Astronomy for Students

by Barry and Helen Setterfield

 

This course is the result of requests we have gotten through the years, most recently from a home school group near us here in southern Oregon. Two three-tiered lessons will be put up each week for six weeks. [Note: that was the original plan; it looks like we will be doing more lessons than 12. We'll continue until a full course has been done]. The tiers are designed for different age groups. Level 1 is for 4th - 6th grades. Everyone should start with this level. Junior high and early high school should go on to Level 2. Level 3, after those two are read, is for upper high school and college students. The levels build on each other, so even though the first level is stated quite simply, it contains the basic information on which the other two levels are built. There are no tests, no homework, although we have presented some challenges for the upper levels and if you email us your results we will be happy to email you back with the answers. This is for the fun of learning, so enjoy.

Lesson 1 -- this page
Lesson 2 -- telescopes
Lesson 3 -- solar system overview and Mercury
Lesson 4 -- Venus
Lesson 5 -- Earth
Lesson 6 -- the Moon

 

Skymap for July, 2012

Lesson One – relative sizes

Level One:

Pick up a leaf.  Hold it up and see how much of a distant tree you can block out with the leaf.  Does that mean the leaf is bigger than the tree?

If you look up at a full moon and measure it against your hand, you will find it seems to be exactly the same size as the sun.  You can cover each one with your fist.  Does that mean the moon and the sun are the same size? 

Here is a diagram showing you the difference:

comparisons

If you look very hard next to the earth, you will see a tiny dot. That is our moon.



We must be very, very far away from the sun for the moon to look like it is the same size to us.  But that is why the moon can cover the sun during a solar eclipse

eclipse


If you want to have fun  with this idea of different sizes and an eclipse, try the experiment here

Right now (June 19, 2012) in the sky, the planet Saturn is high in the southwest, just after dark.   You can see where to look for some planets on this page

They are just tiny twinkling lights, right? 
Nope!
Look at the sizes of Jupiter and Saturn:

Keep scrolling down and you will see how small we are compared to what looks like little twinkling lights in the night sky.


Level 2

Understanding that distance really makes things look smaller, let’s look at some of the sizes in numbers:

  • Let’s make the earth 1 inch wide.
    Our moon would be ¼ of an inch wide.
    Jupiter would be 11 inches wide.
    The sun would be 125 inches wide (almost 10 ½ feet).
    Our galaxy, the Milky Way, would be  about 6 x 1011 inches (600,000,000,000 inches) or   9.3 MILLION miles across

Here, the math is too much:  take a look at this

Project:  how far would you have to back up from the sun (if it were 10.5 feet across)  so that the moon (1/4 inch across) could cover it?  Try it.  It will take more than your yard.  It will take more than the church parking lot.  Get your mom to hold the sun and get your dad to walk down the road with you – a long, straight road.  That will give you an idea of how far our moon is from our sun.

Level 3

Instead of trying to walk the distance from the moon to the sun, above, see if you can figure it out mathematically.  Hint:  8000 miles is equal to one inch.  The sun is about a million miles across.  The moon is 250,000 miles away from us; the sun is 93,000,000 miles away from us.  See what you can do.

Our galaxy, if the earth is one inch, is 9.3 million miles across.  In real life, it is 100,000 light years across.  A light year is not a time, but a distance.  It is how far light, at its present speed, can travel in one year.  Light travels at 186,000 miles a second, so

  1.  How far is one light year?
  2. How many miles across is the Milky Way Galaxy?

The Milky Way is a medium-large galaxy.  We are in a local group of galaxies.  The closest one to us is Andromeda (M31).  It’s about 2.5 million light years away.  It appears as a fuzzy spot to the naked eye.  In a clear, dark sky it is a faintly hazy patch in the morning sky this time of year.

So, how do we know these large astronomical distances?  For nearby stars in our galaxy, you can measure the apparent change in direction of a star when you go across the diameter of the earth’s orbit.  The diameter of the earth’s orbit requires six months.  How much as the star ‘moved’ during that time?  Actually, it’s us who has moved, but because of that movement, the star will be in a slightly different position in the sky.  The diameter of the earth becomes the base of a triangle.  The tip of the triangle will be the star.  We can measure the amount of difference in the ‘top’ of the triangle where the star is from the beginning of a six month period to the end of it.  This difference will give you an angle whose measurement will tell you how far away the star is.  This page gives an excellent explanation

“parsec” is a distance of 3.26 light years. 
“AU” is “astronomical unit” and 1 AU is the distance from the Earth to the Sun – 93 million miles.

If we want to measure distances to objects beyond our part of the Milky Way Galaxy, we use a type of star called a Cepheid Variable star.  These stars have a characteristic light fluctuation curve.  That means it goes through its cycle of being bright and then a little dimmer and then bright again with clockwork precision.  The brighter the star is, the longer it takes to go through that cycle.  We can see these stars in other galaxies, out quite a ways.  As we measure the timing of the brightness changes we know how bright the star is, regardless of what we think we are seeing.  We can then measure how bright it seems to us vs. how bright we know it really is, and that tells us the distance. 

cepheid1

 

cepheid2

The top diagram shows the peaks associated with a cepheid star's change in brightness.

The lower diagram shows how the peaks are longer apart when the star is brighter.



Because the most distant galaxies we can see are too far for us to see Cepheid variable stars, we use exploding stars the same way.  It’s called a Type Ia supernova explosion.  They have a standard brightness we can measure.