A brief History of Astronomy

How is science done?

Observations

Experiments

Explanations

Theories

Laws

Repeat

Assumptions of Early Models

• Geocentric = Earth in the center of the universe

• Everything orbits the Earth

• Stars are located on the Celestial Sphere

• Everything moves in uniform circular motions

Thales (624-546BCE)

• Philosopher (“tails”)• Proposed the first model of

the universe that did not rely on supernatural forces

• Simple model:– Small, flat Earth surrounded

by a sheet of water, with a single vast sphere.

– This sphere carried the stars

Pythagoras (580-500BCE)

• Proposed a more complex model of the universe– the Earth was a sphere– All stars and planets were on their own

concentric spheres beyond the Earth

Plato (427-347BCE)

• Proposed that celestial bodies (planets, etc) moved about Earth at a constant speed, and followed a circular motion with Earth at the centre.

• Asserted that heavenly motion must be in perfect circles and that heavenly objects reside on perfect spheres

Aristotle

Problems with Aristotle

• Retrograde motion… it didn’t make sense with the current model

Key Terms• Retrograde motion= motion that is

backward compared to the norm.

Example: Mars travels in apparent retrograde motion when it moves westward rather than the more common eastward.

Ptomely (100-170 BCE)

• Argued that each planet also revolved in a small circle (EPICYCLE)

• His GEOCENTRIC model (the Ptolemaic model) remained for 1400 years

Key Terms• Epicycle= a small rotation

on which a planet is placed. The epicycle then moves on a larger orbit. Used to explain retrograde motion.

Ptolemaic Model

http://faculty.fullerton.edu/cmcconnell/Planets.html#2

THE COPERNICAN REVOLUTION

• The Greeks and other ancient peoples developed many important ideas of science

• What we now consider science arose during the European Renaissance (14th to 16th century)

• The dramatic change now known as the Copernican revolution spurred the development of virtually all modern science and technology

Nicholaus Copernicus (1473-1543)

• Proposed a sun-centered (HELIOCENTRIC) universe where the Earth travelled around the Sun.

• Held onto the idea of epicycles and constant circular motion

• Proposed that stars were very far away

• Proposed that the Earth rotated on an axis

http://faculty.fullerton.edu/cmcconnell/Planets.html#2

• Feared criticism from the Catholic Church.

• Early supporters were drawn to the aesthetic advantage of his model.

• Belief in circular orbits made it no less complex than the Ptolemaic

• As a result it won few converts for 50 years

• Why was is it considered such a big deal?

• It was a strange and even rebellious notion

• It was a time of major upheaval: Columbus had sailed to “the New World”, Martin Luther has proposed radical revisions in Christianity

• The present PARADIGM (or prevailing scientific theory) is a way of seeing the universe around us. Questions, research and interpretation of results is all in the context of this theory. Viewing the universe in any other way requires a complete shift in thinking.

• Replacing a theory that had been believed to be correct for nearly 2000 years is not easy

• Only when the old theory’s complexity made it beyond usefulness was the intellectual environment at a point that the concept of heliocentric universe was possible

• By his time, tables of planetary motion based on the Ptolemaic model were noticeably inaccurate. But few people were willing to undertake the difficult calculations required to revise the tables.

• He was probably motivated in large part by the much simpler explanation of retrograde motion offered by a Sun-centered system.

Tycho Brahe (1546-1601)

• Considered the best naked-eye observer of all time.

• Observed a supernova and a comet• Was able to show that the stars existed

way beyond the distance of the moon• He was convinced that the planets must

orbit the sun, but was unable to develop a satisfying model

• Accuracy through repetition

Johannes Kepler (1571-1630)

• Worked for Brahe

• Highly religious

• Believed in the Heliocentric model

• Attempted to find a physically realistic model for Mars’ orbit (retrograde motion)

• This finally lead him to discard the circular orbit

Let’s look at what Kepler had for data…

• Page 573 in your textbook

• Use the data table and plug these numbers into your graphing calculator

• Determine if the relationship between Orbital Radius (ie, the distance from the planet to the Sun) and the Orbital Period (how long it takes the planet to orbit the Sun – ie, a year).

What type of relationship?

• Inverse (linear)

• Logarithmic

•

• Exponential

Exponential

• Check your “r” values!

Kepler’s Laws of Planetary Motion

• Three Laws that describe the relationships between the motion of the planets

Kepler’s Laws of Planetary Motion

• 1st Law: The orbits of planets and other celestial bodies around the Sun are ellipses. The Sun is one of the ellipses foci.

• An ellipse is defined as a figure drawn around 2 points called FOCI, such that the distance from one focus to any point on the figure back to the other focus is a constant

Kepler’s Laws of Planetary Motion

• 2nd Law: A line from the Planet to the Sun sweeps over equal areas in equal amounts of time

http://commons.wikimedia.org/wiki/File:Ellipse_Animation_Small.gif

http://www.opencourse.info/astronomy/introduction/05.motion_planets/

Why does this happen?

• Planets move quicker when they are closer to the Sun. This makes it cover more distance.

Kepler’s Laws of Planetary Motion

• 3rd Law: Deals with the length of time that it takes a planet to orbit the Sun (The Period of Revolution).

• All planets orbiting the Sun have the same ratio (k).

• Where r is the planet’s average (mean) distance from the Sun (measured in AU)

• T is the period of revolution of the planet (measured in years)

2

3

T

rk

Kepler’s Laws

• We now know that k is constant not only for planets but for ALL satellites (even artificial!) orbiting the Earth, the Moon, and the Sun.

• Note: k is not a true constant. It is only a constant for things orbiting the same celestial body. Example: All planets orbiting the Sun have the same k value. All moons orbiting Jupiter have the same k value.

• All Kepler’s Laws are true for all satellites.