and planetoids in the solar system have also been studied so we will talk about what we know about these. The sun is the center of the solar system and is the source of life on earth; for this reason, we will devote a chapter on the sun as well.

Outside of the solar system is the Milky Way Galaxy, other galaxies and planets, and entities we are just beginning to understand, such as black holes. Exploration of these areas lags behind other aspects of astronomy but we will talk about how astronomers are studying these parts of the universe and how, in some cases, they are looking for signs of life outside of planet Earth.

In chapter one of the course, we begin the study of astronomy by looking back at how the skies and the different structures seen in them have been understood and studied from the time of antiquity. Astronomy is probably the world’s oldest field of scientific study as mankind has always held an interest in it. We will talk about the evolution of astronomical theories since ancient times; there was a flood of new information and changing theories about planetary motion and other astronomic details still used today that began to be developed around 1600 CE. These will be covered in this first chapter along with the history of the telescope.

Chapter two focuses on the universe and how it is believed to have started in the first place. Surprisingly, other than creationism, there have been few other theories on the

origin of the universe other than the Big Bang Theory. In this chapter, we will talk about this theory and how the universe has come to evolve into what it is today. There are a few alternate theories about how the universe originated and is evolving, such as the ekpyrotic model of the origin of the universe. We will talk about the theories that exist today and about how we have come to believe in the way our Universe was probably born.

The topic of chapter three in the course is the solar system we live in. We will talk about the solar system in general, including what exists as part of the solar system, how vast it is, and what we consider to be the major and minor planets within it. We will discuss the major asteroid belts in the solar system and how they came to be as well as how planets and other satellites orbit around the Sun. Theories on how old the solar system is and on how it was formed will also be covered in this section.

Chapter four talks about our planet Earth and its only natural satellite, the moon. We will talk about the current theories that exist about how the Earth was formed as well as about the astronomical characteristics of the planet, such as its orbit, size, atmosphere, composition, and magnetosphere. The formation of the moon will also be discussed as well as its known features. We will also take the opportunity to cover the earth moon system, including the moon’s phases as seen on earth, the phenomenon of tides, and lunar eclipses. Finally, we will discuss solar eclipses as they have been witnessed on this planet.

Chapter five in the course talks about the three terrestrial planets in our solar system other than our own planet Earth. Each of these planets has similar characteristics in that they tend to be solid and rocky. They are also smaller than the larger gas planets further away in the solar system. These planets have origins best explained by the accretion model, where debris in the solar system coalesced and collapsed into a dense sphere through central gravitational forces. We will discuss the features of each planet as we have come to understand them through the modern study of astronomy.

In chapter six, we will study the Jovian planets, which are the furthest from the sun. There are two types, called the gas giants and the ice giants. As you will see, these are fundamentally different planets from the terrestrial planets due to their distance from

the sun, which affects the influence of the solar winds on their formation and current existence. None of these planets is believed to be habitable, mostly because their orbits lie outside the Goldilocks zone in our solar system.

The focus of chapter seven of the course will be the Sun, which is the star around which our solar system rotates. We know the most about stars from studying our own sun. In this chapter, we will talk about how the sun was created and the features we know to be true of the sun, both inside and on the surface. As the sun continues to evolve, we can make some assumptions as to the future of the sun over the time left in our solar system, which will be covered in this chapter as well.

In chapter eight, we will finish up our discussion of the different types of small celestial bodies. We will have already talked about asteroids in the discussion of the asteroid belts in the solar system but here we will discuss comets, meteoroids, and meteorites. We will also talk about Pluto, which is a minor planetary object far beyond Neptune and is part of the Kuiper belt.

In chapter nine in the course, we will go beyond the study of our own star to talk about stars in general. There are many different types of stars and ways to characterize them, which we will talk about. The lifespan of stars has been determined so we will go over this timeline. Supernovas are the fate of some stars. For this reason, this spectacular end to some stars will be discussed in detail in the chapter.

Chapter ten is about what happens at the end of a star’s life. We will have talked about the lifespan of stars and their overall aging process in earlier chapters. In this chapter, we talk about things like neutron stars, pulsars, and black holes, which are really the aftereffects of a star’s death. Black holes are especially intriguing so we will talk about what they are and what it would be like to be near one.

The focus of chapter eleven in the course is galaxies, including what they are, how they form, and the different types there are. Galaxies are incredibly numerous in our vast universe and have a wide range of sizes. We will talk about the different galaxy structures, the contents of most galaxies, and dark matter, which is what the majority of the universe is made from.

