Alta High Astronomy

Chapter 2

The History of Astronomy

Einstein Einstein improved upon the

work of Newton by explaining how gravity worked!

The Universe is actually made up of a 4 dimensional fabric we call the space-time continuum. Gravity is the effect of heavy massive objects bending or warping that fabric.

Gravity travels at the speed of light 3 x 10 ^8 m/s (186000 mph)

Alta High Astronomy

Alta High Astronomy

The Ancient Astronomers

Most Ancient Astronomy revolved around agriculture Most famous astronomical construct –

Stonehenge on the Salisbury Plain of England Used as a calendar

Other Examples Big Horn Medicine Wheel in Wyoming Caracol Temple on the Yucatan Peninsula

Alta High Astronomy

The Greeks and Muslim Astronomers

Muslim Astronomers Created trigonometry and Algebra – they were the first mathematicians

Greeks – continued their work and developed the first models of the Universe

Alta High Astronomy

The Greeks

The Sun, Moon, and Planets were the known Universe Sun, Moon and Stars were fixed on the Celestial

Sphere Planetes – wanderers planets move both east to

west and appear to move west to east Apparent west to east motion is called retrograde

motion. Aristotle -The Universe is Geocentric –

Everything revolves around the earth and everything moves in a circle.

Alta High Astronomy

Ptolemy

140 AD – Ptolemy constructed the best Geocentric model

In order for the model to fit the data it required the use of epicycles. Epicycles or a circle on a

circle – required over 80 circles The main circle around the

earth is called the deferent and the circle on that circle is called the epicycle

Alta High Astronomy

Copernicus 16th Century – Nicholas Copernicus

rediscovered the early Greek work by Aristarchus which proposed a Heliocentric or “sun centered” universe. This theory was much simpler than

that of Ptolemy – hence it satisfied Occam’s Razor – the philosophy that the simplest solution is usually the best one

Unfortunately this theory contradicted the doctrine of the Catholic Church

Copernicus published this work in Der Revolutionibus – The Revolution meaning the revolution of the planets about the sun. A book that was banned in 1616 by the Church

Alta High Astronomy

Galileo Galilei Also a 16th century astronomer,

Galileo was an italian who first heard of the telescope (no he did not invent it!) and decided to construct one to look at the sky. In 1609 he used his telescope to observe The moon

Had craters and valleys The phases of Venus

Proved it went around the sun 4 moons of Jupiter

Callisto, Ganymede, Io, and Europa

The moons revolved around a planet other than earth

Alta High Astronomy

The Trials of Galileo

1616 – his ideas were declared heretical by the Church

1633 – Placed under house arrest until his death

1992 – Formally Forgiven by the church

Alta High Astronomy

Tycho Brahe

Lived 1546 – 1601 Created the first formal

observatory named Uraniborg on the Island of Hveen in Denmark

Most accurate “naked eye astronomer of his time

Spent more than 20 years charting the known heavens

Hired Johannes Kepler as an assistant to do his mathematical calculations

Alta High Astronomy

Johannes Kepler German Mathematician

– he was not a scientist. He had no theories about the universe he simply wanted a mathematical model to fit the data collected by Brahe.

Hired by Brahe in 1600 he worked for 29 years on the three laws of planetary motion

Kepler’s laws of planetary motion Orbits are elliptical not circular

A planet will sweep out equal areas in equal time.

The time of orbit squared equals the distance to the sun squared ( p2 = a3 )

Alta High Astronomy

Alta High Astronomy

1st Law of Planetary Motion 1st Law Planetary

Orbits are elliptical, not circular Two focal points – sun

is at one foci, every point on the ellipse is the sum of the distances from the foci

Major Axis - length of the long part of the ellipse

Semi-Major Axis - Half the length of the major axis

Alta High Astronomy

2nd Law of Planetary Motion 2nd Law – a planet

sweeps equal areas in equal times This requires planets

to speed up and slow down

Perihelion – When the planet is closest to the sun and moving fastest

Aphelion – When the planet is furthest from the sun and moving slowest

Alta High Astronomy

3rd Law of Planetary Motion

The period of a celestial bodies orbit squared is equivalent to the length of its semi-major axis cubed

Mathematically:p2 = a3

Alta High Astronomy

Sample problem

If Earth is 1 AU from the sun and takes 365 days to go around the sun, how long does it take Mars to orbit the sun if it is 1.5 AUs from the sun?

Alta High Astronomy

Solution

2

2

3

3

m

e

m

e

p

p

a

a

Using the formula:

p2 = a3

Rearrange it for each object

2

3

2

3

m

m

e

e

p

a

p

a 676x

12

3

p

a23

3651

52.1

x

Alta High Astronomy

Eccentricity

Eccentricity – how far off you are from a perfect circle

E = distance between foci/ length of the major axis

Mathematically

E = d/2a

)1( eaP

)1( eaA

APa 2

Alta High Astronomy

Newton

Newton was an English Scientist

He wanted to explain why Kepler’s Laws work

Newton is famous for his three laws of motion which are the basis of Mechanics which is a branch of Physics

Alta High Astronomy

Newton’s Laws of Motion

An object at rest tends to stay at rest, and an object in motion tends to stay in motion. Acceleration is directly proportional to the force applied, and the mass is

inversely proportional to it’s acceleration (F=ma) For every action there is an equal and opposite reaction

Alta High Astronomy

Inertia: A resistance to change, a property of all matter.

Alta High Astronomy

Newton’s Second LawThe Law of Acceleration The acceleration of an object is in the direction of

the force applied. Acceleration is directly proportional to the force

applied. The harder you push an object the faster it goes Acceleration is inversely proportional to the mass of

the object. The heavier the object, the less affect a push has.

Alta High Astronomy

Newton’s Third LawAction - Reaction For every action there is an equal and

opposite Reaction.

Alta High Astronomy

Universal Gravitation

Newton’s Law of Universal Gravitation states that any two objects with mass are attracted to each other

The Force of Attraction is directly proportional to the product of the masses and inversely proportional to the square of the distance between them.

Alta High Astronomy

Universal Gravitation

F = Gm1m2/r2

G – the universal Gravitation constant. Experimentally determined to be

G = 6.67 x 10-11 N•m2/kg2

M1 & M2 – the masses of the objects R2 – the distance between the objects This law explains why an object speeds up when it

is closer to the sun and slows down when it is further away