Astronomy Review Sheet

Astronomy Review Sheet



  • 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


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


  • 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


  • 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


  • 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


Shape of The Earth


  • 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


  • 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


  • 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


  • 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


  • 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


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


  • 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


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


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


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, FC, 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
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