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 ( 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 B.C.

Origins of Astronomy: Eratosthenes ( B.C.).  The first successful attempt to establish the size of the Earth is credited to Eratosthenes ( B.C.).  Eratosthenes observed the angles of the noonday sun in two Egyptian cities, Syene and Alexandria.

Origins of Astronomy: Eratosthenes ( B.C.).  Finding that the angles differed by 7 degrees, or 1/50 th 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 ( 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.