A Brief History of Astronomy and Astrophysics

Kenneth R Lang

• Publisher: WSPC
• Year: 2018
• ISBN:
  9789813233850
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

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This book traces out the unfolding history of important discoveries in astronomy and astrophysics, and anchors our present understanding of the Universe within the findings and personalities of accomplished astronomers. They have used telescopes and instruments to extend our vision to places that cannot be seen with the unaided eye, discovered a host of unanticipated objects, found out how various parts of the night sky are related, and discovered that the Universe is larger, more complex, and older than has been previously thought. This comprehensive historical approach to the present state of astronomy is a unique aspect of the book.

--> Sample Chapter(s)
Introduction
1. The Earth Moves --> Contents:

• Introduction
• Everything Moves:
  • The Earth Moves
  • Gravity Guides Movement and Bends Space-Time
  • Motion within Matter
  • How Light Moves Through Space and Interacts with Matter
  • The Stars are Moving
  • The Universe is Expanding and Breaking Away
• Nothing Stays the Same:
  • Natural History of the Stars
  • How the Sun and Planets Came into Being
  • The Ways Stars Shine
  • The Paths of Stellar Life
  • The Ways Stars Die
  • Darkness Made Visible
  • Primordial Light
  • The Origin of the Chemical Elements
• References and Notes
• Author Index
• Subject Index

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--> Readership: Students and teachers, also suitable for the general public, together with astronomy enthusiasts. -->
Astronomy;Astrophysics;History0 Key Features:

• There are no books of modern astronomy that include the four unique aspects (historical development, motion and change, personal story-centric, and beliefs)
• Includes complete references and notes that can be used for further explorations by curious readers and students
• The general educated reader is going to find this of captivating interest

Part I

Everything Moves

1. The Earth Moves

“Hereafter, when they come to model Heaven
And calculate the Stars [planets], how they will wield
The mighty frame, how build, unbuild, contrive
To save appearances, how gird the Sphere
With Centric and Eccentric scribbled over,
Cycle and Epicycle, Orb in Orb.”

John Milton (1667)¹

A Universe that can be Counted on

Ancient astronomers must have looked out at the bright beacons of Mars and Venus with a sense of wonder and awe. These celestial vagabonds did not move with the stars. They crossed the sky in a regular pattern that might be used to predict when and where they would next appear. Our ancestors called them planetes, the ancient Greek word for “wanderers.” The ordered motion of the planets was surely set in place by some great power,² and when astronomers tried to describe their wandering movements science began.

As described by the Nobel-prize winning physicist, Robert Millikan, astronomy explains: “A Universe that knows no caprice, a Universe that behaves in a knowable and predictable way, a Universe that can be counted on; in a word, a God who works through law.”³

The geometrical models that were constructed to describe the planetary movements depended upon one’s perspective. About 2000 years ago, the Greco-Egyptian astronomer Claudius Ptolemaeus, provided a complex model involving circular motions in an Earth-centered Universe, which was the best description available for about 1400 years. Then the Polish astronomer Nicolaus Copernicus questioned the Ptolemaic model and set the Earth in motion around the Sun.

In the following century, Galileo Galilei brought the Heavens down to Earth by using the newly invented telescope to find otherwise unseen mountains and valleys on the Moon, a star-filled Milky Way, and four moons circling around Jupiter. Galileo’s contemporary Johannes Kepler removed the “perfect” circle from consideration of the planetary motions, and replaced it with elongated paths around the Sun. He discovered laws that would provide the foundation of Isaac Newton’s theory of universal gravitation.

Pure, Everlasting, Heavenly Music

In antiquity, it was thought that both the planets and stars move in heavenly circles about a central unmoving Earth. Their circular movement had no beginning or end, and would continue forever without change. After all, the Sun and Moon are circular in shape, and wheels move easily across the Earth’s ground because they are round.

The ancient Greek philosopher and mathematician Pythagoras of Samos has been credited with the idea that there is music in the spacing of the planets, which emit harmonious sounds related to their distances and speeds of motion. The nearest, slower planets were thought to emit a low sound; the distant faster ones produced a high sound.