In chapter twelve, we will talk about space exploration as it exists today. Mankind has been able to explore space from Earth based telescopes and from probes and landers sent out beyond our borders. These have been valuable in understanding the universe around us. We will also talk about space travel and the difficulties in doing this, the habitability of planets, and the question as to whether or not it is likely that there is other intelligent life in places other than on earth.

CHAPTER ONE: THE HISTORY OF ASTRONOMIC STUDY

In this chapter, we start the study of astronomy by looking back at how the skies and the different structures seen in them have been understood and studied from the time of antiquity. Astronomy is probably the world’s oldest field of scientific study as mankind has always held an interest in it. We will talk about the evolution of astronomical theories since ancient times; there was a flood of new information and changing theories about planetary motion and other astronomic details still used today that began to be developed around 1600 CE. We will cover these in this first chapter along with the history of the telescope.

ANCIENT UNDERSTANDING OF ASTRONOMY

The study of astronomy clearly predates the discovery of the written word; however, there is plenty of evidence to indicate that mankind has long been entranced by the things in the sky. There are prehistoric drawings in caves and elsewhere of comets, eclipses, and other astronomic phenomena. It appears that these were people who were afraid of the sky. Even so, there are depictions of the sky and its objects on many pieces of artwork from as early as 1600 BCE. There is also evidence that some of the artistic objects were probably religious objects as well. Figure 1 shows an ancient North American rock with notches at the point where the sun fell on the Solstice each year:

Native Americans clearly studied the skies themselves. There are petroglyphs dating from about 1000 AD, including one that showed the supernova from the Crab Nebula that occurred in 1006 BCE. Those cultures who had language and writing skills, such as the Babylonians, recorded various astronomical information since about 1800 BCE. They were able to record the timing of certain eclipses and the planetary positions in the sky, among other things. Even Stonehenge in England is part of a project used by these ancient people in order to record when the Solstice and other events would happen.

In ancient cultures, many of the priests and priestesses used astronomy, myth, and the basics of astrology in order to determine when certain rituals should happen and when farmers should plant their crops. Pagan holidays of the time (and since then) were based on astronomical events; the skies were mysterious and mystical to so many that it was likely part of the religions of these people.

The Babylonians around 1800 BCE were the first to scientifically study astronomy and document their findings. Their records were detailed and included the daily, monthly, and yearly positions of the different celestial bodies in the sky. The information was used in religious circles and to possibly predict future catastrophes. They were thought to have first discovered Halley’s comet and were probably the first to have divided the sky into different zones.

The ancient Greeks got their first astronomical information from the Babylonians and used these to further the study of astronomy. Philosophers and scientists, such as Thales from about 480 BCE, were able to use known data and the study of mathematics in order to predict when eclipses could occur. By about 250 BCE, there were philosophers who were able estimate the size of the known planetary objects as well as the distance between Earth and these objects.

The ancient Greeks around the time of Plato had decided that the Earth must be round; this was confirmed by Plato who said that the “perfect shape” was the sphere. They also assumed the spherical shape of the Earth because the moon was also this shape. They maintained at the time that the Earth was the center of the universe. Some believe that the skies were actually a crystal shaped bowl that covered the entire earth. They also

observed ships descending in the ocean at a distance and assumed the round shape of the earth.

During the time of Aristotle (385 BCE 323 BCE), it was believed that there were nesting and concentric spheres containing the Moon, other planets, and the sun that hung above the Earth with the Earth being at the center of these spheres. They did not know what was holding the spheres suspended around the Earth but did not question it much as the rest of the theory made sense. Eratosthenes was a Greek scholar of the time who was an avid astronomer, among other things. He was the first known person to estimate the circumference of the Earth. Even at that time and with limited knowledge of astronomy, he was off by less than a few thousand miles. He also identified the idea of having a Leap Day, based on the tilt of the earth’s axis and the amount of time it takes for the Earth to go around the sun in a year. The ancient people of India also had astronomers who were among the first to decide that astronomy was less of a mystical thing and more of a scientific field of study. They still believed in geocentric ideas, which are that the Earth is the center of the universe but were able to determine that the Earth rotated and that the shine from the Moon and from visible planets came from the sun’s reflected light.

We can’t forget the Mayans when it comes to the study of astronomy. These were isolated people who had an intense interest in the planets, Earth, the sun, and the stars. They used large monuments and shadow casting to study the skies and the sun in particular. They used the information they developed in order to create the Mayan calendar.

