Earth Science 22.1 Origins of Astronomy

1. Earth Science 22.1 Origins of Astronomy Origins of Astronomy

2. 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

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.