The Early Period

1. The Early Period The Beginnings of Classical Astronomy

2. Early Ideas • The Ancient Greeks were the first to try and explain the workings of the heavens in a systematic manner • They used models and observations made only with the naked eye • They also used what they knew about logic and mathematics, especially geometry

3. The Shape of the Earth • Ancient Greeks knew the Earth was round • Pythagoras taught that the Earth was spherical because the believed that a sphere was the perfect shape and this would have been how gods would have created the Earth with the perfect form

4. The Shape of the Earth • Aristotle argued for a spherical Earth based on his observations • He noticed that the shadow cast during a lunar eclipse was curved • He also noticed that if one traveled south they would see stars that were previously hidden below the southern horizon

5. The Size of the Earth • Eratosthenes, head of the library at Alexandria made the first measurements as to the size of the Earth • He devised an experiment to determine the circumference of the Earth

7. The Size of the Earth • After determining the angles Eratosthenes knew that the distance from Alexandria to Syene was 5000 stadia (1 stadia = .1 miles) • The angle was 7° and therefore 7/360 of the Earth’s circumference (about 1/50) • 50 x 5000 stadia = 250,000 stadia or 25,000 miles (Actual = 24,901.55 miles) (24,859.82 miles at the poles)

8. Distance and Size of the Sun and Moon • 75 years before Eratosthenes Aristarchus of Samos had estimated the relative sizes of the Earth, Moon, and Sun and the relative distances from Earth to the Moon and Sun

9. Aristarchus of Samos • Calculated Moon’s diameter to be about 1/3 that of Earth based on eclipse data • Sun was 20 X further away from the Earth than the Moon was • Sun’s diameter was 7 X that of Earth • He also gave thought to the idea that the sun was the center of the heavens, an idea that wasn’t accepted for another 2000 years

10. Aristarchus of Samos • Others argued that if the Earth moved around the sun the positions of the stars should change during the course of the year something called parallax • They did not observe any parallax (it wasn’t successfully measured until 1838)

11. The Motion of the Planets • Geocentric Theory: Earth as the center of the universe • Those bodies that move the fastest are closest to the Earth • Model didn’t explain retrograde motion • Retrograde motion was explained by the planets have two motions

13. Ptolemy • Planets moved on one small circle called the epicycle and one large circle • Helped explain retrograde motion • Still were problems • Models became more and more complicated • Theory survived until the 1500’s

14. Occam’s Razor • A British philosopher • The simplest explanation is probably the correct one • Astronomical models had become unbelievable complicated

15. Contributions from Islam • Used celestial observations to set their religious calendar • Many Arabic words in use today: zenith, the names of the bright stars • They revolutionized mathematical techniques through innovations like algebra • Gave us the Arabic numerals, and the concept of zero

16. Contributions from the Asians • Kept detailed records of unusual celestial events like eclipses, comets, and exploding stars • Were the first to note dark spots on the sun which we now know were sun spots • They became proficient at predicting eclipses

17. Measuring the diameter of Astronomical Objects • Angular size changes inversely with distance • L= 2πDA 360 L= Diameter of the body D= Distance to the body A= Angular Size