The Egyptians also studied astronomy and had many myths and legends that were based on astronomical events. The pyramids themselves are aligned according to the Egyptian models of the sun and Earth together. The Great Pyramid of Giza was particularly aligned with the North Star of the time. They used astronomy practically as well as religiously so they could prepare for potential flood events. Their calendar system was based on their findings, having 30 days per month and a total of 365 days per year. Each week was 10 days in total length with just three weeks per month.

It is hard to go back far enough in time when the Chinese did not also have their hand in astronomical studies. The moon called Ganymede around Jupiter was first discovered by an ancient Chinese astronomer and detailed star charts were developed. They noted supernovas in the starry sky. In recent years, the oldest known star chart was found in a Buddhist cave in China, which dated to before 700 AD.

Less is known about Persian astronomy in ancient times except that it was a popular scientific endeavor in the earliest post Islam Persian times. They discovered the Andromeda Galaxy and, after Ptolemy’s time, made their own corrections of his work. One Persian astronomer was able to create a huge sextant that was able to accurately calculate the Earth’s axis within 2 degrees.

The roundness of the Earth was probably the earliest correct astronomical finding made by early civilizations. As mentioned, they could see ships appear to sink in the distant ocean and could see lunar eclipses, where the Earth casts a shadow over the moon. Eratosthenes was able to use geometry and known distances between two cities in Greece in order to determine that the Earth’s circumference was about 40,320 kilometers. We now know it is more like 40,030 kilometers.

The only visible things in the sky besides stars prior to the development of the telescope were the Moon, the Sun, and the planets of Mercury, Venus, Mars, Jupiter, and Saturn. Plato decided that the planets followed completely circular orbits around the Earth, while Heraclides in 330 BCE revised this model to create the first “geocentric solar system”. This model differs, of course, from the heliocentric model of the solar system we now know to be true. This is that the sun is the center of the solar system rather than the Earth. The first heliocentric model wasn’t proposed until 270 BCE by Aristarchus. Figure 2 shows the difference between these models:

Figure 2.

Despite the validity of the Aristarchus model, others of the time did not agree with it and had three main reasons why it wasn’t true. These included the following:

• The scholars of the time recognized that, for this theory to be correct, the earth would have to move, which was something no one could feel themselves so the idea was met with skepticism.

• They expected a parallax if the Earth had a circular orbit. This is the effect of seeing things move past you at different speeds, depending on how far apart they are from you. They believed then that the stars were stuck on a sphere and were all the same distance apart so there would be no parallax effect.

• They did not see that the concept of anything other than the Earth at the center of the universe was a natural thing so they rejected it on philosophical grounds. Ptolemy was an ancient astronomer who espoused the geocentric concept of the solar system. We will talk more about him in a minute as we explore how the views on astronomy changed over time. In 272 AD, however, the great storehouse of ancient material in the ancient world the library of Alexandria burned to the ground. This destroyed much of what was known and understood about astronomy prior to that time.

This is when a kind of Dark Ages developed and when few other theories came forth until the time of the Renaissance.

PTOLEMY AND PTOLEMAIC ASTRONOMY

Ptolemy was considered an ancient astronomer, living in Alexandria in Egypt at around 200 AD. His main contribution to astronomy was to provide a mathematical basis for the geocentric theory of the solar system. The basis of his initial work came from the ideas of Aristotle. His body of work included a thirteen book series called the Almagest, which were designed to explain the different astronomical concepts known about at the time. Some of it was also based on the star charts of Hipparchus, who also named the different constellations.

The first argument that Ptolemy made was that the earth itself must be an immovable object. He said that, because all things must fall toward the center of the universe and because on earth, all things fall to the ground, the earth must be the center of the universe. He also argued that, if the Earth spun, you would throw something upward and it would fall in a different spot, which was not observed.

According to Ptolemy, the Earth was the center, followed outward by the Moon, Mercury, Venus, Sun, Mars, Jupiter, and finally Saturn. There was, however, the observation that some planets appeared to go backwards some of the time a phenomenon called retrograde motion. This was explained using the idea that there were circles upon circles with planets undergoing small circles called epicycles within a larger circular orbit. Figure 3 shows these epicycles on a Ptolemaic model of the solar system:

Figure 3.

In this model, the larger circles around the earth were called deferents, while the small circles or epicycles moved around these deferents. He still believed that there were crystalline spheres that were where the different celestial bodies, including planets, existed as attached objects. He required the presence of 28 total epicycles to explain the way that the planets moved in his geocentric system. He also invented the “equant”, which was the center of a planet’s epicycle that moved with some degree of uniform angular velocity.

