Transcript Motions of the planets

As the Earth spins on its axis,
the sky seems to rotate
around us. This motion, called
diurnal motion, produces the
beautiful concentric trails
traced by stars during time
exposures. Near the middle of
the circles is the North
Celestial Pole (NCP), easily
identified as the point in the
sky at the center of all the star
trail arcs. The star Polaris,
commonly known as the North
Star, made the very short
bright circle near the NCP.
About 12,000 years ago, the
bright star Vega was the North
Star, and in about 14,000
years, as the Earth's spin axis
slowly continues to precess,
Vega will become the North
Star again.
Homework #1 will be posted later today (check
class website)
It will be due Monday, September 14, 8:00 pm
The origins of astronomy and the physical
sciences stem from:
 observing the sky and nature
 finding repeatable patterns
 seeking natural explanations for these patterns
Patterns Observed in the sky:
(observed facts)
Celestial Sphere
Large imaginary spherical
surface centered on the
Earth.
Stars and other celestial
objects “fixed” on its
surface.
Conceptual Model, not a
physical model
The sun follows the same path around the sky (celestial
sphere), repeating this journey once every year.
This path is called the “Ecliptic”
Why does the sun appear
to move like this?
Locations of planets in the sky
Mercury: always close to Sun in sky
Venus: always close to Sun in sky
 Mars: no restrictions on distance from Sun in sky
 Jupiter: no restrictions on distance from Sun in sky
 Saturn: no restrictions on distance from Sun in sky
What causes these differences?
Motions of the planets
 On short term (diurnal motion), planets appear to move
with the stars, east to west, making a full circuit around the
sky (meridian to meridian) in approximately one day
 Most of the time, planets move slowly eastward each day
relative to the stars: different planets at different rates
What causes these motions?
 Planets are always
close to the “ecliptic”,
the apparent annual
path of the sun through
the sky.
Close grouping of five
planets in April 2002.
This is a pattern that
was well known to the
“ancients”
Why are the planets
restricted to these
locations?
Some planets occasionally reverse their motion
relative to the stars, moving slowly westward
relative to the stars, for a few days
apparent retrograde motion
What causes this?
What causes this?
What causes the observed motions of the
stars, sun, moon, and planets in the sky?
The Greeks developed a model for the
Universe that lasted for nearly 15
centuries.
It did a reasonably good job explaining
these motions.
Claudius Ptolemy (100-170 CE)
Developed a model of the
universe designed to fit
the observational data.
Ptolemy and later scientists were
strongly influenced by the belief of
Plato that …
“all natural motion is circular”
● Earth is at center
(Geocentric)
● Sun orbits Earth
●Planets orbit on
small circles
(epicycles) whose
centers orbit the
Earth on larger
circles (this explains
retrograde motion)
Ptolemy’s
Geocentric Model
Apparent retrograde motion in geocentric model
 Planet orbits lie in
approximately the
same plane (this
explains why the
planets are always
near the ecliptic)
 Inferior planet
epicycles were fixed
to the Earth-Sun line
(this explained why
Mercury & Venus
never stray far
from the Sun).
Geocentric Model
Ptolemy’s model fit the data and made accurate
predictions, but was horribly contrived!
● Although the geocentric model of
Ptolemy gained dominance,
Aristarchus of Samos actually
proposed that the earth rotated
daily and revolved around the sun
Ptolemy’s Geocentric Model
● Relied upon circles upon circles (epicycles &
defferents) to explain the motions of planets
and the sun.
● Tied to Plato & Aristotle’s belief that “all
natural motion is circular”
● With modifications (e.g., additions of epicycles
upon epicycles), remained the standard
through the middle-ages.
Plato proposed that the orbits of
the planets have what shape?
conical
circular
elliptical
equal-angular
epicycles
Plato proposed that the orbits of
the planets have what shape?
conical
circular
elliptical
equal-angular
epicycles
The diurnal (daily) motion of stars is due to
the motion of the earth around the sun
the rotation of the earth
the epicyclic nature of the celestial sphere
the rotation of the celestial sphere
The diurnal (daily) motion of stars is due to
the motion of the earth around the sun
the rotation of the earth
the epicyclic nature of the celestial sphere
the rotation of the celestial sphere
What is the ecliptic?
when the Moon passes in front of the Sun
the constellations commonly used in astrology to
predict the future
the Sun's daily path across the sky
the Sun's apparent path across the celestial
sphere
What is the ecliptic?
when the Moon passes in front of the Sun
the constellations commonly used in astrology to
predict the future
the Sun's daily path across the sky
the Sun's apparent path across the celestial
sphere
About how long does it take the Sun to complete
one “trip” around along the ecliptic around the
entire sky?
One day
One month
One year
The time varies from one trip to the next
This never happens
About how long does it take the Sun to complete
one “trip” around along the ecliptic around the
entire sky?
One day
One month
One year
The time varies from one trip to the next
This never happens
The Revolution Begins!
The Copernican Revolution
●
●
Copernicus, Tycho, Kepler, and Galileo.
Kepler’s three laws of planetary motion
Nicolaus Copernicus (1473-1543)
He thought Polemy’s
model was contrived
Yet he believed in circular
motion
De Revolutionibus
Orbium Coelestium
Copernicus’ Heliocentric Model
●Sun is at center of the Universe
●Earth orbits the Sun like any other planet
●Earth rotates
●Circular orbits for all planets
●Inferior planet orbits are smaller
●Planets move at constant velocities in their
orbits
●Retrograde motion occurs when we “lap” Mars
& the other superior planets
Copernicus’ Heliocentric Model
●Retrograde motion occurs when we “lap” Mars
& the other superior planets
Simpler, more “elegant”
But, it still required some epicycles in order to make
accurate predictions
because
It was still wedded to Aristotle's circular orbit paradigm
Predictions were not much better than those of Ptolemy
Tycho Brahe (1546-1601)
●
Greatest observer of his
day
• Charted accurate
positions of planets
(accurate positions of
the planets were not
fully available)
Tycho Brahe…
was motivated by inadequacy of existing
predictions
made very accurate observations of positions
(this was prior to the development of the
telescope)
advocated a model in which Sun orbits Earth
because he could not observe stellar parallax
The parallax problem troubled the Greeks and
Tycho. It led both to reject a heliocentric universe.
The problem was that stars are too distant to produce a parallax
large enough to be seen with the technology of those time.
1600 – Tycho brought Johannes Kepler to bear
on problem. He assigned him the task of
understanding the motions of Mars.
Kepler had great faith in Tycho's
measurements; they placed strong constraints
on model