Download Slide 1

Ancient Astronomy
Neolithic Astronomy
Stonehenge
~ 2800 – 1700 BCE
The Americas
Several of the major cultures had complex structures
that reveal astronomical alignments
Machu Picchu – the Intihuatana
or “hitching post of the sun”
Marked solstices, equinoxes, and
positions of the Moon.
The Caracol at Chitzen Itza, Yucatan ~ 800 AD
Venus was the ‘Great or Ancient Star”. Observations from slits
in circular structure used to record times of its rising and
setting. Measurements of Venus and moon used to establish a
calendar and for timekeeping.
N. America – Medicine Wheels in western plains
Stone alignments in a spoked
wheel pattern some with large
cairns that marked sight lines
All are on high places with a
clear view of the horizon.
Medicine Wheel in Big
Horn Mtns, Wyoming
Date from ~ 1400 - 1700
Geocentric or Ptolemaic model
Problem – apparent backward
motions of planets -- retrograde
The Heliocentric Model
Copernicus 1473 - 1543
Kept circular orbits and epicycles, explained retrograde motion
Galileo 1564 – 1642
Galileo’s famous observations with the telescope
Surface of the Moon – mountains, craters
Phases of Venus –
Saturn had “ears”
4 moons of Jupiter
Milky Way made of 1000’s of stars
Sun was blemished
Galileo’s 3 decisive discoveries for Heliocentric model
:
1. Surface features on the Moon
2. Jupiter’s moons and motions
3. “Full” phases of Venus
None of these made sense in geocentric model,
but they were all perfectly OK in Copernicus’
universe. The phases of Venus are especially
decisive.
Tycho Brahe 1546 – 1601
Brahe couldn’t measure stellar parallax and
therefore couldn’t confirm Copernican
model -- proposed his own
Thomas Digges – English
mathematician, astronomer
1546 – 1595
Contemporary with Brahe and
Galileo, but his model goes a
step further
"This orb of stars fixed infinitely up extends
itself in altitude spherically, and therefore
immovable the palace of felicity garnished
with perpetual shining glorious lights
innumerable, far excelling over [the] sun
both in quantity and quality the very court of
celestial angels, devoid of grief and
replenished with perfect endless joy, the
habitacle for the elect."
Johannes Kepler 1571-- 1630
The Astronomical Unit ( 1 AU)
distance Earth to the Sun
93 x 106 mi , 150 x 106 km
Kepler’s 3 laws of planetary motion
1. The orbits of the planets are
ellipses with the Sun at one focus
2. The line joining the planet and
the Sun sweeps out equal areas
in equal times.
3. Period of revolution proportional
to distance from Sun
If a = average distance from the Sun
and P = orbital period, then
P 2 = (constant) x a 3
If P in years, a in astronomical units
then P 2 = a 3
Kepler’s third law -- the numbers
a3
P2
a (AU)
P (yr)
MERCURY
0.387
0.241
0.058 0.058
VENUS
0.723
0.615
0.378 0.378
EARTH
1.000
1.000
1.00
1.00
MARS
1.524
1.881
3.54
3.54
JUPITER
5.203
11.86
141
141
SATURN
9.54
29.46
868
868
Isaac Newton 1642 -- 1727
Astronomer, mathematician, physicist – father of physics, mechanics
Laws of Motion and Law of Gravity
The 3 Laws of Motion and the Law of Gravity
Some basic concepts:
Speed – how fast an object is moving mi/hr, km/s
Velocity -- vector – speed + direction
Acceleration – rate of change in velocity
Inertia – property of an object resists change in state of rest or motion
1st Law -- An object remains at rest or in motion (in a straight line) unless
acted upon by an outside force
2nd Law -- The acceleration of an object is directly proportional to the force
acting on it.
F = ma
Defines: Force
Mass (m) - total amount of material in an object
weight depends on the force of gravity on an object with mass m
3rd Law -- (Law of Reaction) Whenever a force is exerted on an object there
is an equal and opposite reaction
Law of Gravity -- the force of attraction between two objects is directly
proportional to their masses and inversely proportional to the
square of the distance between them
( attractive force between M and m )
FGrav = G x M x m / (distance) 2 .
Laws of Motion + Gravity explained Kepler’s Laws
Newton’s form of Kepler’s 3rd law:
P2
= 4 p2/ G x a3/ (M1 + M2)
Why the difference ?
Orbital Motion is due to inertia plus gravity – the force of acceleration towards
the center -- centripetal acceleration