OPERNICUS AND THE COPERNICAN REVOLUTION

Things in astronomy were relatively stagnant after Ptolemy and the Ptolemaic theory was the official Roman Catholic Church doctrine for many centuries until the time of Copernicus, who lived from 1473 to 1543 in Poland. He is credited with reinventing the heliocentric theory of the solar system and was successful in doing this, largely because he published his data and because it came at a time when the Church’s influence on Northern Europe was weaker.

This was when physicists were discovering things like inertia in movement, which could not be explained if the earth did not move. In addition, the idea that the Earth rotated was a better explanation for the apparent diurnal motion of the stars. The heliocentric theory also did not require so much effort to explain the phenomenon of retrograde motion of the planets. This shift to heliocentrism was called the Copernican revolution. Copernicus believed that the earth was just the center of the local gravity we experience and was the center of gravity for the Moon as well. The also proposed that the Sun was the center of our solar system and that all of the planets rotated around it. This meant that retrograde motion was caused by the earth’s orbit rather than any true planetary motion change.

One of the mistakes that Copernicus made was that he assumed that the rotation of the planets were completely circular, when they are in fact elliptical. He also needed to use some creative mathematics to explain the phenomena seen when circular motion is assumed. He needed to produce complicated tables to explain his theories and didn’t put the entire thing together until his final work was published while he lay dying, called “On the Revolutions of the Heavenly Spheres”. He ultimately did have to use the concept of epicycles and deferents to explain the thing he noted.

Based on Copernican theory, with the sun as the center of the solar system, the planets were now arranged in the way they are known to be: Mercury, Venus, Earth and its Moon, Mars, Jupiter, and Saturn, which are not all of the known planets because Uranus had not been identified at the time. The stars were felt not to be a part of the

solar system at all and do not rotate anywhere in relation to it. The earth was felt to rotate once in 24 hours so that retrograde motion was just an illusion. Retrograde motion and the new order of the way planets rotate could explain why some planets seem brighter at some times and not as bright at other times. When they are further from us, the planets will seem dimmer. In addition, retrograde motion was explainable using geometric principles.

Even though the rotation of earth explained epicycles, he could not get around using them anyway. This was because he couldn’t get past the “perfect circle” revolution theory. Because he thought the planetary revolutions around the sun were perfect circles, epicycles had to be used to justify the different phenomena seen. The Copernican revolution happened largely after Copernicus died. It is believed that he did not publish his book until he was on his deathbed because of the fear of retribution by the Church he faced. The Church at the time still believed in Aristotle and in Ptolemy so it was a huge thing to challenge these long held belief systems.

Besides the issue of heliocentrism and circular versus elliptical orbits, Copernicus had to decide if the planets were made of the same basic stuff that makes up planet earth. Prior to this, it was thought that the other planets were made of some type of special or different substances than earth so this needed to be challenged as well.

While Copernicus was able to make his ideas stick, they weren’t much different from the heliocentric concepts put forth by Aristarchus on the island of Samos off of Turkey. He believed in the rotation of the earth as well as is rotation around the sun itself in 200 BCE. Most of his writings were either lost or discounted by the influence of Aristotle. There were some “commonsense” issues he couldn’t explain based on what he knew. He couldn’t explain why things didn’t fly off of earth as it spun around or why birds didn’t just get left behind as it moved around the sun. He also didn’t understand why there wasn’t a parallax effect. The reason there is no parallax effect with the stars is that they are so far away that it just isn’t something that can be easily seen. There is a parallax effect but it isn’t big enough to notice. These were the main reasons why Aristarchus was not believed.

According to Copernicus, the earth has three separate motions. It rotates every 24 hours to make the day and night. It revolves around the sun in a revolution once a year. It also has annual tilting movement of the earth on its axis. The earth’s different motions, especially rotation, was the main explanation for retrograde motion. He also proposed that the distance between the sun and earth is very small compared to the distance between the earth and any of the other stars in the universe.

The six book compendium that came out of his life’s work was originally proposed as a mathematical theory rather than a true astronomical one. This was an attempt to distance his theories from anything that remotely looked like a threat or challenge to the Church or to the Bible itself. It came at a time when the Church was trying to modify the Julian Calendar and it was said that the book could help do this.

