In the Preview section of the text, we are given a sense of scale.


Earth is our planet (a round body that orbits the Sun and has cleared out any material from its orbit). Much of our study of astronomy is based on a study of the Earth.  It is not always possible to go to other planets, stars, etc. - so we compare processes that we can observe on the Earth with features of other bodies.



The Moon is Earth's only natural satellite (a body orbiting a planet).  It is about a quarter the size of the Earth, and a quarter million miles away.

The Moon is very different from Earth because it is smaller, is cooler on the inside and, therefore, does not have plate tectonics.  The Moon gives us clues to what the early Earth may have looked like.



Planets are bodies that...

  • ...orbit the Sun.
  • ...have enough gravity to be pulled into a round shape.
  • ...have cleared out most of the debris from their orbits.

There are eight planets in our solar system.

  • Mercury
  • Venus
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune

Mercury, Venus, Mars, Jupiter and Saturn can be seen in the night sky without the use of a telescope.  The planets are divided into two groups: the four inner planets and the four outer planets.

The planets were discovered because they move against the background stars, whereas stars all  appear to move together across the night sky.

You can remember the order of the planets with the following sentence:

Mary visits every Monday and just stays until noon.

  •   Mary       -   Mercury
  •   Visits      -   Venus
  •   Every      -   Earth
  •   Monday  -   Mars
  •     and         -   asteroid belt
  •   Just        -   Jupiter
  •   Stays     -   Saturn
  •   Until       -   Uranus
  •   Noon     -   Neptune
  •     .(period)  -   Pluto


The Sun

The Sun is a star - a huge ball of hydrogen and helium.  It shines by nuclear reactions that take place in its core.

The Sun's diameter is 100 times larger than the Earth's diameter; and the Sun is more than 300,000 times more massive than the Earth.


The Solar System

The Solar System is made up of the Sun and all of the bodies that orbit it - planets, moons, dwarf planets, comets, and asteroids.


Units of Measure

An Astronomical Unit (AU) is the distance between the Earth and the Sun.  It is equal to about 93 million miles (150 million km).  we use AUs to measure distances within the Solar System.

A Light Year (ly) is the distance that light can travel in one year.  This is equal to about 6 trillion miles (10 trillion km).  We use light years to measure distances between stars.

Scientific Notation

Because of the large numbers used in measuring distances in Astronomy, we often use scientific notation (or powers-of-ten notation).  To do this, we use ten to an exponent to tell the number of zeros (or decimal places).  For example...

  • 93,000,000 miles = 9.3 x 107 miles    (1 AU)
  • 6,000,000,000,000 miles = 6 x 1012 miles    (1 ly)

When you multiply two of these numbers, you add the exponents.  When you divide them, you subtract the exponents.


The Milky Way Galaxy
A galaxy is a huge system of stars that are held together by gravity.  Our galaxy is called the Milky Way Galaxy.  It is about 100,000 ly across.  It spins, with stars orbiting its center.  Our Sun takes about 240 million years to make one complete trip.

The galaxy also contain huge clouds of gas and dust (much bigger than out solar system), which are where new stars are formed.


Galaxy Clusters, Superclusters, and the Universe

A group of galaxies held together by gravity is called a galaxy cluster.  The one to which our galaxy belongs is called the Local Group, which contains about 40 galaxies and is about 10 million ly across.

A group of galaxy clusters held together by their gravity is called a supercluster.  Ours is called the Local Supercluster, which contains a few dozen galaxy clusters and is about 150 million ly across.

The Great Attractor appears to be a cluster of superclusters, and is about 300 million ly across. 

The Universe contains everything.  We can see about 14 billion ly in any direction - although the universe may be larger than this.



Gravity is the force of attraction between all objects.  Everything that has mass also has gravity.  Gravity connects all objects in the universe, no matter how far apart they are.


Atoms and Atomic Forces

Matter is made up of atoms.  Atoms are made up of...

  • Protons - positively charged particles in the atom's nucleus.
  • Neutrons - particles with no charge in the atom's nucleus.
  • Electrons - negatively charged particles orbiting the atom's nucleus.

There are three main forces that operate on atoms...

  • Electric Force - occurs between charged particles.  Opposite charges attract each other, similar charges repel.
  • Strong (Nuclear) Force - bind the protons and neutrons together in an atom.
  • Weak Force - plays a role in radioactive decay (when atoms break down into other elements).

The Unknown Universe

The stars and galaxies that we can observe seem to be influenced by stronger gravity than is created by the stars and galaxies we can see.  Galaxies seem to contain large amounts of dark matter - matter that we cannot see because it does not emit any radiation that we can detect. 




Homework from the Text:

  • Read the Preview section (pages 1- 14).
  • Answer the Review Questions (#1-9) on page 15.