Quoting from Alan Lightman's, "A Modern Day Yankee In A
Connecticut Court and other essays on Science".
Conversations with Papa Joe
The First Evening
Key Words & Phrases:
Crystal Clear Skys
Evolution of Stars
Spinning Earth - Focault Pendulum
Inverse Square Law
Cepheid Variables (Shapley, Levett)
Galaxies (and more galaxies)
HOW DO WE KNOW THE EARTH SPINNING?
Wikipedia - Foucault pendulum
The Foucault pendulum or Foucault's pendulum is a simple
device named after French physicist Leon Foucault and
conceived as an experiment to demonstrate the Earth's
rotation. The pendulum was introduced in 1851 and was the
first experiment to give simple, direct evidence of the
earth's rotation. Foucault pendulums today are popular
displays in science museums and universities.
HOW DO ASTRONOMERS MEASURE DISTANCES?
Wikipedia -- Kepler's laws of planetary motion
The ratio of the square of an object's orbital period with
the cube of the semi-major axis of its orbit is the same for
all objects orbiting the same primary. This captures the
relationship between the distance of planets from the Sun,
and their orbital periods. Kepler enunciated in 1619 this
third law in a laborious attempt to determine what he viewed
as the "music of the spheres" according to precise laws, and
express it in terms of musical notation. So it was known as
the harmonic law.
Kepler's Third Law states: T^2 ~ r^3
The discovery of phases of Venus by Galileo in 1610 was
important. It contradicted the model of Ptolemy which
considered all celestial objects to revolve around the Earth
and was consistent with others, such as those of Tycho and
In Galileo's day the prevailing model of the universe was
based on the assertion by the Greek astronomer Ptolemy
almost 15 centuries earlier that all celestial objects
revolve around Earth (see Ptolemaic system). Observation of
the phases of Venus was inconsistent with this view but was
consistent with the Polish astronomer Nicolaus Copernicus's
idea that the solar system is centered on the Sun. Galileo's
observation of the phases of Venus provided the first direct
observational evidence for Copernican theory.
Wikipedia -- Astronomical unit
The AU was originally conceived as the average of Earth's
aphelion and perihelion; however, since 2012 it has been
defined as exactly 149,597,870,700 m.
The astronomical unit is used primarily for measuring
distances within the Solar System or around other stars. It
is also a fundamental component in the definition of another
unit of astronomical length, the parsec.
Wikipedia -- Parallax
To measure large distances, such as the distance of a planet
or a star from Earth, astronomers use the principle of
parallax. Here, the term parallax is the semi-angle of
inclination between two sight-lines to the star, as observed
when Earth is on opposite sides of the Sun in its orbit.[a]
These distances form the lowest rung of what is called "the
cosmic distance ladder", the first in a succession of
methods by which astronomers determine the distances to
celestial objects, serving as a basis for other distance
measurements in astronomy forming the higher rungs of the
Wikipedia -- Henrietta Swan Leavitt - Standard Candles
Henrietta Swan Leavitt (July 4, 1868 - December 12, 1921)
was an American astronomer. A graduate of Radcliffe College,
she worked at the Harvard College Observatory as a
"computer", tasked with examining photographic plates in
order to measure and catalog the brightness of stars. This
work led her to discover the relation between the luminosity
and the period of Cepheid variables. Leavitt's discovery
provided astronomers with the first "standard candle" with
which to measure the distance to faraway galaxies.
Wikipedia -- Harlow Shapley
Harlow Shapley (November 2, 1885 - October 20, 1972) was an
American scientist, head of the Harvard College Observatory
(1921-1952), and political activist during the latter New
Deal and Fair Deal.
Shapley used Cepheid variable stars to estimate the size of
the Milky Way Galaxy and the Sun's position within it by
using parallax. In 1953 he proposed his "liquid water belt"
theory, now known as the concept of a habitable zone.
Wikipedia -- Cosmic distance ladder
The cosmic distance ladder (also known as the extragalactic
distance scale) is the succession of methods by which
astronomers determine the distances to celestial objects. A
real direct distance measurement of an astronomical object
is possible only for those objects that are "close enough"
(within about a thousand parsecs) to Earth. The techniques
for determining distances to more distant objects are all
based on various measured correlations between methods that
work at close distances and methods that work at larger
distances. Several methods rely on a standard candle, which
is an astronomical object that has a known luminosity.