TYCHO BRAHE AND HIS WORK

Most people don’t recognize Tycho Brahe as a famous astronomer. He lived in the late 1500s and was a serious observer of the phenomena in the sky. He was the main force behind the Danish Observatory, which sed sextants to study the skies because there were not telescopes then. He used these sextants in order to measure the different positions of the start and planets, creating a then modern database. He was a mathematician who used trigonometry to show that the sun was relatively far away compared to the moon. The idea of a moving earth was confusing to everyone at the time, including Brahe. The earth’s size had been known for a long time and it just seemed to be too big of an object to have any force strong enough to actually move it. Because he couldn’t detect any parallax, he had to assume that the stars were indeed very far away. He tried to estimate the size of stars but couldn’t get around the blurring of starlight in the atmosphere. This led to a projection of their size as being larger than is true.

Brahe also identified that there were differences in the way the inner planets rotate around the sun compared to the way the outer planets rotate around the sun. He went awry when he decided that there was a hybrid condition where some planets orbited the

sun, while others orbited in a more geocentric way. He could not himself decide that the earth was a moving thing.

The totality of Brahe’s contributions to astronomy were these:

• He made the most precise astronomical observations to date using his sextants.

• He provided input on the movement of planets that helped Kepler modify and create our current solar system depiction.

• He was able to carefully observe a supernova or exploding star but did not know what it was. It was thought at the time to be a “new star” rather than a “dying star”.

• He was able to carefully observe a 1577 comet and was able to measure the parallax effect for it. This proved it was further from the earth than the moon and not an atmospheric phenomenon.

• He determined there was no parallax with stars so he felt the earth was motionless or that the stars were too far away to measure their parallax. His final conclusion was that the earth didn’t move.

• He proposed a model for the universe that was a hybrid between the heliocentric and geocentric models.

LAWS OF PLANETARY MOTION

Johannes Kepler of the 1600s was a student of Tycho Brahe so he was able to use the data collected by the earlier astronomer in order to create his Laws of Planetary Motion. These were the laws every astronomer was missing because it proved the heliocentric theory but did not require the falsities of epicycles to explain what phenomena were being seen. He used the idea of elliptical orbits rather than that of circular orbits around the sun.

Kepler’s laws require a great deal of mathematical understanding. Orbital motions about the earth required several different key terms. The concept of the perihelion or the point where the earth or a planet is closest to the sun is offset by the aphelion, which is the

point where the Earth’s orbit is furthest from the sun. The aphelion does not necessarily happen in wintertime and the perihelion is not necessarily a summertime thing. In fact, it is the opposite of what you think. Figure 4 describes these states:

Figure 4.

Other terms to know are the apogee and the perigee. These relate to the lunar orbit. The perigee is when the moon is closest to the earth, while the apogee is the point when the moon is the furthest from the earth.

By the end of the 1600s, Johannes Kepler had determined the three laws of planetary motion. He used Brahe’s observations to make theses laws and was able to prove an entire heliocentric system. The laws themselves are still considered valid but his explanations have largely been discounted. The three laws briefly are these:

1. The planetary motions occur in an elliptical fashion around the sun.

2. An imaginary line from the sun’s center to the center of a planet will sweep the same area in the same amount of time spent in the movement.

3. The ratio of the squares of the periods of any two planets is the same as the cubic ratio of their average distances from the sun.

Let’s look at these laws in detail, which are described in figure 5:

Figure 5.

The Law of Ellipses is Kepler’s first law. It described an elliptical rather than circular orbit around the sun. There are two central foci that determine the ellipse’s shape. If they are close together, the ellipse will look more and more like a circle rather than an elongate ellipse. The sun is always one of these foci.

The Law of Equal Areas describes the speed at which a planet moves about the sun. This is not a static thing but always changes. The planet closest to the sun moves faster while the planet further from the sun moves slower. When you draw a line from the sun to any two points traveled over the same period of time, the area within this triangular area will always be the same.

The Law of Harmonies is Kepler’s third law. It looks at the orbital period around the sun and the radius of its orbit compared to the other planets in the solar system. It made a mathematical calculation that the ratios of the period squared to the average distances from the sun cubed are essentially the same for every planet around the sun. It was

further determined that this ratio holds even if the rotating object is not a planet but some other orbiting celestial body or satellite.

ISAAC NEWTON EXPANDS ON KEPLER

Isaac Newton lived in the late 1600s and was most interested in how mathematics could be applied to the physical sciences, such as astronomy. At the time, there was a lot of interest in planetary motion so this is what he studied from a mathematical perspective. He published his law of universal gravitation in 1687.

