Patrick B Hall
Chapter 1- Our place in the universe
This chapter is a basic fluff over the concept of astronomy and space, not much to take away from this
chapter. It goes over our place in the universe as well as some basic concepts like the Big Bang, solar systems,
galaxies and so forth. At the end of this small chapter it goes over the idea that we can only observe so much of
the universe because light takes time to travel. Also it touches on the idea that the universe is expanding why it
looks like its expanding away from us. Lastly it covers the concept that we’re always moving in different frames of
Universe » Observable Universe » super cluster » cluster » galaxy » star system » solar system
Telescopic observations of galaxies show that the entire universe is expanding, meaning that the
average distances between galaxies are increasing with time.
The rate the universe is expanding leads to the age of the universe to 14 million years.
This is confirmed since we can’t see past 14 million light years away.
Stars take the two first elements H and He and turn them into heavier elements through fusion.
The further we look in distance the further we look back in time.
We are always in constant motion around the universe. Earth rotates, orbits the sun and our solar system
rotates the center of the galaxy which in turn travels about the universe.
We are “star stuff” – Carl Sagan (1934-1996)
Chapter 2 – Discovering the universe for yourself
In chapter two we begin to go deeper into the subject than chapter 1. We learn that our ancestors
originally thought of the universe as celestial sphere around the earth and the start were on this sphere. This was
very convenient to explain the things they observed. Later it covers why things in our sky rise and set, the seasons,
phases of the moon and the slow motion wobble of our axis.
Originally the pattern of stars were called constellations
but now astronomers call reigns in the sky with well-
defined borders a constellation, the stars just help us
locate these region.
Even though the stars look like they land on a celestial
sphere, in reality they very in distances away from earth.
Although false the idea of a celestial sphere is used to
understand the stars, the sphere has 4 important areas.
1. North Celestial pole is the point in the sphere
directly above the North Pole.
2. South Celestial pole is the point in the sphere
directly above the South Pole. 3. Celestial Equator is a projection of the earth’s equator on the sphere.
4. Ecliptic is the path the Sun fallows as it appears to circle around the celestial sphere.
Local sky is what you see when you go outside and look up at the sky, it has keep feature you need to
1. The horizon is the point where the earth meets the sky.
2. The imaginary line stretching from N to S is called the Meridian.
3. The Zenith is the point directly above you.
In order to point out an object in the sky you need its direction along the horizon and the altitude in the
sky (how high it is).
Altitude is measured in angular degrees the horizon being 0° and the zenith being 90°.
We divide each degree into 6o arcminutes and each arcminute into 60 arcseconds.
Stars near the poles do not rise or set, rather they spin about the north celestial pole. We call these stars
Because earth rotates west to east stars appear to rise in the east
and set in the west this includes our Sun.
One thing that stumped our ancients was apparent retrograde
motion, which was that the plants appears to loop and change
direction. Also they couldn’t observed stellar parallax so they stuck
with the earth centered model.
You can determine your latitude by measuring the altitude of the
celestial pole in the sky.
Polaris is our north star now, but as the earth slowly wobbles it will
move, in the south the Southern Cross points to their south celestial
The tilt of the earth is the main reason for the seasons, our ellipse
orbit isn’t that extreme and the tilt causes different amounts of
sunlight to hit different parts of the earth throughout the year.
We use equinoxes and solstices to mark the procession of the seasons, they divide our obit around the
sun in four parts.
Just like a spinning top, as earth spins its axis slowly rotates. This is called precession and it takes roughly
26,000 years to complete. As it changes so does our “North Star”.
The moon orbits every 27 days.
We see Lunar Phases because half of the moon is always lit up by the sun, and since it also orbits us we
different fractions of the moon in darkness.
The moons phases are directly related to our move along our sky. For example the full moon must rise
around sunset and reach its highest point at midnight and set at sun rise since it’s opposite the sun.
Waxing mean increasing and waning means decreasing.
A Lunar Eclipse is when the Earth lies between the moon and the sun so that the shadow of the earth
falls on the moon hence Lunar Eclipse.
A Solar Eclipse is when the moon lies between the Sun and the Earth so that the moon covers the Sun.
“We are all connected; To each other, biologically.
To the earth, chemically. To the rest of the universe
atomically.” – Neil deGrasse Tyson Chapter 3 – The Science of Astronomy
This chapter goes a bit into the history of Astronomy, science and the Copernican Revolution. We learn
how to tell good science from bad science and how to identify and use theories. Also we learn about Johannes
Kepler (1571-1630) and his three laws. Half ways through this chapter we learn about how Galileo Galilei (1564-
1642) solidified the Copernican Revolution.
The Ancients and Science
Ancient Greeks needed to explain the apparent retrograde motion they came up
with the Ptolemaic model, this model had each planet rotating on a
small orbit which was tethered to larger orbit around earth.
The Copernican Revolution started with the European
Kepler came up with the idea that the plant fallowed
orbits that were not perfect circles but rather ovals called
The ellipse had two “centres” or foci.
The line connecting the two is called the Major Axis and
half that would be called the Semi Major Axis.
Kepler came up with three laws of planetary motion:
1. Kepler’s first law: The orbit of each planet about the sun is an
ellipse with the sun at one focus.
2. Kepler’s second law: As a planet moves around its orbit it sweeps out equal areas
in equal times.
3. Kepler’s third law: p = a3
Perihelion is the short end of the orbit and the
Aphelion is the long end.