Both Plato and Aristotle subsequently developed the concept of the music of the spheres. In his Republic, written around 380 BC, Plato advocated circular planetary motions at different uniform speeds in proportion to their distance from the central, spherical Earth.

In his De Caelo (On the Heavens), Plato’s student Aristotle provided a mechanism for the motion by attaching the known planets to seven rotating, crystal-like spheres with a common center, all in counter-rotation to an eighth swift, outermost sphere of stars. Such a stellar sphere would explain why the stars seem to slide across the night sky, and why travelers to new and distant lands see new stars as well as new people.

For Aristotle, the Earth was a place of decay and change, the home of our temporary and impure lives. Natural motions on Earth, as distinguished from forced motions there, travel in straight lines, and that motion always ends. A stone falls straight down and stops, a fire rises straight up and disappears, and every human journey ends. In contrast, the indestructible, pure and eternal planets and stars are in everlasting motion. They seem to last forever and never stop moving.

To the ancient Greeks, the outermost celestial sphere formed the edge of the observable Universe. This sphere contained the fixed stars that remained firmly rooted within the night sky without ever moving with respect to each other. They all moved together as the celestial sphere wheeled around the central Earth once every day. The planets were supposed to move in the opposite direction at a slower pace.

Following Plato’s suggestion, astronomers spent centuries trying to describe the observed planetary movements, their appearances, using circular motion at constant speed around a central Earth, but they never could reproduce the temporary backwards motion of Mars, known as a retrograde, or its faster and slower motions observed during different parts of its path in the sky.

In the second century AD, the Greek mathematical astronomer Claudius Ptolemaeus created a geometrical model that could “save the appearances” presented by the planets. Ptolemy, as he is known, worked in the fine library at Alexandria, Egypt, where he was able to consult the work of previous astronomers. He showed that the observed planetary movements could be described by a system of moving circles in motion around the Earth, like the gears of some fantastic cosmic machine. Each planet was supposed to move with constant speed on a small circle, or epicycle, while the center of the epicycle revolved on a larger circle whose center was displaced from the Earth. A planet in uniform circular motion about a center slightly offset from the Earth would appear to a terrestrial observer to be moving with varying speed, faster when it is closest to Earth and slower when further away.

With this complex arrangement, Ptolemy was able to use circles upon circles to reproduce and predict the apparent motions of the planets with remarkable accuracy. He succeeded so well that his model was still being used to predict the locations of the planets in the sky more than a thousand years after his death.

Then, in the mid-16^th century, the Polish cleric Mikolaj Kopernik, better known as Nicolaus Copernicus, set the Earth and other planets moving about a stationary, non-moving Sun.

Copernicus’ Vision of Sun-Centered Motion

Nicolaus Copernicus was born on February 19, 1473 in the city of Torún in the Province of Royal Prussia, a region of the Kingdom of Poland. His father was a merchant from the capital Cracow, and his mother, Barbara Watzenrode, was a member of a wealthy and powerful Torún family.

Upon his father’s death, when Copernicus was just 10 years old, his mother’s brother, Lucas Watzenrode, looked after his education and career. At age 19 he matriculated at the University of Cracow, where his studies included astronomy, mathematics, philosophy, physics, and the works of Aristotle and Ptolemy. Four years later, in 1496, Copernicus began a three-year study of law at the University of Bologna, Italy, a prominent European legal institution. Here he learned Papal decisions regarding authority, judgments, rights, and penalties within the jurisdiction of the Church, known as canon law. To round out his education, Copernicus then began a two-year study of medicine at Padua University in Italy, a leading faculty of medicine.

In 1503, at the age of 30, Copernicus returned home to join the staff of his Uncle Lucas, now the Catholic Bishop of Varmia, which was an area covering about five thousand square kilometers in the far northeast, Baltic coast of Poland, near Gdansk. He spent the next seven years as companion, secretary, and personal physician to his uncle, taking part in administrative, ecclesiastic, economic, and political duties that benefited from his education in canon law.

Before the end of 1510, Copernicus left service with his Bishop-Uncle, and moved to take up duties as the Canon of the Cathedral at Frombork. This town is located at the Vistula Lagoon on the Baltic Sea, far from the centers of European society, and it is in this remote location that the isolated genius would reside for most of his remaining 33 years. His life ended on May 24, 1543 at the age of 70, without ever being married or having children.