The ladder analogy arises because no single technique can
measure distances at all ranges encountered in astronomy.
Instead, one method can be used to measure nearby distances,
a second can be used to measure nearby to intermediate
distances, and so on. Each rung of the ladder provides
information that can be used to determine the distances at
the next higher rung.
Wikipedia -- Hubble's law
Hubble's law, also known as the Hubble-Lemaitre law, is the
observation in physical cosmology that galaxies are moving
away from the Earth at velocities proportional to their
distance. In other words, the further they are the faster
they are moving away from Earth. The velocity of the
galaxies has been determined by their redshift, a shift of
the light they emit to the red end of the spectrum.
Hubble's law is considered the first observational basis for
the expansion of the universe and today serves as one of the
pieces of evidence most often cited in support of the Big
Bang model. The motion of astronomical objects due solely to
this expansion is known as the Hubble flow. It is often
expressed by the equation v = H0D, with H0 the constant of
proportionality-Hubble constant-between the "proper
distance" D to a galaxy, which can change over time, unlike
the comoving distance, and its speed of separation v, i.e.
the derivative of proper distance with respect to
cosmological time coordinate. (See uses of the proper
distance for some discussion of the subtleties of this
definition of 'velocity'.)
Hubble constant is most frequently quoted in (km/s)/Mpc,
thus giving the speed in km/s of a galaxy 1 megaparsec
[3.26 ly] away, and its value is about 70 (km/s)/Mpc.
However, the SI unit of H0 is simply s^-1 and the SI unit
for the reciprocal of H0 is simply the second. The reciprocal
of H0 is known as the Hubble time.
Tests of Big Bang Cosmology
EVOLUTION OF STARS - MAIN SEQUENCE
Wikipedia -- Main sequence
In astronomy, the main sequence is a continuous and
distinctive band of stars that appears on plots of stellar
color versus brightness. These color-magnitude plots are
known as Hertzsprung-Russell diagrams after their
co-developers, Ejnar Hertzsprung and Henry Norris Russell.
Stars on this band are known as main-sequence stars or dwarf
stars. These are the most numerous true stars in the
universe, and include the Earth's Sun.
After condensation and ignition of a star, it generates
thermal energy in its dense core region through nuclear
fusion of hydrogen into helium. During this stage of the
star's lifetime, it is located on the main sequence at a
position determined primarily by its mass, but also based
upon its chemical composition and age. The cores of
main-sequence stars are in hydrostatic equilibrium, where
outward thermal pressure from the hot core is balanced by
the inward pressure of gravitational collapse from the
overlying layers. The strong dependence of the rate of
energy generation on temperature and pressure helps to
sustain this balance. Energy generated at the core makes its
way to the surface and is radiated away at the photosphere.
The energy is carried by either radiation or convection,
with the latter occurring in regions with steeper
temperature gradients, higher opacity or both.
The main sequence is sometimes divided into upper and lower
parts, based on the dominant process that a star uses to
generate energy. Stars below about 1.5 times the mass of the
Sun primarily fuse hydrogen atoms together in a series of
stages to form helium, a sequence called the proton-proton
chain. Above this mass, in the upper main sequence, the
nuclear fusion process mainly uses atoms of carbon, nitrogen
and oxygen as intermediaries in the CNO cycle that produces
helium from hydrogen atoms.
Worth While Rabit Hole -- Solar neutrino problem
Main-sequence stars with more than two solar masses undergo
convection in their core regions, which acts to stir up the
newly created helium and maintain the proportion of fuel
needed for fusion to occur. Below this mass, stars have
cores that are entirely radiative with convective zones near
the surface. With decreasing stellar mass, the proportion of
the star forming a convective envelope steadily increases.
Main-sequence stars below 0.4 Solar masses undergo convection
throughout their mass. When core convection does not occur,
a helium-rich core develops surrounded by an outer layer of
In general, the more massive a star is, the shorter its
lifespan on the main sequence. After the hydrogen fuel at
the core has been consumed, the star evolves away from the
main sequence on the HR diagram, into a supergiant, red
giant, or directly to a white dwarf.