Kepler’s ideas on planetary motion were prevalent in Newton’s time and were accurate representations of how the planets move. Newton expanded on these by having three of his own laws, mainly related to motion and forces related to motion. He argued that these three laws applied also to astronomic or celestial bodies. These were Newton’s three laws explained:

• First Law of Motion this indicates that a body in motion stays in motion unless acted on by an outside force of some kind. Stationary objects also remain that way unless a force is applied and direction of movement will always be straight unless a force sidelines this straight motion in some way.

• Second Law of Motion this indicates that an object’s ability to accelerate is in proportion to the amount of force applied to it. If there is no force, the velocity will stay the same. There is constant acceleration in planetary motion which affects the direction of the planet’s movement. The gravity of the sun will force the planet in a path around it.

• Third Law of Motion this indicates that any force applied on another object will have an equal and opposite force applied back to the first object. When you sit on a chair, you exert a force that is directly opposed by the chair itself on your bottom. The sun in the sky feels the force of each planet but does not change much because of the large size differential between the two objects.

From all of these laws came Newton’s law of universal gravity. It considers both the mass of two objects as well as the distance between them. As the masses of the objects

increase, so does the force of gravity between them. As the distance between the two objects increases, the force of gravity will decrease. The only reason there is force between the sun and planets is because of the large masses involved.

GALILEO’S CONTRIBUTIONS TO ASTRONOMY

Galileo Galilei was an Italian astronomer and scientist who lived from 1564 to 1642 AD. He improved on the telescope and developed his own theories on motion. Rather than using mathematics, he was able to use observation in order to confirm the elliptical orbits of the planets in the solar system. His telescope showed the craters on the moon and indicated that the Milky Way was made of many different stars, which challenged the theories on the “perfection” of the universe. He also saw phases in the different planets and moons of Jupiter. The moons of Jupiter orbiting this planet also destroyed the geocentric theory of the solar system.

One of his most interesting experiments was the one where he dropped balls of different sizes and weights from a tall building. He found that all objects conformed to gravity with the same acceleration regardless of the size or mass of the object. He showed the parabolic fall of objects thrown in the air. He was fascinated with timekeeping and kept pendulum type clocks that he himself invented.

While he did not actually invent the telescope, Galileo was probably the first to use it effectively to study the skies. He also improved on the telescope constantly. His first experience with the telescope was in 1609 AD and started to make his own lenses. Through his telescopes, he was able to see things up to about 9 times magnification, which was better than had existed before then. He saw the rings of Saturn for himself. He found four of the largest moons of Jupiter, which are today called the Galilean moons. He was the first to see Neptune but did not know what it was.

Around 1616, there was still the position of the Church that the earth was still the center of the universe so, when this was challenged by Galileo, he was sent to Rome and told not to teach his theories. He persisted and published a mathematical approach to the idea of heliocentrism but was placed under house arrest because of what the Church felt was heresy.

ASTRONOMY AFTER THE TELESCOPE

The history of the telescope is interesting and dates back to 1608 in the Netherlands when an eyeglass maker named Hans Lippershey first tried to patent one. Galileo soon discovered this object and seized upon it, gradually improving on it so that it could be applied to astronomical study. In 1611, Kepler further modified the telescope with designs that made use of more than one lens and using lenses of different types.

THE FIRST OPTICAL TELESCOPES

Optical telescopes use a combination of lenses that pass through light. These were the first telescopes created. While lenses have been discovered dating more than 4000 years ago, it isn’t known what they were used for at the time. Many researchers in ancient times looked at the properties of lenses, including Ptolemy, who wrote a book called Optics, where he studied things like refraction, reflection, and color. They began to be used to correct nearsightedness in the latter part of the thirteenth century.

As mentioned, the first patent on the telescope was applied for by Hans Lippershey, who was an eyeglass maker in 1608 AD. Another Dutch instrument maker named Jacob Metius also applied for a similar patent a few weeks later. Neither patent was awarded because it was felt that these things already existed ubiquitously so that no patent was necessary.

These initial telescopes had a convex lens plus a concave lens put together so that the image was upright but that only had a three-times magnification. They were very popular and were made by many different people throughout the Netherlands shortly after their invention. Figure 6 shows what these telescopes looked like: Figure 6.