Galileo attacked the three point of the old model:
1. Earth could not be moving because
objects would fall off. Galileo did
experiments with rolling balls to show
observations that worked like newton’s
2. Heavens are perfect and unchanging.
Galileo saw comet, supernovas, sunspots
and craters on the moon.
3. No observed stellar parallax. Galileo shows that stars were too far away to observe stellar
Also Galileo observed Venus’s phases like the moon and the moons of Jupiter orbiting Jupiter.
Science as a Whole
Modern science seeks explanation for observed phenomena that rely solely on natural causes. Science
progresses through the creation and testing of models of nature that explain the observations as simply as
possible. Another key element of good science is Occam’s Razor. Which is that the amount of accuracy of a
model is only important if it’s inversely proportional to its simplicity. In theory you could make a earth centered
model that explains everything accurately but it would be immensely complicated. Bias can also hinder science
in the fact that it can stop good and objective science from progressing because it is too far out of the paradigm.
When a model based on a primary hypothesis is tested and stands up to experiment by many people it gets
elevated to a theory. “Knowing how to think empowers you far beyond
those who know only what to think.” ― Neil
Chapter 4 – Making Sense of the Universe
Chapter 4 basically gets into the actual math and physics being the Astronomy. Newton’s three laws of
motion as well as Newtonian gravity. 4.2 gets into conservation of angular momentum and conservation of
energy with we’ll see again later. Then we wrap it up a little bit about the tides and rightfully named tidal forces.
1. An object moves at constant velocity is there is no net force Equations
acting upon it
2. The sum of all forces is equal to the mass time acceleration
⃗ ⃗⃗ ⃗⁄
3. For any force, there is always an equal and opposite reaction
force. ⃗ ⁄
The law of conservation of momentum states that the total angular
momentum of system will stay the same
Because there are no objects around to give or take angular momentum
earth’s orbit will remain the same, all though the moon is slowly taking ∑ ⃗ ⃗
away the angular momentum of our rotation. ⃗⃗
Conservation of energy: Energy can be transferred from one object to another or transferred from one
type to another, but the total amount of energy is always conserved.
Energy can be subdivided into three types:
Kinetic – Motion, falling rocks or anything moving possesses Kinetic energy.
Radiative - Is basically light, all light carries energy. This includes x-rays and gamma rays.
Potential – Is energy to be later converted into other forms. For example if you send Kinetic
energy in moving a rock up on a shelf, the rock gain gravitational potential energy since it have
the potential to fall.
Thermal energy is basically heat. Thermal energy the collective kinetic energy of many particles inside
Thermal energy is also measured in science in Kalvin (K).
Every mass attracts every other mass through gravity.
The strength of this force is inversely proportional to the square of the distances between their centres.
Basically the force get exponentially weaker as the two
Gravitational encounters are when two bodies get
close enough for the gravity to affect them.
Tidal forces are the tug and pull of the moon on the
earth as the moon travels around the earth. This
creates the tides. Also when the sun and the moon are lined up the tides are extra-large.
Tidal forces are also making bulges on the ground as well.
“In modern times, if the sole measure of what’s out
there flows from your five senses then a precarious
life awaits you.”
― Neil deGrasse Tyson
Chapter 5 – Light
In this chapter we dive into the world of light and how to works and how we know it. Since light is so
important to Astronomy, it helps Astronomers look into parts of the universe a simple image couldn’t do. We learn
how light works then how it interacts with various things; matter, atoms and objects in space.
Properties of light
When white light enters into a prism it breaks into a spectrum of visible light.
Wavelength is the distances between the peaks and the frequency is the amount of times the wave
repeats in a second.
The longer the wavelength the lower the frequency and lower the energy. It’s important to note that this
relationship is only possible because the speed of these waves is fixed at ⁄ .
Another property of light is its wave particle duality. Meaning it has properties of both particles and wave.
Light comes in packets or particles called photons, these photons have a specific wavelength,
frequency and energy.
Light has different frequencies with visible light in the middle from 700nm - 400nm
Red side we got infrared, microwaves and radio waves in that order.
Violet side we have ultraviolet, x-rays then
Light and matter
Light and matter can interact in four
Emission: Like how a light bulb
emits light (often confused with
Absorption: if you were to place
your hand near that light bulb your hand absorb light. The absorbed energy would in turn warm your hand.
Transmission: Some forms of matter can transmit light, meaning light can pass right through.
Reflection/scattering: Light can bounce off matter, leading what we call reflection. (like a
We can learn about matter thought light, by studying how matter, effects light.
There are three different kinds of spectra; continuous, emission and absorption.
Continuous spectra is produced when something emits all visible light
Emission spectra comes from a source like hot gas the type of light it emits will tell us the chemical
makeup of it.
Absorption spectra come when you have white light that passes through something. The object will
absorb some light. The light that’s missing from the white light will tell us what the object is made up of.
Atoms and light
Basically why thing absorb and emit light is because of a property of
This property is that they can absorb and emit energy
When an electron is hit with a photon it gains energy,
the electron reacts by moving up an energy level.
The thing is electrons are not stable here and will
eventually go back down to ground level, when they
do they release the difference in energy as light.
The only thing is though as we learned earlier each photon
can have a different energy (or frequency) and elections need only a certain amount of energy to
move up or down levels.
Also as you go up in energy levels the
less energy needed to move up.
Planets, Stars and people all emit
thermal radiation or black body radiation.
Our bodies are too cold to emit visible
light rather we emit infrared light.