Ancient Astronomy

Neolithic Astronomy

Stonehenge

~ 2800 – 1700 BCE

The Americas

Machu Picchu – the Intihuatana

or “hitching post of the sun”

Several of the major cultures had complex structures

that reveal astronomical alignments

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

a (AU) P (yr) a 3 P 2

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