Transcript The Earth in the Universe - Sierra College Astronomy Home Page

The Earth in the Universe
Observations and Models
Presented By
Professor Harry L. F. Houpis
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The Celestial Sphere
 Celestial sphere is the sphere of heavenly objects
that seems to center on the observer.
 Celestial pole is the point on the celestial sphere
directly above a pole of the Earth.
 In the Northern Hemisphere the north celestial pole
is very near to the star Polaris.
 In the Southern Hemisphere, there is no obvious
star near the south celestial pole.
 Height of the celestial pole above the horizon is equal
to your latitude
 All celestial objects move from east to west on a
minute-by-minute basis.
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The Celestial Sphere
A constellation (from the Latin, meaning
“stars together”) is an area of the sky
containing a pattern of stars named for a
particular object, animal or person.
The earliest constellations were defined
by the Sumerians as early as 2000 B.C.
The 88 constellations used today were
established by international agreement.
An asterism is an unofficial arrangement
of stars.
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The Sun’s Motion: A Year
 Timing
 The Sun appears to move constantly eastward among the
stars (on a day-to-day basis).
 The time the Sun takes to return to the same place among
the stars is about 365.24 days.
 Consequently, the stars rise about 4 minutes earlier each
day.
 Paths in the Sky
 The celestial equator is a line on the celestial sphere
directly above the Earth’s equator
 The ecliptic is the apparent path of the Sun on the celestial
sphere
 The zodiac is the band that lies 9° on either side of the
ecliptic on the celestial sphere and contains the
constellations through which the Sun passes
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Observation: The Planets
 Five planets are visible to the naked eye:
Mercury, Venus, Mars, Jupiter, Saturn.
 Planets lack the simple, uniform motion of the
Sun and Moon.
 These planets always stay near the ecliptic.
 Mercury and Venus never appear very far from
the position of the Sun in the sky.
 Planets sometimes stop their eastward motion
and move westward against the background of
stars. This is called retrograde motion.
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Pre-Copernican Models
Models of Claudius Ptolemy (circa A.D. 150)
 To account for retrograde motion, the epicycle is
introduced – the epicycle is the circular orbit of a
planet, the center of which revolves around the
Earth in another circle.
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Pre-Copernican Models
Models of Claudius Ptolemy (continued)
Needed to “tie” Mercury and Venus to the
Sun.
To account for other non-uniform variations
in the planets’ motions while maintaining
the principle that the Universe was
constructed of circles and spheres, the
following constructs were introduced:
 Deferent and equant (note that the Earth is no
longer at the center of the Universe)
 “Embedded” spheres
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Pre-Copernican Models
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Pre-Copernican Models
Models of Tycho Brahe (born 3 years after
Copernicus died)
Placed all the planets around the Sun
except the Earth.
The Earth-less Solar System then orbited
around the stationary Earth
Other Variations and Models
See http://www.csit.fsu.edu/~dduke/models
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Criteria for Scientific Models
 Three criteria for scientific models:
 Model must fit the data
 Model must make predictions that can be tested and be of such
a nature that it would be possible to disprove it
 Model should be aesthetically pleasing - simple, neat, and
elegant (Occam’s razor)
 Ptolemy’s and Tycho’s models meet the first two criteria
for a good scientific model fairly well but it is much less
successful with the third (aesthetically pleasing).
 400 years before Ptolemy, the Greek philosopher
Aristarchus proposed a moving-Earth solution to
explain celestial motions.
 Ptolemy and others discredited Aristarchus’s model, but
used wrong assumptions to do so.
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Nicholas Copernicus
Copernicus, a contemporary of
Columbus, worked 40 years on a
heliocentric (sun-centered) model
for two reasons:
 Ptolemy’s predicted positions for
celestial objects had become less
accurate over time.
 The Ptolemaic model was not
aesthetically pleasing enough.
His system revived many of the ideas of the
ancient Greek Aristarchus.
