The Discovery of the Universe BC Aristotle 85 – 165 AD Claudius Ptolemy 1473 – 1543 AD Nicolaus Copernicus AD Tycho Brahe AD Johannes Kepler AD Galileo Galilei AD Isaac Newton

Plato & Aristotle by Raphael ( ) The School of Athens (1511)

Alexander’s Empire 320 B.C.

EratosthenesEratosthenes (220 B.C.) Using a deep well in Syene (nowadays Aswan) and an Obelisk in Alexandria, he measured the angle cast by the sun at noonday in midsummer at both places. He measured the sun to be vertical in Syene and making an angle equal to 1/50 of a circle at Alexandria he measured the circumference of the earth to be 25,000 miles.

Empedocles( B.C.): Greek Theory of Matter

Euclid (325 – 270 BC) Proved only five solid shapes can be made from simple polygons (square, triangle, hexagon) Plato revised the four element theory with five elements, adding quintessence or aether.

The Universe According to Aristotle – 399 BC Socrates 427 – 347 BC Plato Plato suggested that man was born with knowledge. (Rationalism: knowledge comes before experience) Aristotle argued that knowledge comes from experience. (Empiricism: knowledge comes after experience) Aristotle relied more on his perceptual senses to understand the universe and even performed a few experiments.

Aristotle’s Geocentric Universe The Earth is at the Center of the Universe The Sun, Moon and the planets move around the Earth on crystal spheres (the sphere was considered to be a perfect shape) The stars are on a single sphere that was just beyond the orbits of the planets. The universe remains unchanged since it was created.

Aristotle’s Geocentric Universe This view of the universe is supported by observational evidence (our senses show that the Earth appears to be stationary). –Air, clouds, birds, and other things unattached to the ground are not left behind as they would be if the Earth was moving. –There is no steady wind created by a spinning Earth. 2. The Universe was thought to be relatively small. After all Icarus was able to fly too close to the Sun (thought to be the size of a shield and just beyond the range of a good archer).

Aristotle on Motion There is a tendency for objects to remain at rest unless disturbed. Motion occurs as: –Natural motion in which objects moved naturally towards the center of the Earth and the Universe. –Violent motion in a direction not towards the center of the Earth. Violent motion requires the application of a force. Objects on Earth move in straight lines. Objects in the Heavens move in circles (Heavenly motion). Objects fall through the air at a speed proportional to their weights.

Problems with the Aristotelian or Geocentric Model..... Retrograde Motion of the Planets: Every once in a while, a planet will slow down its motion, will appear to "stop" for a short while, and will then start moving in the opposite direction, to the west. Eventually, it will stop again, and resume its movement towards the east. The phenomena of retrograde motion is unexplained by the Aristotelian model.

The Lack of Parallax When viewing stars from an orbiting Earth, parallax should be evident when viewing the same star from different parts of the orbit. The lack of parallax is evidence of a nonorbiting Earth and a geocentric universe.

Aristarchus of Samos ( BC) Aristarchus was the first to propose a heliocentric universe, larger than any of the geocentric universes proposed by his predecessors. Aristarchus explained the lack of observed parallax by postulating that the stars were infinitely far away. From a lunar eclipse, he concluded that the radius of the Moon was 0.5 times the radius of the Earth (actually 0.28 times). Measured the Moon's angular diameter to be 2° (later 0.5°) Calculated the Earth -Moon distance to be Earth radii (actually 60.4). By noticing that the Sun and Moon have equal angular diameters during a solar eclipse, he calculated that the distance to the Sun was 19.1 times the distance to the Moon (actually 390 times).

Aristarchus introduced six hypotheses, from which he determined first the relative distances of the sun and the moon, then their relative sizes: 1) The moon receives its light from the sun. 2) The earth is positioned as a point in the center of the sphere in which the moon moves. 3) When the moon appears to us halved, the great circle which divides the dark and bright portions of the moon is in the direction of our eye. 4) When the moon appears to us halved, its [angular] distance from the sun is then less than a quadrant by one-thirtieth part of a quadrant. (One quadrant = 90 degrees, which means its angular distance is less than 90 by 1/30th of 90, or 3 degrees, and is therefore equal to 87 degrees.) (This assigned value was based on Aristarchus' observations.) 5) The breadth of the earth's shadow is that of two moons. 6) The moon subtends one fifteenth part of a sign of the Zodiac. (The 360 degrees of the celestial sphere are divided into twelve signs of the Zodiac each encompassing 30 degrees, so the moon, therefore, has an angular diameter of 2 degrees.)

Knowledge of the Universe at the End of Classical Antiquity The earth is a sphere (Pythagoras, Aristotle, Ptolemy) The diameter of the earth is approximately 8,000 miles (Eratosthenes) The moon shines by reflected light of the sun; solar eclipses are due to the moon covering the sun, while lunar eclipses are due to the earth’s shadow covering the moon. (Anaxagoras) The diameter of the moon is approximately 1/4 the diameter of the earth (i.e.~2,000 miles) (Aristarchus, Hipparchus) The moon is approximately 60 earth radii away (~240,000 miles) (Aristarchus, Hipparchus) The Sun is very far away (20 times the distance to the moon) (Aristarchus). The number is wrong, but it suggests that the solar system is very large. The Sun is very large compared to both the moon and the earth (20 times the moon, 6 times the earth). (Aristarchus) Again, the wrong figure, but qualitatively correct. The earth is very small compared to the universe (“as a point when compared to the celestial sphere”). (Aristotle, Ptolemy) The “equinoxes” (the intersection between the ecliptic and the celestial equator) are precessing (slowly changing position). (Hipparchus)

Claudius Ptolemy

Claudius Ptolemy improved the Aristotelian model to account for the retrograde motion of the planets. Ptolemy reasoned that the retrograde loop was caused by the planet moving in a circular epicycle as it moved in a circular orbit. Epicycles preserved the geocentric nature of the model and kept the planets moving in “perfectly circular” orbits. Ptolemy's system involved at least 80 epicycles to explain the motions of the Sun, the Moon, and the five planets known in his time.

Ptomely’s Epicycles

The Ptolemaic model accurately predicted the motions of the planets and the Sun and the Moon that it remained the preferred model among astronomers until the mid 1600’s......

End of part 1a