Astronomy and the enlightnement

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Information about Astronomy and the enlightnement

Published on February 14, 2009

Author: david.s.graff



A history of some of the topics of astronomy developed during the enlightenment, with emphasis on the nebular theory of the solar system as developed by Kant and Laplace.

Talk for a general audience.

Astronomy and the Enlightenment -------------------- Nebular Theory of Formation of Solar System David S. Graff

The Enlightenment in a sentence • Human reason can combat ignorance, superstition, and tyranny and to build a better world. – Science is an important tool by which we use reason to conquer ignorance and superstition.

The solar system as seen by the ancients • “Fixed” stars rotate around the earth every 24 hours • “Planets” move with respect to fixed stars in complicated motion • Planets and Moon move in band surrounding ecliptic – Zodiac

Ecliptic in LEO

Images of the Ecliptic QuickTimeª and a TIFF (Uncompressed) decompressor are needed to see this picture. sun Stan Ricard via APOD

QuickTimeª and a TIFF (Uncompressed) decompressor are needed to see this picture.

Geocentric Universe

Geocentric Universe in 3-d • All planets in same plane • All planets appear to move through zodiac zodiac

Newtonian Synthesis • Law of Universal Gravitation determines – How Planets move around Sun – How Moon moves around Earth – How Moons of Jupiter move around Jupiter – How thrown ball moves on earth

Unsolved Problem • Structure of the Solar System: Why do all planets lie in the Zodiac band around the ecliptic?

Newton’s Unsolved Problem The Order of the Solar System • All planets move in Zodiac = near ecliptic • All planets are in plane of ecliptic – Exception: Pluto • All planets orbit in the same direction around Sun • All Planets spin in same direction as ecliptic orbits – Exception: Uranus, Venus • All Moons in same direction as ecliptic – Exception: Uranus • Thousands of Asteroids all aligned like planets.

Solar System Movie Rick Pogge & Ari Soloman

All planets in same plane (except Pluto)

All planets in same plane (except Pluto)

Moons of Jupiter • Orbit in plane, near parallel to ecliptic

Rings and Moons of Saturn • Make a plane around Saturn, near parallel to Ecliptic

Solar system didn’t have to be a plane

But it is…

This order is not accounted for by laws of Universal Gravitation • Newton:  …it is not to be conceived that mere mechanical causes could give birth to so many regular motions…. This most beautiful system… could only proceed from the counsel and dominion of an intelligent and powerful Being.

Philosophical Challenge to the Enlightenment • Newton: Unexplained order in the Universe proof of the existence of God • Enlightenment: Human reason can conquer ignorance. • Modern: Universe follows laws of physics. Unexplained order is a challenge to scientists.

Nebular hypothesis • Solar system formed from a big cloud of gas. • This explains order in the solar system. • Still believed today. IMMANUEL KANT

Nebula • A giant cloud of gas QuickTimeª and a TIFF (Uncompressed) decompressor (Nebula) are needed to see this picture. cools and condenses.

Fragments • Small knots of the Nebula QuickTimeª and a TIFF (Uncompressed) decompressor are needed to see this picture. further collapse to form dense cores

Spin up • As gas collapses, it spins faster and faster like water down a drain. – Conservation of angular Momentum

Gas Disk • Rapidly spinning gas collapses into dense disk around star • Planets form from disk • Orbits of planets preserves orbits of gas in disk – Same Plane – Same Direction – Circular Orbits

Modern Proof of Nebular Theory • Forming Solar Systems in Orion

Photos of forming stars

More photos

The Nebular Theory in other systems • Planets and Moons like mini-solar systems. Planets and their moons also form from spinning clouds of Gas “What the sun with its planets is on a large scale a planet with a sphere of attraction extending far out is on a small scale”

QuickTimeª and a TIFF (Uncompressed) decompressor are needed to see this picture.

Solar system: great circle on sky = plane in space Saturn QuickTimeª and a TIFF (Uncompressed) decompressor are needed to see this picture. Venus Mercury

Milky Way Galaxy is also a Great Circle on sky

Milky Way is Plane in space                                                                 

Nebular Hypothesis also explains how galaxies form • Big cloud of gas (100 Billion x mass of sun) • Slowly Spinning • Cools and shrinks over 10 Billion Years • Spins faster and gets flatter • Forms cold disk of gas • Gas further fragments to form stars

Nebular Hypothesis explains Galaxies … all the development in the comprehensive systems within a huge sphere… like the one which the same basic material which developed the planets created around the sun on a small scale... The Milky Way is the Zodiac of this higher world order. •Suggests the same laws of physics act on the very large scales of galaxies as on the scales of individual planets, and on the very small scale of water going down a bathtub drain.

Summary of Nebular Theory • Solar System began as big cloud of gas – Simplest possible initial condition • Gas cools, collapses, and spins into disk • Spinning disk fragments into planets • Planets preserve disk orbits:

Wider Implications • Order in the solar system is a fossil of its formation, not Divine intervention • Order determined entirely by universal laws of physics • Initial conditions much simpler than present day (Diffuse cloud of Gas) • Cosmology is concerned not primarily with present day state of the Universe, but with history of Universe: – How did Universe move from simple conditions to present complexity?

Expanding Horizons • With discovery of wide shape of Milky Way, astronomy moves outside solar system. • Solar system evolved (changed with time) from simpler state to present state. – This process must have taken a long time (e.g., for gas to cool) – Solar system must have a very large, but finite age • Important precursor to Darwin’s theory of evolution. • Science not only able to study the here and now, but also the long ago and far away.

Laplace’s Atheist interpretation • Laws of Physics describe order of the solar system – All the effects of Nature are only the mathematical consequences of a small number of immutable laws. • Laplace, Exposition du système du Monde • God is not needed to reorder Universe – Sire, I had no need of that Pierre-Simon Laplace hypothesis • Laplace to Napoléon

God as the creator of the laws of Physics The more perfect nature is in its developments, the better its universal laws lead to order and harmony, then the more certain the proof of the Divinity from which nature takes these relationships. Its productions are no longer the effects of contingency and results of accidents. Everything flows from it according to unchanging laws which thus must display nothing other than nature's skill, because they are exclusively features of the wisest of all designs from which disorder is prohibited. – Immanuel Kant, Allgemeine Naturgeschichte

Astronomy and the Enlightenment -------------------- Nebular Theory of Formation of Solar System David S. Graff PhD

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