Astronomy Review Sheet

PREVIEW:

Earth

Home planet
Consists of oceans, jungles, volcanoes

The Moon

Earth’s nearest neighbor
Mass is only 1/80 that of Earth’s
Unable to retain an atmosphere
Little to no erosion
Unable to retain heat
Has changed very little over the Assignment of time

The Planets

Sister bodies of Earth
Move slowly against the pattern of background stars because of their orbital motion
Named after gods and goddesses

Properties

Inner Planets

Mercury: Made up of craters; airless
Venus: Dense; extremely high temperatures
Earth: Made up of oceans, volcanoes, jungles, deserts
Mars: Made up of canyons and deserts; cold temperatures

Outer Planets

Jupiter: storms that have lasted centuries
Saturn: Rings made up of icy fragments
Uranus: Spin is tipped
Neptune: Methane clouds fill the atmosphere

The properties of other planets allow us to better understand Earth:

Atmospheres and circulation features
The amount of volcanic activity
The strength of their magnetic field
The amount of water present and what forms it is in

Order of Astronomical Objects closest and farthest from the Sun

Venus
Earth
Mars
Asteroid Belt
Jupiter
Icy Dwarf Planets

The Sun

About 5 billion years old, estimated to last 5 billion more years
Generates energy in the core by nuclear reactions that convert hydrogen into helium
Energy flows out into space, illuminating and warming the planets

The Solar System

Asteroid: objects too small to have pulled themselves into a round shape
Asteroid Belt: region between Mars and Jupiter in which asteroids orbit the sun
Kuiper Belt: realm in which uncounted icy bodies, large and small, reside

Astronomical Sizes

Astronomical Unit (AU): the average distance from the Earth to the Sun
Lightyear (Ly): The distance light travels in a year

The Milky Way

A cloud of several billion stars with a flattened shape like the Solar System
Stars intermingle with immense clouds of gas and dust
Stars are the site of stellar birth and death

Galaxy Clusters and the Universe

Galaxies assemble themselves into what is known as a galaxy cluster
Local Group: a cluster of galaxies in which the Milky Way belongs

Basic Order: Earth, Solar System, Milky Way, Local Group

Forces and Matter

Gravity: A universal force of attraction between ALL objects

Matter

Composed of submicroscopic particles called atoms
Nucleus: The central core of an atom made up of electrons (-), protons (+), and neutrons (equal number of electrons and protons)
Quarks: basic particles that make up electrons and protons
Quarks are attracted to each other by the strong force

Electromagnetic Force

Can either attract or repel
Opposites attract
Like charges repel

Dark Matter

Everything unknown, increased by dark energy

Scientific Method

Propose an idea
Test out the idea

CHAPTER ONE: CYCLES OF THE SKY

Cyclic behavior in the sky implies that events predictable

The Celestial Sphere

Horizon: A dome in which the sky meets the ground
Celestial Sphere: An imaginary sphere surrounding Earth that represents the sky
When standing on Earth, the ground blocks the bottom half of the celestial sphere
The celestial sphere represents a way of thinking about the motion and the location of the stars in the sky

Constellations: Fixed patterns in the sky that bear resemblance to certain animals

Stars move through Space
As seen from Earth, these motions are extremely slow
We are seeing the same sky that ancient people saw
Constellations were used as mnemonic for keeping track of the seasons and navigation

Motions of the Sun and Stars

Stars rise along the east and set in the west

North and South Celestial Poles: Two points on the celestial sphere that do not move

Lie exactly above the north and south poles of Earth
The Celestial sphere rotates around the celestial poles
Stars appear to circle the north celestial pole in a counterclockwise direction in Earth’s northern hemisphere

Motion of The Earth

Earth orbits in the same direction that it spins
Earth’s spin causes the daily motion of the sky and stars
Earth orbital motion changes what we see in the sky over the Assignment of a year
Earth’s motion allows us to see stars previously hidden. These movements are called annual motions.

Elliptic and Zodiac

Elliptic: A line that runs around the celestial sphere

An eclipse can occur when the new or full moon is on this line

Zodiac: A belt shaped region of the sky surrounding the elliptical, contains 12 constellations

The Seasons

Seasons are caused by the Earth’s rotation on its axis
A surface facing directly toward a source of radiation is heated more when tilted
The tilt of Earth’s axis causes the eliptic to be tilted with respect to the celestial equator
The axis remains oriented in the same direction as the Earth orbits the sun

Equinoxes and Solstices

Lag of the seasons: The result of the oceans and land being slow to warm up and cool down
Equinox: The time period in which the Sun is on the equator and there is an equal amount of length in days and nights
Spring (Vernal) Equinox: near March 20th
Fall (Autumnal) Equinox: near Sept. 22
Solstice: Time period in which the Sun is in the celestial equator and the longest and shortest nights occur
Summer Solstice: June 21
Winter Solstice: December 21- Earth’s tilt is closest to the Sun

The Sun’s Changing Position

Zenith: The point in the sky straight overhead
The sun rises due east and sets due west when on the celestial equator
During the Vernal Equinox the sun rises due east and sets due west
From the Vernal Equinox up to the Summer Solstice, the sun’s rising and setting points shift north each day

The Moon

Goes through lunar phases that take approximately 29.5 days
Waxes: grows
Wanes: shrinks
Cycles of the Moon against the stars is caused by the Moon’s orbit around the Earth
Half of the moon is always lit by the sun

