V.Earth and Space A. Models for the Universe B. The Solar System

V. Earth and Space

A. Models for the Universe

B. The Solar System

A. Models for the Universe

1. Earliest Ideas

2. Mathematical Models and Shifting Paradigms

3. 20th Century

4. The Current View

A. 1. Earliest Ideas

Early observations of the skyEarly explanations of the skySignificance of the sky:

AstrologyFeatures of the sky:

– Sun & moon– Stars– Planets:

“Wanderers” with seemingly irregular “retrograde” motion that puzzled the ancients

– http://csep10.phys.utk.edu/astr161/lect/retrograde/retrograde.html

A. 2. Mathematical Models

Ptolemy (~100)– Wrote The Almagest– Geocentric model of the universe– “Epicycles” provided an empirical solution to the

problem of retrograde motion– His model accurately predicted the motion of the

planets, and was the basis of medieval astronomy/astrology (with periodic corrections) for some 1400 yrs

– http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html http://seds.lpl.arizona.edu/nineplanets/psc/theman.html


Copernicus (~1500)– Proposed a heliocentric model for the universe– Did not eliminate circular orbits– Required fewer epicycles than Ptolemy's

model; however, predictions from Copernicus’ model were not as accurate as from Ptolemy's

– http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html


Galileo (~1600)– Observations with his telescope provided

empirical support for the Copernican model of the solar system

– http://csep10.phys.utk.edu/astr161/lect/history/galileo.html


Kepler (~1600)– An assistant of Tycho Brahe– Developed a heliocentric model of the solar

system based on elliptical planetary orbits– “Kepler’s Laws of Planetary Motion”– http://csep10.phys.utk.edu/astr161/lect/history/kepler.html


Newton (late 1600s)– Developed mathematical “laws of motion” to explain

velocity & acceleration– Developed calculus– Developed a “theory of gravitation” based on the concept of

“force at a distance”• Every object in the universe attracts every other object with a force

directed along a line that connects the centers of the two objects. The force is proportional to the masses of the two objects and is inversely proportional to the squares of the distances between the two objects.

– Demonstrated that Kepler’s laws could be derived from the laws of motion & gravitation

– http://csep10.phys.utk.edu/astr161/lect/history/newton.html


Maxwell (~1850)– Developed a field theory of electromagnetism– Explained the “lines of force” observed in a

magnetic field (for example, if you sprinkle powdered iron around a magnet)

– http://www.phy.hr/~dpaar/fizicari/xmaxwell.html

– http://micro.magnet.fsu.edu/electromag/java/magneticlines/

A. 3. 20th Century

Einstein (Early 1900s)– Einstein’s ideas indicated that Newton’s

“laws” of motion & gravity were, in fact, only approximations that did not account for the behavior of matter, space, and time at large velocities or in large gravitational fields

•Einstein links:•http://csep10.phys.utk.edu/astr161/lect/history/einstein.html

•http://encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761562147&pn=1

•http://www.pbs.org/wgbh/nova/einstein/

A. 3. 20th Century

Einstein (cont.)– Special Theory of Relativity

• The speed of light in a vacuum is constant, regardless of the perspective of the viewer

• As an object approaches the speed of light, its size and time approach zero, and its mass approaches infinity

• Mass and energy are equivalent (E = mc2)




A. 3. 20th Century

Einstein (cont.)– General Theory of Relativity

• Gravitation and acceleration are equivalent• Mass causes the curvature of space-time,

accounting for the existence of gravity• Several phenomena such as the “bending” of light

rays by strong gravitational fields and certain anomalies with the orbit of Mercury could not be explained by Newton’s laws, but were explained by the Einstein's General Theory




A. 3. 20th Century

– Quest for a “unified field theory”• Physicists recognize four “fundamental forces:”

electromagnetism, strong nuclear force, weak nuclear force, and gravitation

• A “Unified Field Theory” (sought by Einstein & other physicists) would be a single set of equations that would predict the behavior of these forces in space-time

A. 3. 20th Century

Hubble (1924)– Demonstrated that many of the “nebulae” were, in fact,

isolated clusters of thousands of stars (galaxies)– The Earth’s sun is only one star in the Milky Way galaxy – Hubble also demonstrated that other galaxies are “racing

away” from the Milky Way: an expanding universe– The major evidence for an expanding universe was the

“Doppler shift:” The wavelength (color) of light from distant galaxies is “shifted” toward lower wavelengths (toward the red end of the spectrum)

– http://encarta.msn.com/encnet/refpages/refarticle.aspx?refid=761572208

A. 4. The Current View

In the mid-20th century, the astronomer George Gamow proposed the “big-bang” hypothesis (name coined by Fred Hoyle in 1950) to account for the expanding universe

http://archive.ncsa.uiuc.edu/Cyberia/Expo/cosmos_nav.html


Proposed sequence of the “big bang”– The universe begins as a singularity (point)– In the first 10-34 sec, the universe “inflates” (expands) and is very

“hot,” so energy & matter are indistinguishable (“quark soup”)– By 1 sec, the universe has expanded and cooled to the point that

stable protons and neutrons can form– By 300,000 years, the universe had expanded and cooled (to about

4000°K) enough so that light could pass “through” empty space without bumping into matter

– The universe continued to expand and cool, to its present temperature of about 4°K. The first galaxies were formed about 1 billion years ago

– The estimated age of the current universe is between 12 and 15 billion years

– http://archive.ncsa.uiuc.edu/Cyberia/Expo/cosmos_nav.html


Experimental evidence for the big bang– There is a “background” of microwave radiation

throughout the universe, predicted by Gamow and others

– Some features of the earlier universe have been observed by recent telescopes

– There is slight variability in the background temperature of the universe, a feature that is necessary to explain the existance of galaxies and galactic clusters

http://archive.ncsa.uiuc.edu/Cyberia/Expo/cosmos_nav.html

B. The Solar System

1. The nebular hypothesis

2. The sun

3. The inner planets

4. The outer planets

5. Comets

http://seds.lpl.arizona.edu/nineplanets/nineplanets/overview.html

B. 1. The Nebular Hypothesis

The concept that the solar system formed from a rotating cloud of interstellar gas (mostly hydrogen) and dust

Possibly the remnant of a supernova http://csep10.phys.utk.edu/astr161/lect/solarsys/nebular.html

B. 2. The Sun

Consists of ~75% hydrogen and 25% helium

Heat is produced from nuclear fusion (Hydrogen nuclei fuse to form a helium nucleus, with a release of large quantities of energy)

http://seds.lpl.arizona.edu/nineplanets/nineplanets/sol.html

B. 3. The Inner Planets

“Rocky” planets with compositions rich in iron and other metals

MercuryVenusEarthMarsThe asteroid belt

B. 4. The Outer Planets

Jupiter, Saturn, Uranus, and Neptune are classified as “gas giant” planets, rich in hydrogen, with extensive satellite and ring systems

Pluto is a small rocky planet beyond Neptune (usually) but with a highly eccentric orbit. It is believed perhaps to once have been a moon of Neptune which escaped its gravitational pull

B. 5. Comets

Balls of ices and dust that orbit the sun in highly eccentric orbits

When approaching the sun, part of the comet evaporates and foms the “tail” seen from earth

http://seds.lpl.arizona.edu/nineplanets/nineplanets/comets.html

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V.Earth and Space A. Models for the Universe B. The Solar System