Astronomy Review Sheet

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 course 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

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 course 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