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Nicholas Copernicus
 Model Specifics:
 The Earth rotates under a stationary sky (which gives
the same observations as a rotating celestial sphere
and a stationary Earth).
 The Earth revolves around a stationary Sun, which
appears to move among the background stars.
(Projection is the key concept)
 His model explains the generally west to east motion of
the planets.
 Observed retrograde motion of planets (such as Mars)
is explained more simply and conclusively.
 Copernicus had the Moon revolving around the Earth.
All others circled the Sun.
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Nicholas Copernicus
Model Specifics (continued)
 The Sun’s apparent motion north and south
of the equator is explained by having the
Earth’s equator tilted with respect to the
planet’s orbit around the Sun.
 Closeness of Mercury and Venus to Sun is
easily explained.
However, the Copernican model turns
out to be no more accurate (relative to
the observations) than the geocentric
models
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Tycho Brahe
Tycho was born 3 years after
Copernicus died.
Tycho built the largest and most
accurate naked-eye instruments
yet constructed.
He could measure angles to within
0.1º, close to the limit the human
eye can observe.
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Tycho Brahe and Johannes Kepler
 In 1600, a year before Tycho died, Kepler
accepted a position as Tycho’s assistant,
working on models of planetary motion.
 Tycho’s best data had been gathered for Mars.
 Based on circles and epicycles Kepler’s best
model for Mars matched Tycho’s data to an
accuracy of 0.13º.
 Yet, this error exceeded the error in Tycho’s
measurements, which bothered Kepler.
 Kepler’s persistence led him to abandon circles and
try other shapes. The shape that worked for Mars and
all other planets was the ………..
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Johannes Kelper
The Ellipse
 The ellipse is a geometrical shape every point of
which is the same total distance from two fixed
points (the foci).
 Eccentricity is the distance between the foci divided
by the longest distance across (major axis).
 Astronomers refer to the semi-major axis distance
and eccentricity.
Consequences
 Kepler developed his three laws
 The heliocentric theory worked far better than the old
geocentric theory with regard to predictions
 Kepler’s empirical laws were later explained by the
use of Newton’s dynamical laws of motion and
gravitation – the first unified theory of physics
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Galileo Galilei
Galileo was born in 1564 and
was a contemporary of Kepler.
Galileo built his first telescope
in 1609, shortly after hearing
about telescopes being
constructed in the Netherlands.
Galileo was the first person to
use a telescope to study the
sky.
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Galileo Galilei
Galileo made 5 important observations:
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Mountains and valleys on the Moon
Sunspots
More stars than can be observed with the naked eye
Four moons of Jupiter
Complete cycle of phases of Venus
Though Galileo’s first three observations do
not disprove the geocentric theory, they cast
doubt on the assumption of perfection in the
heavens.
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Galileo Galilei
Satellites of Jupiter
 In 1610 Galileo discovered that Jupiter had four
satellites of its own, now known as the Galilean
moons of Jupiter.
 Jupiter and its orbiting moons contradicted the
Ptolemaic notions that the Earth is the center of all
things and if the Earth moved it would leave behind
the Moon.
The Phases of Venus
 Galileo observed that Venus goes through a full set
of phases: full, gibbous, quarter, crescent.
 Venus’s full set of phases can be explained by the
heliocentric theory.
 The Ptolemaic theory predicts that Venus will
always appear in a crescent phase, which is not
borne out by the observations.
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Summary
 The Pre-Copernican Era
 Ancient Greek desire to explain the Universe with the “most
perfect” of all geometries: circles and spheres
 “Common sense” and human intuition were all that were
needed to explain the observations
 The Earth was the center of the Universe and the heavens
followed a different physics
 The “science” of the times was not held hostage to
observations – models were only for calculation purposes and
not necessarily a required reality
 The Copernican Era
 What’s with the circles? – “around the corner” vs “direct
consequences” inquiries
 How common is common sense and how reliable is intuition?
 Unification is the holy grail of science
 Models are held hostage to observations and fundamental
symmetries (realities?) are sought
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