Earth Science 22.1 Origins of Astronomy

Origins of Astronomy

Earth Science 22.1 Origins of Astronomy

Earth is one of many planets and many smaller bodies that circle the sun.

The sun is part of a much larger family of perhaps 400 billion stars that make up our galaxy, the Milky Way.

In turn, there are billions of galaxies within this universe. The Milky Way Galaxy

Ancient Greeks:

Astronomy is the science that studies the universe.

It deals with the properties of objects in space and the laws under which the universe operates.

The “Golden Age” of early astronomy (600 B.C.-A.D. 150) was centered in Greece.

The early Greeks used philosophical arguments to explain natural events.

The Greeks also relied on observations.

Beginnings of Astronomy

The Greeks used instruments such as the astrolabe, shown at right, to track the position of the sun, moon and stars.

The Greeks developed the basics of the mathematical sciences; geometry and trigonometry.

They in turn used mathematics to help them determine the sizes of the sun and moon and to determine the distances to them from Earth.

Origins of Astronomy: Aristotle

The famous Greek philosopher, Aristotle (384-322 B.C.), concluded that Earth is round because it casts a curved shadow on the moon when it passes between the sun and the moon.

Aristotle’s belief that the Earth is round was largely abandon in the Middle Ages.

Aristotle 384-322 B.C.

Origins of Astronomy: Eratosthenes (276-194 B.C.).

The first successful attempt to establish the size of the Earth is credited to Eratosthenes (276-194 B.C.).

Eratosthenes observed the angles of the noonday sun in two Egyptian cities, Syene and Alexandria.

Origins of Astronomy: Eratosthenes (276-194 B.C.).

Finding that the angles differed by 7 degrees, or 1/50th of a circle, he concluded that the circumference of the Earth must be 50 times the distance between the two cities.

The cities were 5000 stadia (157.6 meters) apart. This would make Eratosthenes calculation of Earth’s circumference about 39,400 kilometers.

This measurement is very close to our modern day knowledge putting the earth’s circumference at 40,075 kilometers.

Origins of Astronomy: Hipparchus

Probably the greatest of the early Greek astronomers was Hipparchus, best known for his star catalog.

Hipparchus determined the location of almost 850 stars, which he divided into six groups according to their brightness.

Hipparchus

Geocentric Model:

The Greeks believed in a geocentric universe, in which Earth was a sphere that stayed motionless in the center.

In a geocentric model, the moon, sun, and the known planets all go around the Earth.

Geocentric Model

Geocentric Model:

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

Beyond the planets was a hollow, transparent sphere on which stars travelled daily around the Earth. This was called the celestial sphere.

Geocentric Model

Geocentric Model:

To the Greeks, all of the heavenly bodies, except seven, appeared to remain in the same relative position to one another.

These seven wanderers included the sun, the moon, mercury, Venus, Mars, Jupiter, and Saturn.

Each was thought to have a circular orbit around the Earth.

The Greeks were able to use this model to explain all the movements of the heavenly bodies they observed.

Geocentric Model

Heliocentric Model:

Aristarchus (312-230 B.C.) was the first Greek to propose a sun-centered, or heliocentric, model of the universe.

In the heliocentric model, Earth and the other planets orbit the sun.

Heliocentric Model

Heliocentric Model:

Aristarchus used geometry to calculate the relative distances from Earth to the sun and from Earth to the moon.

He later used these distances to calculate the size of the sun and moon.

Heliocentric Model

Heliocentric Model:

Aristarchus theory was closer to the truth but he came up with distances that were much too small compared to what we now know.

He did however determine that the sun was many times more distant than the moon and many times larger than the Earth.

Although there was evidence to support the heliocentric model, the Earth-centered geocentric model dominated western beliefs for nearly 2000 years. Heliocentric Model

Ptolemaic system:

Much of our knowledge of Greek astronomy comes from Claudius Ptolemy.

In A.D. 141, Ptolemy presented a model of the universe that was called the Ptolemaic system.

The accuracy with which his system predicted the motion of the planets allowed it to go unchallenged for nearly thirteen centuries.

Ptolemaic System

Ptolemaic system:

Just like the Greek model, the Ptolemaic system had planets moving in circular orbits around a motionless Earth.

However, the motion of the planets against the backdrop of stars seemed odd.

Ptolemaic system:

Each planet, if watched night after night, moves slightly eastward among the stars.

But periodically, each planet appears to stop, reverse direction for a time, and than resume an eastward motion.

Ptolemaic system:

This apparent westward drift is called a retrograde motion.

This rather odd apparent motion, we now know, results from the combination of the motion of the Earth and the planet’s own motion around the sun.

Ptolemaic system:

Ptolemy explained retrograde motion by saying that planets moved along smaller circles, which in turn moved along their orbits around the Earth.

He called these smaller circles epicycles.

Ptolemaic system:

Ptolemy’s theory, we now know, was incorrect; the planets do not orbit the Earth.

Yet his theory was able to account for the planet’s apparent motions for many centuries.