1. The History of Astronomy Part Two Aristarchus, Parallax And the size of the Earth

2. Aristarchus ca.310-230BC Aristarchus through careful observation he calculated: 1.That the Moon was about 1/3 (.35) the size of Earth ( It is.27, pretty close!) 2.That the Sun was 20x farther away than the Moon (It’s really about 387x farther away) 3.As the sun looks the same size as the Moon it must be really larger 7x larger than Earth (It’s really 110x larger than Earth 4. That the planets including Earth orbited the Sun.

3. Parallax Parallax is the apparent motion of an object due to the motion of the observer

4. Against Aristarchus Both Religious and Scientific Arguments were made against Aristarchus’ Heliocentric (Sun-centered) model Religious: He was tried for impiety for proposing that the Earth wasn’t at the center of the universe. (He was found innocent) Scientific: There was no sign of stellar parallax which should occur if the Earth moved. Thus he was proved wrong! The Geocentric Model was for the next 2000 years believed to be true

5. Eratosthenes (276-195BC) was the first person to accurately calculate the size of the Earth. used the shadow of an obelisk in Alexandria to find the angle of the Suns rays at Alexandria on the Summer Solstice (82.8°) At Syene, in southern Egypt on the same day, he knew that the Sun shone straight down a well thus the angle there was 90º

6. The difference in the angles was about 7.2° or 1/50 th of a circle Thus the distance between Alexndria and Syene must be 1/50 th of the Earth’s Circumference. A little multiplication gave him the Circumference to be about 25,000 miles, very close to its true size.

7. Columbus and the size of the Earth Using an incorrect number for the size of the Earth (from Strabo) of 18,000 miles… Columbus thought the spice islands where 7000 miles closer than they were. If it hadn’t been for the Americas Columbus and his men would have starved as some of Isabella’s advisors had warned they would!