I have just started the space unit in school and have 3 questions. I hope someone can answer and explain. I would appreciate! :)

1) "If you look into the night sky, you will see a cloudlike band of stars that stretches across the sky. Because of its milky appearance, this part of the sky is called the "Milky Way". The "Milky Way" is the disk of the "Milky Way Galaxy".

I don't get this part! Are "Milky Way" and "Milky Way Galaxy" the same thing or are they different? What is a "disk" and what does it mean by "The 'Milky Way' is the disk of the 'Milky Way Galaxy'."?

For example, in the following picture, which part is the disk?
http://news.nationalgeographic.com/news/2005/08/images/050817_milkywaybar.jpg

2) As you observe the night sky, why do stars apear to move westward across the sky?

I would say earth's rotation causes stars to appear moving westward around a central point (Polaris). And earth's revolution causes them to shift slightly to the west each night, over a period of a year. Am I right?

3) "The earth is 4.6 billion years old. How many revolutions around the center of galaxy has it travelled? The sun is located in one of the spiral arms and completes one revolution in about 200 million years."

For the sun, I can calculate that it has completed 460000000/200000000=23 revolutions around the center of the galaxy? When the sun completes one revolution around the center of the galaxy, does the earth completes one revolution around this center, too?

1) yes, they are the same thing. In the "photo" you are "seeing" the disk face on. the milky way looks like a wide swath across the night sky because we are inside it .
2) right on the first part. on the second it's the earths movement around the sun that causes the star field to shift from our point of view.
3) yes because the earth travels with the sun ( as does our whole system) although IIRC the latest data suggests that the period for one revolution around the galaxy is 225 millon years.
edited for typo/srr

1) Then, for example, in my picture, is the disk the whole thing shown there, including the spiral arms?

And I thought that everything we see with our unaided eyes is in the Milky Way Galaxy, then how come the quote says "this part (cloudlike band) of the sky is called the Milky Way", why it seems to only occuply a small section of the sky?

"The Milky Way is the disk of the Milky Way Galaxy"
This part of the quote is confusing me and it seems to me that they are 2 different things.........like the Milky Way is a part of the Milky Way Galaxy...If they are the same thing, whis actually is this statement trying to say?

2) So is it true that the stars APPEAR to move westward across the sky for 2 reasons??
(i) earth's rotation - apparent westward motion of stars around Polaris during a single night?
(ii) earth's revolution - apparent shifting motion of stars to the west from night to night?

"the milky way" is that milky band you see stretch across the sky. it is like labelling a constellation really, but it is the same thing as the milky way galaxy. the 'milky way' band is what we see in the sky of the milky way galaxy. they say it is the disc because you are looking at it from a horizontal view, like looking at a plate sideways so it seems thin. they just worded it in a confusing way.

The term "disk" is a bit vague. You could say the disk is the entire galaxy. Alternatively, you could say the disk is just the "central" region, with densely-packed stars.

Regardless, the bright band of stars you see in the sky is the central region of the galaxy, where the stars are just a bit closer together, on average, than if you look towards the outer spiral arms of the galaxy. That bright band of stars is what is called the "Milky Way". It is not quite the same thing as the "Milky Way galaxy", which refers to the whole thing, including our solar system.

1) Then, for example, in my picture, is the disk the whole thing shown there, including the spiral arms?

And I thought that everything we see with our unaided eyes is in the Milky Way Galaxy, then how come the quote says "this part (cloudlike band) of the sky is called the Milky Way", why it seems to only occuply a small section of the sky?

"The Milky Way is the disk of the Milky Way Galaxy"
This part of the quote is confusing me and it seems to me that they are 2 different things.........like the Milky Way is a part of the Milky Way Galaxy...If they are the same thing, whis actually is this statement trying to say?

2) So is it true that the stars APPEAR to move westward across the sky for 2 reasons??
(i) earth's rotation - apparent westward motion of stars around Polaris during a single night?
(ii) earth's revolution - apparent shifting motion of stars to the west from night to night?
1) yes to the first part but we see most of the entire universe when we sky gaze, including other galaxies. as far as the milky way being seen( from earth) as a band, it is from our perception. Try to imagine yourself on the side of a swimming pool looking down at the water. That is the view that your picture represents. Now lower yoourself into the water untill almost eye level (looking across the surface of the pool). that is the view that we see from earth. does that help?
2) the stars nightly rotation around polaris is due to the earth's rotation, yes. but the seasonal shift of the stars is due to the earths revolving around the sun.

I'd like to note that some astronomers are starting to think that the milky way is a barred spiral galaxy not a multable spiral arm galaxy.

2) the stars nightly rotation around polaris is due to the earth's rotation, yes. but the seasonal shift of the stars is due to the earths revolving around the sun.
First, I would like to thank all for explaining to me!

2) The apparent motion of the stars due to earth's rotation is moving westward.

Is the (seasonal) shift of the stars from night to night due to earht's revolution also shifting westward as well?

Is the (seasonal) shift of the stars from night to night due to earht's revolution also shifting westward as well?

You can see the direction of the revolution of Earth around the Sun in this graphic

http://members.aol.com/pakulda/images/emsmmo.gif

First, I would like to thank all for explaining to me!

2) The apparent motion of the stars due to earth's rotation is moving westward.

Is the (seasonal) shift of the stars from night to night due to earht's revolution also shifting westward as well?
The seasonal shift is not apparant from night to night.

1) yes, they are the same thing. In the "photo" you are "seeing" the disk face on. the milky way looks like a wide swath across the night sky because we are inside it .
2) right on the first part. on the second it's the earths movement around the sun that causes the star field to shift from our point of view.
3) yes because the earth travels with the sun ( as does our whole system) although IIRC the latest data suggests that the period for one revolution around the galaxy is 225 millon years.
edited for typo/srr

Is that period of revolution 225 million years +/- 2 or 3 days? :D

I'd like to note that some astronomers are starting to think that the milky way is a barred spiral galaxy not a multable spiral arm galaxy.
Actually the Milky Way Galaxy is probably both; a barred spiral with multiple arms.
Here is a graphic I have made showing the Milky Way galaxy, with its barred centre and multiple arms (including the Cygnus arm, Perseus arm, Orion arm, Sagittarius arm, Norma-Crux arm)...
http://img66.imageshack.us/img66/2008/milkyway4xs.png
http://img66.imageshack.us/img66/4069/galaxy7ie.jpg

Thanks for the links