The History Of Astronomy
History of Astronomy
1. Ancient times (Before 3000 B.C):
a. Earth is Flat – astronomical bodies pass beneath Earth at
night
b. Astronomy is severe limited by the practical interests of the people
and their still primitive outlook on the universe
** Some people still believe the world to be FLAT!!! 
www.flat-earth.org
2. Babylonians (3000 BC)
a. 1st Evidence Earth is Round
b. Basic Constellations and Astrology
3. Aristotle (384 BC – 322 BC) famous Greek writer, philosopher,
physicist who was one of the 1st to attempt to develop a rational
understanding of the universe.
Aristotle
History of Astronomy (Cont.)
3. Aristotle cont. (384 BC – 322 BC)
a. Geocentric Theory – Earth is the Center of the Solar system
(even universe) and planets, sun, and stars revolves around it
- All stars are fixed points which rotate on a single
sphere
b. Believed there were only a few basic Substances
1. Earthly Realm – Air, Earth, Fire, and Water
2. Heavenly Realm – Quintessence (Not found on Earth)
c. Also agreed Earth was round BUT obviously could not be moving
1. Rotation- NO sense of motion...no strong winds…..No
displacement of thrown object.
2. Revolution – No stellar parallax - the shift of a nearby object
with respect to more distant objects as seen from two different
locations
5
Stellar Parallax – If the Earth revolved, the closer
stars should shift among the background of further
stars
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History of Astronomy (Cont.)
4. Eratosthenes (276 - 195 B.C.)
a. 1st person able to measure the circumference of the Earth
(came within a couple thousand miles of actual value)
- used geometry and ratios to determine the size of Earth
5. Ptolemy (90 – 168 AD)
a. worked out a complex system of Epicycles and perfect circles for
the orbits of planets, sun, and moon around Earth
Geocentric Model
History of Astronomy (Cont.)
6. Nicolas Copernicus (1473 –1543 AD)
Problems with Geocentric Theory:
1. Model was becoming more and more inaccurate at making
predictions as time passed
2. Orbits of planets had to become increasing comlpex in order
to fit observations
a. 1st to propose a Heliocentric Theory- Planets and moons
revolve around the Sun.
- Orbits still perfect circles with epicycles
- Much better at predicting future planet locations but
still not completely accurate
b. Invented idea of Astronomical Unit (A.U.) – Distance of
Earth from the Sun (About 93 million miles)
NICOLAS COPERNICUS
Copernicus Heliocentric Model
Primitive Heliocentric Model
Distances of Planets from the Sun
Planets
Copernicus
Modern
Mercury
Venus
Earth
Mars
Jupiter
Saturn
0.38
0.72
1.00
1.52
5.22
9.18
0.387
0.723
1.00
1.52
5.20
9.54
Distances Measured In Astronomical Units (A.U.)
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History of Astronomy (Cont.)
7. Johannes Kepler (1571- 1630)
a. Proposed that orbits of planets are not perfect circles but
rather ellipses
b. Developed 3 simple laws to explain thousands of years of
planetary observations
** 1st law- Orbit of all planets are ellipses with the sun at one
focus
** 2nd law – Law of Equal Areas ; which defines planetary
speed as they go around the Sun
** 3rd law – As planet distance from the Sun increases,
orbital period increases
PROBLEM: What is force that locks the planets in space orbit??
How to Draw an
Ellipse
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KEPLER’S 1st LAW
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KEPLER’S 2nd LAW
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KEPLER’S 2nd LAW
History of Astronomy (Cont.)
8. Galileo Galilei (1564-1642 A.D.)
- Founder of Modern Mechanics and Astronomical use of the
Telescope
a. Proved Aristotle Wrong
1. Many more stars too faint to be seen with eye
2. Moon has mountains and craters like Earth….
Earth and Space made of same material
3. Discovered imperfection in Sun (SUNSPOTS)…Sun is not
perfect
b. Provided more evidence for a Heliocentric Solar System (Venus
exhibits a full cycle of phases which is only possible in a Heliocentric
system + Jupiter appears as a mini solar system which means that
Kepler’s Laws apply for all planets)
History of Astronomy (Cont.)
8. Galileo Galilei (1564-1642 A.D.) cont.
c. Experimented with gravity and discovered that gravity is
uniform on Earth
d. Developed law of Inertia…an object at rest remains at rest. An object
in motion will remain in motion in the absence of friction and
gravity.
Galileo Galilei
Galileo’s Sketch of the Moon Using a Telescope
SUNSPOTS
Phases of Venus as Seen from Earth
Phases of Venus….Evidence for a Heliocentric Solar system
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History of Astronomy (Cont.)
9. Sir Isaac Newton (1642- 1727 A.D.) – mathematician and physicist
who is considered the father of modern physics. He brought together the works
of Kepler and Galileo with his basic physical laws and his laws of gravity.
a. Gravity is a force in the whole universe, not just on Earth.
b. Gravitational force between two object decreases as the distance
between the objects increases
More Mass = More Gravity
c. Gravity is the force that keeps the planets in orbit around the
Sun.
d. Three basic laws of Physics
1st Law- Law of inertia – an object at rest will stay at rest an
object in motion will stay in motion unless
acted upon by an outside force
History of Astronomy (Cont.)
9. Sir Isaac Newton (1642- 1727 A.D.) cont.
d. Three basic laws of Physics (cont.)
2nd Law – More massive objects require more force to be
accelerated (F= M x A) AN additional aspect of this law is that is
provides for a way to calculate an objects weight. Weight equals the
force of Earth’s gravity on the object times its mass (W=M x g)
3rd Law – For Force (Action) on one body there is an equal but
opposite force (Reaction) acting on another body
ex. - If an apple falls from a tree, not only does the apple fall to
Earth but Earth falls towards the apple as well.
F (Earth) = F (Apple)
MA (Earth) = MA (Apple)
Sir Isaac Newton
Newton’s 1st Law of Inertia
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Newton’s 1st law applied to Planets
Newton’s 2nd Law
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Newton’s 3rd Law
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History of Astronomy (Cont.)
9. Sir Isaac Newton (1642- 1727 A.D.) cont.
Aspects of Newton’s Laws of Motions
1. A force of gravity is present between any two masses regardless of
size. Most objects are not massive enough to sense their force.
2. Using his laws of gravity Newton mathematically proved Kepler’s
Laws to be correct
3. Calculated the existence of other planets due to orbital anomalies of
outer known planets
*** Although Newton is able to describe and calculate with great precision
the forces of gravity he comes short in explaining the nature of gravity
History of Astronomy (Cont.)
10. Albert Einstein (1879 – 1955 A.D.) American Physicist who
came up with many theories which expanded our understanding of
the universe
a. Space and time are not separate units but rather one
unit = space-time
b. Gravity is the curvature of space-time. The more
massive the object, the greater the curvature. (This is
the basis of Einstein’s General Relativity Theory)
*** Einstein explained the true nature of gravity. That’s it not a
force but rather a course that mass takes around more massive
objects. This theory was proven correct by the deflection of
starlight observed during a 1919 eclipse.
c. Most Famous for the Equation E=MC2
confirmed by the creation of the atomic bomb
Lesser Known Einstein Equation
Einstein’s Theory of Relativity
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Less Massive Object
More Massive Object
Gravity as the
Warping or Curving
of Space
Time…..The More
Massive the Object
the Greater the
Curving
Gravity as the Warping or Curving of Space-Time
Final Thoughts on Einstein