Eclipses

Lunar Eclipse: occurs when the Earth passes between the Sun and the Moon and casts its shadow on the Moon
Total Solar Eclipse: occurs when the Moon passes between the Sun and Earth and blocks our view of the Sun
Solar eclipses happen very rarely in different locations once every century
Can only occur during a new or full moon
Can only be seen within a narrow path
Do not occur every lunar month because the Moon’s orbit is tilted with respect to Earth’s orbit
Annular Eclipse: Sun is only partly covered

CHAPTER TWO: THE RISE OF ASTRONOMY

Shape of The Earth

Aristotle

Presented arguments of Earth’s round shape through the spherical phases of the moon
A traveler who moves south will see the stars disappearing below the horizon

Sizes of the Moon and Sun

Aristarchus

Used geometric methods to estimate the relative sizes of the Moon, Sun, and Earth, and the relative distances to the Moon and Sun
Angular Size: Apparent size of an object
If distance and diameter are equal, and if the Sun’s rays were parallel, the diameters of the Earth and Moon would be the same
Argued that the Sun, not the Earth, was the center of the Universe
Parallax: apparent shift of objects against distant backgrounds
Smaller distances create larger parallax, vice versa
Other astronomers assumed that stars were much closer to the Earth than they actually were, leading to measurements that were not precise

Eratosthenes

Succeeded in making the first measurement of Earth’s size
Because the Sun is far away from Earth and much larger, its light travels in nearly parallel rays towards the Earth

Motion of the Planets

Planets move against the background stars because of a combination of Earth’s and their orbital motion around the Sun
Retrograde Motion: the reversal of planetary motion in the opposite direction (west to east)
Geocentric Model: All celestial objects orbit the Earth, Earth is the center of the Universe

Ptolemy

Created a better model that predicted the planetary motions with better accuracy
Fashioned a model in which each planet moved in epicycles, each planet moved in a small circle
Retrograde motion occurs when epicycles carry planets in the opposite direction

Copernicus

Presented the ability of the heliocentric model to explain retrograde motion
An inner planet on a smaller orbit moves faster and overtakes an outer planet that is moving more slowly on a larger orbit
An observer on an inner planet, it appears as though the outer planet has reversed its motion against the night sky
Copernicus applied geometry to measure the radius of each planet’s orbit

The Renaissance

The Catholic Church regarded heliocentrism as heresy

Scientists were skeptical of the heliocentric model as it did not:

Answer the parallax problem
No physical sensations on Earth could be detected
Sky still moved around Earth

Tycho

Observed the “exploding star” or Supernova
Found no evidence of stellar parallax

Kepler

Used Brahe’s measurements to depict that Mars followed an elliptical orbit rather than a circular orbit
Assigned semi-major axis, half the long dimension of an ellipse, as a measure of distance from the sun
Compared orbital sizes with orbital periods and deduced laws of planetary motion

Kepler’s Laws

Planets move in elliptical orbits around the sun at one focus of the ellipse
The orbital speed of a planet varies so that a line joining the Sun and the planet will sweep over equal areas in equal time intervals
When a planet is near the sun, it moves more rapidly than when it is far away
The amount of time a planet takes to orbit the Sun is related to its orbit’s size, such that the period, P, squared is proportional to the semimajor axis, a, cubed
A planet far from the Sun has a longer orbital period than a planet closer to the Sun
P² = a³

Kepler’s laws:

Method of comparing motions of different planets
Measurement of orbital period yields distance from Sun
Provides insight into the nature of the force (gravity) holding planets in their orbits

Galileo

First person to use a telescope
Observed how bodies move and fall
Deduced the first laws of motion

Newton

Invented calculus
Developed 3 laws of motion
Developed the universal law of gravitation

CHAPTER THREE: GRAVITY AND MOTION

Gravity: A universal force that acts on all objects

Gravity holds astronomical bodies together
Controls the motions of astronomical bodies

Inertia: The tendency of a body to remain at rest or in motion to keep moving in a straight line at constant speed

In the absence of a force, inertia keeps an object moving at constant speed
The more inertia, the more mass

Motion: Change in an object’s position

Acceleration: Change in an object’s speed or direction due to a force

An object traveling on a curved path will travel at constant speed and accelerate

Velocity: An object’s speed in a given direction

Newton’s Laws

An object at rest will remain at rest, or an object in motion will continue moving in a straight line unless a force acts upon the object
Net Force: Total of all forces acting on a body
Balanced forces lead to no change in motion
The acceleration of a body is proportional to the net force exerted on it, but is inversely proportional to the mass of the body
When two objects interact, they create equal and opposite forces on each other
The resulting magnitude of the acceleration is not necessarily the same
A heavier or larger object will accelerate more than an object that is lighter

Law of Gravity

Every mass exerts a force of attraction on every other mass
The strength of the force is directly proportional to the product of the masses divided by the square of their separation
If the masses of either object increases and the other factors remain the same, the force increases
If the distance between the two masses increase, the force decreases

Orbital Motion

The masses of orbiting objects determine the gravitational force between them
Centripetal force must be applied to any object moving in a circle
The centripetal force, F_C, depends on the mass and speed at which an object swings in a circle as well as the object’s distance from the center of the circle

Surface Gravity

In a vacuum all objects accelerate downward at the same rate by a force called surface gravity
Gives a measure of the gravitational attraction at a planet or star’s surface
This acceleration determines not only how fast objects fall, but also indicates what a mass weighs
Influences shape of celestial object and whether it can sustain an atmosphere

Escape Velocity

To overcome a planet’s gravitational force, a rocket must achieve a critical speed known as escape velocity
The faster an object is thrown upwards, the higher it goes, and the longer it takes to fall back
Escape velocity is the speed it must achieve to NEVER fall back
A larger mass will have a larger escape velocity, vice versa
An object with a smaller radius will have a larger escape velocity