It is said that as soon as Galileo heard about the original Dutch spyglass, he went home and modified the design within a single day in order to have an instrument that was even more powerful. He soon had instruments that could provide him with a 23-times magnification but that was more than a meter long. It was with this type of instrument that he was able to see the many early astronomical discoveries he saw. The name of “telescope” was first coined based on Galileo’s instrument. They were referred to as Galilean telescopes. Further refinements were made using two convex lenses. The first one of this kind was proposed by Kepler in 1611 but wasn’t commonly used for many year’s later. These were called Keplerian telescopes; they could be used to confirm precise mathematical calculations sometimes made by astronomers. One such telescope was able to see the moon Titan of Saturn in 1655. Figure 7 shows Kepler’s telescope and some of the other types:

Figure 7.

The problem with these types of telescopes are that they needed to be very long in order to have high magnification levels, which makes them difficult to use. Scaffolds or masts had to be used to make use of these telescopes. Telescopes as long as 150 feet were wieldy and often collapsed if perturbed in any way.

After the need for stronger telescopes became necessary, some telescopes were created that did not have tubes at all. Part of the instrument was hung on a tree or pole, while the eyepiece was held in the hands or was mounted on some type of stand. These were known as aerial telescopes and were up to six hundred feet in total length. They were effective but hard to use.

Reflecting telescopes were the next kind to be developed. They were difficult to perfect at first because the image was often blurry but they used concave mirrors in order to create an enlarged image. James Gregory created what he called the Gregorian telescope in theory but could not put it into practice because he did not have the technical skill to do this.

Isaac Newton used the theories of the Gregorian telescope to create his own version, called the Newtonian telescope in 1672. This used refraction and mirrors to create a reflecting telescope out of copper and tin. The telescope of this type he presented to the Royal Society of London was a magnification of 38-times power. This type of reflecting telescope was even further modified by Laurent Cassegrain to make what was called the Cassegrain reflector in 1672. In this telescope, there was a secondary mirror that further reflected light back to the hole you look through.

After about 50 years, John Hadley was able to perfect the design of reflecting telescopes by using precision parabolic mirrors made of polished metal. Some of these were able to be constructed at the time that were about eight feet in total focal length. The downside of these mirrors were that they reflected light poorly and tarnished easily.

Then came what were called “achromatic refracting telescopes”. The problem that needed correcting was that light passing through a lens will bend according to the color frequency, which made the image blurry and inaccurate. This led to ideas, such as creating more than one lens that would put light that had separated out into the color spectrum back into a single color again. These achromatic refracting telescopes were

first made in the early 1700s. They were short and compact, and were able to put some of the colors into one focal area rather than spread out into the color spectrum.

LARGE REFLECTING TELESCOPES ARE MADE

Telescopes that were large and reflective in nature were not possible until it was discovered how to deposit a layer of silver onto a glass telescope mirror. This was invented around 1856. The advantage was that, if they became dull, a new layer of silver could be deposited on the same mirror. This heralded the modern age of telescopes by the turn of the Twentieth Century. The famous 60 inch Hale telescope was created at a high altitude location in 1908. Figure 8 shows this Hale telescope dome:

Figure 8.

In 1948, a larger 200 inch Hale reflector telescope was made on Mount Palomar. This was the largest telescope in the world until a Russian telescope was made that was slightly bigger several decades later.

In the 1980s, the era of adaptive optics began, where image analyzers were used to sense where aberrations in the image were seen. These aberrations could then be corrected in order to reduce blurring of the image. By the 1990s, giant telescopes were created using adaptive or active optic techniques. Some of these include two Keck telescopes, two Gemini telescopes, and a large binocular telescope. They involve technology that accounts for distortions in real time in order to create a very clear image.

NON-OPTICAL TELESCOPES

There are other types of telescopes that have been invented that do not use light waves in the visible spectrum. These are based on the idea that objects in the sky also give off electromagnetic radiation that is out of the range of visible light waves. Telescopes have been developed since the time of World War II that involve waves from the radio wave to gamma wave frequencies. Figure 9 shows the electromagnetic spectrum so you can see that the range of visible light is very small: Figure 9.

Radio telescopes first became a possibility in 1931 when it was discovered that the Milky Way Galaxy emitted radio waves. This led to the first radio telescope, built in 1937. The dish was more than 30 feet in diameter; It was able to identify radio waves coming from

the sky but researchers could not identify the source of the waves. Still larger dishes were created, including the 1000 foot Arecibo telescope, built in 1963. This is shown in figure 10 as a depression that fits into the ground:

Figure 10.

