Up: Astronomy 9 Lecture Notes

ASTRONOMY 9: HISTORY OF COSMOLOGY

Handout #17

J. E. Baker
UC Berkeley, Spring 2000

Cosmology and Astronomy after Newton

Newtonian mechanics and law of gravitation show great success!
Physical laws are the same on earth and in heavens

1.

The universe is made of galaxies!

(a)

Thomas Wright (1750, England)

Idea of the ``galaxy'' or ``island universe'', a great system of stars
Believes our galaxy is spherical, with stars distributed in a shell on the surface

(b)

Immanuel Kant (1724-1804, Germany)

Important philosopher, wrote Critique of Pure Reason
Also cosmological speculation, Universal Theory...
Remarkably ``modern'' idea of galaxies
Reads misleading review of Wright's book which suggests a disk-shaped galaxy
Stars circle in great disks, much like a big solar system
Looking through the plane of the disk $\Rightarrow$ Milky Way
Postulates ``nebulae'' (fuzzy patches) are distant galaxies
Even argues galaxies are themselves hierarchically clustered into ever-larger structures, approaching the perfection of God
Argument is largely philosophical; no conclusive scientific evidence regarding nature of the nebulae for another 150 years!

(c)

Lord Rosse (1845)

Builds 72-inch telescope in Ireland
Discovers some nebulae are distinctly spiral

2.

Cosmology and Determinism

(a)

Pierre Simon Laplace (1749-1827, France)

Used Newton's calculus to explain almost all ``anomalies'' in planetary motion
Mercury still shows a mysterious shift of 43 arc-seconds per century in the location of its perihelion (point closest to Sun)
Shows that solar system is (almost) stable $\Rightarrow$ no need for divine intervention; determinism
1796: Nebular hypothesis: solar system formed from spinning disk-shaped nebula of gas and dust
To Napoleon, on God: ``I have no need for that hypothesis.''

3.

New Views of the Sky

(a)

Christiaan Huygens (1629-1695, Holland)

Discovers that Saturn's rings are rings
Argues that light is made of waves
Fights with Newton (says light is made of particles)
Evidence for both; not resolved until 20th-century quantum physics!
Also argues Newton's law of gravity is unsatisfactory because there is no physical mechanism

(b)

Charles Messier (1730-1817, France)

1760s: Catalogs 101 fuzzy objects which were not comets (so comet-hunters wouldn't be confused!)
Example: M31 is known today as the Andromeda galaxy

(c)

William Herschel (1738-1822, Germany and England)

Self-taught expert in telescope design
Sister Caroline discovers several comets
William finds a ``comet'' in 1781, turns out to be a new planet; named after George III, later changed to Uranus
Builds 49-inch diameter reflecting telescope in Slough, England
Catalogs 848 new binary stars (in orbit around each other)
Catalogs 2500 ``nebulae'', some resolve into stars!
1780s: finds whole solar system is moving relative to other stars, towards Hercules!
Halley had also found some indication of this in 1718, based on Tycho's earlier positions
No more ``fixed stars''!
New limits on parallax $\Rightarrow$ stars even farther away than previously thought

4.

Final Proof of Earth's Motion

(a)

James Bradley (1693-1792, England)

First direct evidence of Earth's orbital motion around Sun!
Looking for parallax, but failed to measure it
Instead, discovered ``aberration of starlight'': small yearly shift in positions of all stars
Imagine Earth is moving through a ``rain'' of starlight
When moving through the rain, it appears to fall from a direction different from vertical
The faster you move, the more horizontal it is
Allows Bradley to measure the speed of light: 295,000 km/s (=183,000 mi/s); note modern value is 299,792.458 km/s!

(b)

Fredrich Wilhem Bessel (1784-1846, Germany)

Predicts existence of Neptune based on anomalies in orbit of Uranus
1838: First detection of annual stellar parallax, more proof the Earth moves!
General formula for parallax:

$\begin{displaymath}\tan\alpha = \frac{1\ \mathrm{AU}}{d} \end{displaymath}$

$\alpha$ is half the angular shift during six months
For distance d big compared to 1 AU, $\tan\alpha \approx \alpha$ is a very good approximation, where $\alpha$ is in radians ( $360^\circ = 2\pi$ rad)
New unit of distance: the parsec (distance a star must have so that its parallax is 1 arc-second = 1/3600th degree):

$\begin{displaymath}\frac{d}{1\ \mathrm{parsec}} \approx \frac{1\ \mathrm{arcsec}}{\alpha} \end{displaymath}$

Note 1 parsec $\approx$ 206,265 AU $\approx$ 3.26 light-years.
For 61 Cygni, measured $\alpha = 0.3$ arcsec $\Rightarrow$ d = 3.3 parsecs

(c)

Jean-Bernard-Leon Foucault (1819-1868)

Foucault pendulum: first laboratory proof of Earth's rotation!
Heavy iron ball swinging from 220-foot wire
Pendulum wants to swing in the same plane (inertial frame)
Earth is rotating, so this plane rotates relative to observer on Earth!
Example: at north or south pole, rotates 360 $^\circ$ in 1 day

5.

Why is the sky dark at night?!

Called ``Olbers' paradox'' after Heinrich Olbers (1826)
But idea had been around since Kepler (1610)!
Also discussed by Edmund Halley (1720) and Jean-Phillipe de Cheseaux (1744)
Basic idea: in an infinite, homogeneous, Newtonian universe where stars have been around forever, all lines of sight should intersect the surface of a star
So the sky should be as bright as the surface of the sun!
More to say about this later ...

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Up: Astronomy 9 Lecture Notes

jonathan baker
2000-03-06