HISTORY OF ASTRONOMY

Early Astronomy

Astronomy is the science studying outer space and the universe.

The “Golden Age” of astronomy was centered in Greece.

Early Astronomy

The Greeks took measurements of distant objects such as the Sun and the moon.

Famous Greek philosopher, Aristotle, concluded Earth is round because it always cast a curved shadow on the moon.

Another Greek astronomer, Hipparchus, determined the location of almost 850 stars.

Aristotle (384-322 B.C.)

Hipparchus (190-120 B.C.)

Geocentric Model

The Greeks believed in a geocentric universe, in which the known planets and the Sun revolved around the Earth.

The path of an object as it goes around another object in space is called an orbit.

Beyond the planets was an invisible sphere on which the stars traveled daily around Earth celestial sphere

The Greeks attempted to explain the movements of all celestial bodies in space by using this geocentric model.

Ptolemaic System

The Greek astronomer, Ptolemy, presented a geocentric model of the universe called the Ptolemaic System, with fixed stars in the background.

Although Ptolemy’s theory was wrong in that the planets do not orbit Earth, it was able to account for the planets’ apparent motions, which he called epicycles.

Ptolemy (90-168 A.D.)

Geocentric Universe

The idea that the earth is at the center of the universe and all things fall around her is the simplest and longest lasting universal view that we have had.

What the Geocentric Universe is made of

Sun Moon Stars 5 Planets

Mercury Venus

Mars Jupiter Saturn

How did they discover the existence of these planets?

Assumptions about the Universe

1. The earth is the center of all motion.

2. The earth does not move.3. The earth is flat.4. Planets move in circular orbit.

5. Stars are immutable.

Heliocentric Model

The first Greek astronomer to propose a heliocentric universe, in which Earth and the other planets orbit the Sun, was Aristarchus.

Though much evidence was provided to support a heliocentric universe, the Earth-centered (geocentric) view dominated Western thought for 2000 years.

Aristarchus (312-230 B.C.)

The Birth of Modern Astronomy

After Ptolemy, very few advances were made in astronomy.

The first great astronomer to emerge after the Middle Ages was a man from Poland named Nicolaus Copernicus.

He believed the Earth was a planet, just like the other 5 known at the time, and supported the heliocentric model of the universe.

Copernicus (1473-1543)

Father of Modern Astronomy

The Birth of Modern Astronomy

After the death of Copernicus came a Danish astronomer, Tycho Brahe.

Brahe became interested in astronomy from viewing the astonishing effects of a solar eclipse.

Had an observatory built where he designed instruments in order to view and measure locations of celestial bodies. Still believed in a geocentric universe based on

his observations of stars.

Gained recognition for being extremely precise on his observations. Brahe (1546-1601)

Summary of Brahe's Contributions

He made the most precise observations that had yet been made by devising the best instruments available before the invention of the telescope.

His observations of planetary motion, particularly that of Mars, provided the crucial data for later astronomers like Kepler to construct our present model of the solar system.

The Birth of Modern Astronomy

Before Brahe died he hired an assistant, Johannes Kepler, who carried on and inherited all of Brahe’s works. Didn’t agree Brahe’s view of a

geocentric universe.

Discovered 3 Laws of Planetary Motion:

1. The path each planet takes around the Sun is oval-shaped and known as an ellipse.

Kepler (1571-1630)

The Birth of Modern Astronomy

2. Each planet revolves so an imaginary line connecting it to the Sun sweeps over equal area in equal time intervals. A planet travels faster when it is closer to the Sun and slower when it is farther from the Sun.

The Birth of Modern Astronomy

3. The square of a planet’s orbital period (the time it takes to orbit the Sun) is proportional to the cube of its average distance to the Sun:

The orbital period of revolution (P) is measured in Earth years. The planet’s distance (a) to the sun is expressed in astronomical units (AU’s), which is the average distance between the Earth and the Sun, approximately 150 million km. or 93 million miles.

P2 = a3

The Birth of Modern Astronomy

Galileo Galilei was a great Italian scientist during the Renaissance.

His most important contributions were his descriptions of the behavior of moving objects.

Everything prior to Galileo was studied and examined without a telescope.

He constructed his own telescope and used it to study the sky, making many important discoveries supporting Copernicus’s view of the universe.

Galileo (1564-1642)

Galileo: the Telescope & the Laws of Dynamics

Galileo Galilei (1564-1642) was a pivotal figure in the development of modern astronomy, both because of his contributions directly to astronomy, and because of his work in physics and its relation to astronomy.

He provided the crucial observations that proved the Copernican hypothesis, and also laid the foundations for a correct understanding of how objects moved on the surface of the earth (dynamics) and of gravity.

The Telescope

Galileo did not invent the telescope (Dutch spectacle makers receive that credit – Hans Leppershey), but he was the first to use the telescope to study the heavens systematically.

It is said that what Galileo saw was so disturbing for some officials of the Church that they refused to even look through his telescope; they reasoned that the Devil was capable of making anything appear in the telescope, so it was best not to look through it.

Sir Isaac Newton and the Unification of Physics & Astronomy

Sir Isaac Newton (1642-1727) was by many standards the most important figure in the development of modern science. Many would credit him and Einstein with being the most original thinkers in science development.

Sir Isaac Newton: The Universal Law of Gravitation There is a popular

story that Newton was sitting under an apple tree, an apple fell on his head, and he suddenly thought of the Universal Law of Gravitation. As in all such legends, this is almost certainly not true in its details, but the story contains elements of what actually happened.

Albert Einstein andthe Theory of Relativity

Motion is relative not absolute.

Other striking consequences are associated with the dependence of space and time on velocity: at speeds near that of light, space itself becomes contracted in the direction of motion and the passage of time slows. Although these seem bizarre ideas (because our everyday experience typically does not include speeds near that of light), many experiments indicate that the Special Theory of Relativity is correct and our "common sense" (and Newton's laws) are incorrect near the speed of light.

SUMMARY

Earth is the center of motion

Earth is does not move

Earth IS flat

Planets move in circular orbits

Stars are immutable

Galileo

Galilean satellites

Phases of Venus

Universal Law of Gravitation

Newton Laws of Motion

Einstein Theory of Relativity

KeplerLaws of Planetary Motion

Brahe Supernova