The Birth of AstronomyLecture 3

1/24/2018

Fundamental Questions of Astronomy (life?)

• What is the shape of the Earth?

• How big is the planet we live on?

• Why do the stars move across the sky?

• Where is Earth in relation to the Sun?

• How do objects orbit the Sun?

• Why do objects orbit the Sun?

The Scientific Method

• Based on natural inquisitiveness and skepticism of human nature.

• Scientific Method:a. Make high quality observations of natural phenomenon

b. Come up with a theory that explains the observations

c. Use the theory to predict future observations

d. Make further observations to test the theory

e. Refine the theory, or if it no longer works, make a new one

You can prove a theory WRONG but not RIGHT

How do we know the Earth is round?

• Pythagoras (500 BCE), a philosopher and mathematician believed circles and spheres to be “perfect forms” and suggested that Earth should therefore be a sphere.

• Aristotle (384–322 BCE) explained that travelers who go south are able to observe stars that are not visible farther north, and the height of the North Star decreases as a traveler moves south.

• The Greeks also noted that as a ship leaves port, it eventually disappears from view.

• On a Flat Earth:• all stars should be visible in the sky, • the North Star should stay in the same spot in the sky and• ships leaving port should not slip over the horizon.

• Eratosthenes was able to even measure the radius of the Earth.

• He measured the projection of the Sun’s shadow at two different locations.

• At Syene, the sun projected the shadow straight down.

• At Alexandria, the shadow as projected at some angle he was able to measure.

• The Greeks had a good understanding of geometry and he was able to calculate the diameter of the Earth

Why do Stars move across the sky?

• Hipparchus (150 BCE), measured, as accurately as possible, the positions of objects in the sky, compiling a star catalog with about 850 entries.

• By comparing his data with previous 150 years of data, he noticed that the position of the North Star was changing.

• He concluded that the direction around which the sky appears to rotate or “wobble” changes slowly but continuously.

• If the sky is appears to be wobbling, then it must be something physical about the Earth that is slowly changing.

• This motion is called precession.

Antikythera Mechanism

• Greek analog computer for tracking stars (60-70 BCE)

• Has 37 gear wheels enabling it to:• follow the movements of the

moon and the sun through the zodiac,

• to predict eclipses,

• and even to model the irregular orbit of the moon

Where is the Sun in relation to the Earth?Ptolemy’s Model

• Ptolemy, last great Roman astronomer (140 AD), who lived in Alexandria.

•He compiled many Greek and Roman astronomical data (including Hipparchus), to create a predictive model of the planets and the Sun.

•Major goal of any scientific model!

• Ptolemy’s geometric representation of the Solar System put the Earth at the center of the universe. Geocentric Model.

Ptolemy’s Model: Retrograde Orbits

Planets generally move in one direction relative to the stars, but sometimes they appear to loop back.

This is called retrograde motion.

Retrograde orbits were one of the most difficult observations to incorporate into a theory

East West

Ptolemy’s Model: Epicyle

• Planets orbit on Deferent circles.

• Retrograde motion is due to epicycle of on deferent orbits.

• Ptolemy did not know that orbits are not circular, but elliptical.

• Deferent circles are not centered on Earth, but at Equant point.

• Each planet has a different Equant point.

Ptolemy’s geocentric model (A.D. 140)

Retrograde Motion Explained

Where is the Sun in relation to the Earth?Copernicus

• Europeans rediscovered many old Greek scientific texts that Ptolemy and others wrote.

• Copernicus (1473 –1543), on his death bed, published On the Revolution of Celestial Orbs.

• People thought that orbits of planets were circular. Copernicus put the centers of these circular orbits at the Sun. Heliocentric model.

• Planets orbit the Sun and moons orbit planets.• Got rid of Epicycles and Equant points (all objects orbit a common point),

• Earth rotates and Celestial Sphere is stationary.

Illustration from Copernicus's work showing heliocentric model

• By taking the Sun as the center of our world, it directly contradicted the Church. Earth is just another planet.

• If Earth is orbiting the Sun, why don’t we feel this motion?• If the giant celestial sphere is rotating, the strain

on those objects would be tremendous.

• Copernicus could not prove that Earth revolves about the Sun. Ptolemy’s model, if adjusted, could still work. Both models had good prediction power.

• There was little motivation to carry out observations or experiments to distinguish between competing cosmological theories

Predictive Power of Scientific Theories

• A good theory should be able to predict physical phenomena based on current knowledge.

• A Heliocentric model predicts that, if Venus circles the Sun, the planet should go through the full range of phases just as the Moon does.

How do objects Orbit the Sun?Galileo

• Galileo Galilei (1564-1642) studied the motion of objects and invented many things (telescope, thermometer, pendulum clock, ect…).

• Natural state of an object is at rest. (not true)

• Galileo showed that objects come to rest if some outside force is acting on it. Implying planets orbit the Sun because no outside force is slowing them down.

• Also showed that objects accelerate uniformly when falling. A feather and a hammer fall at the same speed.

The Galilean Telescope

• Galileo heard of the recent invention of the telescope and created his own.

• Galileo devised a stable mount and improved optics, achieving a magnification of 30×. Strong enough to resolve many faint objects in our universe.

• Galileo found four moons revolving about Jupiter.

• Discovered sunspots. Deduced Sun rotated on its axis.

• He observed the phases of Venus.

The Galilean Moons

• A geocentric model stated that everything orbited the Earth.

• If Jupiter has moons that orbit it, doesn’t that make Jupiter the center of everything?

• Demonstrated that there could be centers of motion that are themselves in motion.

• People thought that the Moon could never keep up with the Earth as it rotated the Sun.

Galileo and Sunspots

• By watching the spots on the surface of the Sun, Galileo deduced that the Sun rotates once per month.

• Additionally it rotating East to West, just like the planets rotating it.

• This is a very crucial observation that future scientist used when formulating a mathematical model of our solar system.

Phases of Venus

The Last Straw

• After Galileo’s work, it became increasingly difficult to deny the Copernican view.

• The Roman Catholic Church, still reeling from the Protestant Reformation, was looking to assert its authority and chose to make an example of Galileo.

• He had to appear before the Inquisition to answer charges that his work was heretical, and he was ultimately condemned to house arrest.

• First steps in proving that the universe is VAST and Earth is insignificant.