Download The Roots of Astronomy Stonehenge

Early Astronomers
Physics 113 Goderya
Chapter(s): 4
Learning Outcomes:
The Roots of Astronomy
• Already in the stone and bronze ages,
human cultures realized the cyclic
nature of motions in the sky.
• Monuments dating back to ~ 3000 B.C.
show alignments with astronomical
significance.
• Those monuments were probably used
as calendars or even to predict eclipses.
Stonehenge
Summer solstice
• Constructed: 3000 – 1800 B.C.
Heelstone
• Alignments with
locations of sunset,
sunrise, moonset
and moonrise at
summer and winter
solstices
• Probably used as
calendar.
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Pythagoras
 580BC - 500 BC
• Calls the heavens
“cosmos”.
• Suggest that Earth
is spherical.
 The First Scientific
Theory
• The daily motion of the
sun and the starry
heavens is due to the
earth rotating in a circle
Eudoxus
 408BC - 355 BC
• Homo centric
model of the
Universe
• What is wrong
with the model?
Observation of
Mars
Homocentric hollow Spheres
Aristotle
• 348 BC
• Lunar Eclipses
• Shape of Earth
– Shadow during
Lunar eclipse
• Earth is round
A
B
– Different sky for A
and B.
– The ocean liner in
the sea
2
Aristarchus
Earth Shadow
• 300 BC - 200 BC
– From earth shadow in Lunar
eclipse:
determines Moon-Earth
diameter
– From Trigonometry and phases
Sun
of the Moon:
Sun-Earth-Moon distance
– Proposes Heliocentric
Universe
1/3
Moon
Moon
Earth
20
1
Eratosthenes
• 276 BC - 195 BC
– Size of the earth.
• when the sun is
y overhead at
directly
noon on some
summer day in
Syene the sunray
made a 7-degree
angle for an obelisk
in Alexandria.
Eratosthenes’s Experiment
(SLIDESHOW MODE ONLY)
3
Calculations
– Eratosthenes calculation
Distance between Syene and Alexandria =
500
00 miles
il
One full circle = 360 degrees
Obelisk angle difference = 7 degrees
7/360 = 500/C
C = (500 . 360 ) / 7 = 26,000 miles
Hiparchus
• 200 BC - 100 BC
– Catalogs the
positions of stars
– Brightness scale
for stars:
Magnitude scale
– Motion of Sun:
Tropical year
Sidereal year
Ptolemy
• 100 AD -200 AD
– Wrote Astronomy text
Almagest.
– Geocentric model
• Earth center of the
universe and the
moon, mercury and
Venus are in
between the earth
and sun.
4
Later refinements (2nd century B.C.)
• Hipparchus: Placing the Earth away from the centers of the
“perfect spheres”
• Ptolemy: Further refinements, including epicycles
Epicycles
Introduced to explain retrograde
(westward) motion of planets
The Ptolemaic system was considered
the “standard model” of the Universe
until the Copernican Revolution.
Epicycles
(SLIDESHOW MODE ONLY)
5
Copernican Revolution
• C
Copernicus
i
(1473
(1473-1543)
1543)
• Supported heliocentric model of the
Universe.
• Explained Retrograde Motion.
– The apparent retrograde motion of
Mars is created by the fact that the
earth passes Mars. This occurs every
26 months.
• Determined distances to the planets.
Copernicus’ new (and correct) explanation
for retrograde motion of the planets
Retrograde
(westward)
motion of a
planet occurs
when the
Earth passes
the planet.
This made Ptolemy’s epicycles unnecessary.
Tycho Brahe (1546 - 1601)
• He proposed a model of the
solar system.
– The Sun and the Moon orbit the
earth while the other planets orbit
the Sun.
• IInvented
d iinstruments ffor
planetary motion studies.
• Discovered exploding stars and
comets.
• Debated the validity of the
heliocentric model.
• Realized that the Universe is
changing and is complex.
6
Johannes Kepler (1571 – 1630)
• Used the precise observational tables of
Tycho Brahe (1546 – 1601) to study
planetary motion mathematically.
• Found a consistent description by
abandoning both
1. Circular motion and
2. Uniform motion.
• Planets move around the sun on elliptical
paths, with non-uniform velocities.
Galileo Galilei (1564 - 1642 )
• Discovered the law of falling
bodies.
– All objects fall to earth at the
same rate in the absences of air
resistance. In other words all
f lli objects
falling
bj t experience
i
th
the
same acceleration due to gravity
in vacuums.
• First person to use the
telescope for Astronomy.
– Studied in detail the moon
surface, sunspots, suns rotation,
Jupiter’s satellite, Saturn (not
including the rings) the Milky
Way and the phases of Venus.
Issac Newton (1642 - 1727)
• Discovered the laws of
Motion (Details to
come later)
• Discovered the law of
Gravity (Details to come
later)
7
The Modern Ideas –Einstein
and Weird physics
•
•
•
•
Weird Physics - Einstein
Theory of Special Relativity
General Theory of Relativity
Space-Time
Space
Time continuum
Source: Wikipedia
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