365 Starry Nights; An introduction to Astronomy for every
night of the year
by Chet Raymo
365 Starry Nights is a unique and fascinating introduction
to astronomy designed to give you a complete, clear picture
of the sky every night of the year. Divided into 365
concise, illustrated essays, it focuses on the aesthetic as
well as the scientific aspects of stargazing. It offers the
most up-to-date information available, with hundreds of
charts, drawings, and maps-that take you beyond the visible
canopy of stars and constellations into the unseen realm of
nebulae and galaxies.
This simple yet substantial text is full of critical
information and helpful hints on how to observe the stars;
describe their position; calculate their age, brightness,
and distance; and much more. Whether you observe the sky
with a telescope or the naked eye, 365 Starry Nights makes
the infinite intimate and brings the heavens within your
grasp. Keep this invaluable, informative guide close at
hand, and you'll find that the sky is the limit 365 nights a
A Modern Day Yankee In A Connecticut Court: And Other
Essays On Science
by Alan Lightman
A second collection of brief essays, reflections, and tales
by Harvard/Smithsonian Astrophysical Observatory physicist
Lightman (Time Travel and Uncle Joe's Pipe). We meet the
spectral Uncle Joe in this collection, too, in the first and
longest essay in the book. Here, Lightman sets out to do no
less than explain the past century of astrophysical
discoveries--from star measurements to gravitational
theories to black holes--as sort of an update for his
no-nonsense relative. Alas, the device of the ghostly
ancestor with a skeptical but inquiring mind was already
cloying in volume I, so let us hope Lightman buries him next
time around. For the rest, Lightman succeeds admirably in
some predictable terrains: gravitational waves,
extraterrestrial life, Halley's comet, what happened in the
first moments of the Big Bang (impressed by Stephen
Hawking's equations to define those initial conditions), and
other matters astrophysical. Here Lightman demonstrates a
gift for colloquial reductions of the complex that would be
intelligible by junior high students. But the book's charm
more often lies in the unexpected and the personal: an
amusing tale that established a student's incompetence in an
electronics lab; a bedtime conversation with his daughter
about the camel's hump and the moon in the sky; the origins
of snowflakes; a neurophysiological description of an
encounter climaxed by a smile; a walk around Walden Pond.
Lightman also displays a serious political side, inveighing
against the new breed of star warriors who are elated at the
thought of unleashing potent weapons in space--individuals
who have never known war in their lifetimes. In homage to
Swift, Lightman's "Modest Proposal" is to annihilate some
measly third-world nation, already hopelessly in poverty and
debt, so that the rest of the world could get a close-up
view. The title essay is a bit about waking up in 1880 and
being arrested for spouting all sorts of nonsense about
being a creature of the 20th century. The trial is not going
well as our hero realizes that he can't explain how a
television set or a refrigerator works. But then comes a
moment when he has to write something and produces a
ballpoint pen. Acquittal promptly follows. A pleasantly
mixed bag, then, with references provided for those who
would like to read more.
The Cosmological Distance Ladder: Distance and Time in
by Michael Rowan-Robinson
The scale of cosmological distances has been a topic of
dramatic controversy during the past decade. Experts
estimating the size of the universe, as measured by the
Hubble constant, have differed by as much as a factor of
two. Just how big is the universe, and why have distance
measurements varied to greatly? Michael Rowan-Robinson sheds
new light on the origins of this controversy, critically
reviewing the main techniques of measuring distances between
astronomical bodies both within and outside our galaxy.
Stars, galaxies, and cluster of galaxies all play a major
part in the distance ladder, and knowledge of distance is
essential for all branches of astronomy. As we examine the
geometrical speculations of the Greeks and the first correct
estimates of the relative distances of the planets from the
Sun by Copernicus we realize that this is also a history of
mankind's expanding horizon. Offering a fair, balanced
review and a clear synthesis of the variety of techniques
and methods for measuring cosmological distances (including
the work of Gerard del Vaucouleurs, Allan Sandage, Gustav
Tammann, and others), Rowan-Robinson integrates the various
distance-measuring methods and presents a new, revised
distance scale for the known universe. He supplies a unique
perspective on modern astronomy itself as he pursues and
expanding scale of distance from the solar system outward.
Extensively illustrated with photographs and line drawings.