There are also telescopes that use microwaves, which have a greater frequency. Some of these must be deployed into space, however, because of the fact that the signal is greatly weakened by the water vapor in the atmosphere.

Infrared radiation is usually absorbed by the atmosphere but telescopes using these wavelengths can be used high in the mountains where there isn’t much water vapor in the atmosphere. The telescopes at the top of Mauna Kea in Hawaii are this kind of telescope. An IRAS satellite telescope was launched that also looked at infrared waves for about 9 months in 1983 until its coolant ran out. Still, it was able to survey the entire sky and found about 245,000 different infrared sources.

Ultraviolet telescopes run in the high visible light frequency range. Most of this must come from satellite based telescopes because the ozone layer will absorb a lot of this UV radiation. These must be coated with magnesium fluoride or lithium fluoride instead of silver or aluminum. One of these, the International Ultraviolet Explorer launched in 1978 was able to survey the skies for 18 years.

There are x-ray telescopes that also must be deployed on satellites to be used above the level of earth’s atmosphere. These types of telescopes were first attempted to be utilized in 1948 using sub orbital rockets. They were able to see x rays coming from the sun as well as from galactic sources, including the Crab Nebula. There are several x ray telescopes of this type in play in recent years for the study of astronomical bodies.

Gamma rays are the highest frequency waves that are easily absorbed by the Earth’s atmosphere so they must be used on satellites deployed into space. These telescopes were first launched in 1967. There are certain Cerenkov radiation imaging telescopes that detect very high energy gamma waves from the ground that exist around the world.

• There has been interest in celestial objects since prehistoric times.

• Many of the early understanding of the skies was related to mystical and religious thinking.

• One of the first civilizations to document astronomical study was the Babylonians.

• All of the major ancient cultures studied astronomy in some way and for different reasons.

• Ptolemy was an early astronomer who first proposed and solidified the geocentric theory of the solar system.

• It took many centuries for Copernicus to have the theory of a heliocentric solar system and to have that stick as a prevailing theory.

• The telescope was invented around 1600 and was first based on lenses and optics.

• Reflecting telescopes use mirrors instead of lenses. They have become more powerful now that the mirrors are coated with things like silver rather than just made of a metallic substance that could tarnish over time.

• Many of the telescopes in use today are not based on the visual spectrum of electromagnetic waves but collect waves all along the electromagnetic spectrum.

CHAPTER ONE QUIZ

1. When did the study of astronomy, even casually, begin?

A. Prehistoric times

B. Babylonians in around 1600 BCE

C. Native Americans in around 1000 BCE

D. Ancient Greeks in 400 BCE

2. Which civilization probably started documenting astronomical information first?

A. Greeks B. Babylonians

C. Egyptians

D. Persians

3. What was the main accepted paradigm in the Copernican Revolution?

A. That the earth rotated on a tilted axis.

B. That there was no such thing as true retrograde motion of the planets

C. That the planets as listed prior to that were out of order.

D. That the sun was the center of the solar system.

4. According to the Copernican theory, what best explains the phenomenon of retrograde movement of the planets?

A. Planets rotate on an axis.

B. Planets undergo actual retrograde movement at times.

C. The rotation of earth gives the illusion of retrograde movement.

D. The planets participate in epicycles as they rotate around the sun.

5. Why couldn’t Tycho Brahe determine the size of stars accurately?

A. They were too small to detect.

B. They were too far away.

C. Their light is diffused too much in the atmosphere.

D. There is too great a parallax effect.

6. What finding did Tycho Brahe get right based on his observations of the planets and stars?

A. That the comets exist outside of the atmosphere.

B. That the geoheliocentric model of the solar system was accurate.

C. That the earth was nonmoving.

D. That the stars were very far away.

7. Which of Newton’s laws most discusses the issue of the acceleration of an object?

A. Newton’s first law

B. Newton’s second law

C. Newton’s third law

D. Newton’s law of universal gravitation

8. What did Galileo not discover as part of his observations of the skies?

A. The contents of the Milky Way.

B. The moons of Jupiter.

C. The geocentric model of the solar system.

D. The elliptical orbits of the planets.

9. Which telescopes have the greatest resolution in detecting sources of electromagnetic radiation?

A. Those that are satellite-based

B. Those that are reflecting telescopes

C. Those that are small rather than large

D. Those that detect radio waves

10. Because of the absorption of waves by the atmosphere, which type of telescope is most likely to need to be deployed onto a satellite?

A. Radio wave telescope

B. Microwave telescope

C. Infrared telescope

D. Gamma ray telescope