In Copernicus’ day, just about everyone thought that the Sun and stars were eternally wheeling about the immobile Earth, the center of the Universe. It certainly looked like the Sun was moving around the Earth and across the sky, and even in modern times people still say that the Sun rises and sets, to clock our daily rhythm. The distant stars were similarly thought to revolve around the Earth, and that was why they were seen moving across the dark night sky. Copernicus’ great insight was to place the Earth and other planets in uniform, circular motion around a central, stationary Sun.⁴

In the opening remarks of his Commentariolus, or Little Commentary, circulated around 1510, Copernicus stated that the heavenly bodies move with uniform speed in a “perfect” circle, as Plato and Aristotle had proposed. He then examined Ptolemy’s widely used planetary theories in which a planet moves on a small circle around a bigger one, but not really at uniform speed around any circle’s center. It only appears in uniform motion when viewed from outside the circle’s center, at an “equant” point chosen for that purpose.

Although Ptolemy’s theory was good enough to predict planetary motions and positions, Copernicus commented that: “A theory of this sort seemed neither sufficiently absolute, or complete enough, nor sufficiently pleasing to the mind,” and he therefore sought a more reasonable arrangement of circles that would “explain all the observed irregularities in planetary motion while keeping everything moving uniformly about its proper center, as required by the principle of perfect motion.”⁵ He accomplished this by setting the Earth free to revolve about the Sun, which became the immovable center of the Universe.

The Earth became just one planet among five others. They all whirled around the Sun, the source of our light and warmth. It was the daily rotation of the Earth that made the Sun apparently move across the daytime sky and the stars swing by at night.

In Copernicus’ Sun-centered model, the Earth and the five other planets visible to the unaided eye swung in the same direction, in uniform circular motion with a period of revolution that increased with the planet’s distance from the Sun. In order of increasing distance, they are Mercury, Venus, Earth, Mars, Jupiter and Saturn. [The more distant planets Uranus and Neptune were not discovered until 1781 and 1846, respectively, and that required the use of the telescope that was not invented until the early 1600s.]

As Copernicus noticed, the further a planet is from the Sun, the longer it takes the planet to complete a circuit around the Sun. This vision is conveyed in this extract from his Revolutions: “In no other way do we find a wonderful commensurability and a sure harmonious connection between the size of the orbit and the planet’s period of revolution.”⁶ [Copernicus used the relative planetary distances from the Sun expressed in terms of the Earth’s mean distance, the “common measure” of the Universe, but no one knew its precise value for an additional three centuries.]

How do the Planets Move?

So who was right? Does the Earth move around the Sun, or is the opposite true? There was no definitive observational test at the time.

Both Ptolemy’s Earth-centered motion and Copernicus’ Sun-centered one provided different explanations for the temporary backwards motion that had been observed for Mars (Fig. 1.1). It apparently looped back in the wrong direction for weeks at a time, seemingly disrupting its uniform progress across the night sky. The planet gradually came to a stop in its eastward motion, moved toward the west, and then turned around again and resumed moving toward the east. Jupiter and Saturn also displayed such a temporary reversed motion in the westward “retrograde” direction before continuing on in the eastward “prograde” direction.

Fig. 1.1. Retrograde loops This photograph shows the apparent movements of the planets against the background stars. Mars, Jupiter and Saturn appear to stop in their orbits, then reverse direction before continuing on — a phenomenon called retrograde motion by modern astronomers. (Courtesy of Erich Lessing/Magnum.)

In Ptolemy’s Earth-centered model, combinations of uniform circular motions explained these looping, retrograde paths of the planets. As mentioned in Copernicus’ Little Commentary, and emphasized in his Revolutions, the apparent backwards motions can instead be explained by the uniform motion of the Earth and other planets at different speeds around the Sun.

In Copernicus’ interpretation, planets moving at a slower speed than the Earth would sometimes appear to move ahead of Earth, and sometimes fall behind. During the relatively short time that the...

Table of contents

• Cover
• Halftitle
• Title
• Copyright
• Dedication
• Contents
• Introduction
• Part I. Everything Moves
• Part II. Nothing Stays the Same
• References and Notes
• Author Index
• Subject Index