Yesterday morning at 7AM my girlfriend called to tell me she could see
a huge moon that appeared to be right outside her office window.  I
left the house then to go to work and saw the huge moon, yet once I
traveled just a little ways, it was no longer visible as it was below
my field of vision to the left.

This morning I left for work at the same time (7AM) and it was still a
full moon, but much smaller & higher in the sky and I could see it the
whole way to work (13 miles).  I called my girlfriend and asked if it
was in her window this morning, and she said no, she could not see it.

Please tell me, in simple terms, why this is.  I guess I thought the
moon would pretty much be in the same place from one 24-hour period to
the next.

Greetings Natalie5111!

There are a lot of people who thought the moon just stays on it's
place in the sky when actually it doesn't. The simple answer to your
question is: The moon can never stay in one position in the sky
because it revolves around the Earth as the latter rotates. However, I
believe that for you to clearly understand what I mean, I will need to
give you a more visual approach. A little "science experiment" would
be fun in this case.

You can get a plate to symbolize the Earth and a saucer as your moon.
Now, get a small tape or anything that you can use to make one marker
on the side of the plate. Pretend that the mark is your girlfriend's
house. Now, try to move the saucer around the plate. You can clearly
see that it passes the mark as you move or "revolve" it around the
plate. This means that if you are in your friend's house patiently
watching the moon in the sky, you can clearly see how it changes
positon as it revolves around the Earth.

Now, let's suppose that the moon is just in a fixed position in the
sky. Using your plate Earth and saucer moon, don't move the saucer
around the plate anymore. This time, try to rotate the plate on it's
place. This time, you can clearly see that the mark passes the saucer
as you rotate the plate. The effect is the same as when you "revolve"
the saucer around the plate.

So you see, the moon cannot stay in place up there in the night sky
because of the two factors that I mentioned. Whether it's stationary
or not, it will always move across the night sky. This is the reason
why your friend can't find the moon though her window anymore.

With regards to the huge moon that your friend saw, it is an optical
illusion. The moon doesn't change it's size as it changes position in
the sky. This "big moon" is often seen when the moon is near the
horizon. The simple answer to this question is: The moon looks big
near the horizon because we compare it to objects and structures on
the ground. Our eyes are used to comparing sizes of things we normally
see or feel, e.g. a man against a truck, or a house beside a bigger
structure, etc.  We use objects or things as a reference point to
determine size. The moon looks big because we unknowingly compare it
against the trees, mountains, buildings etc. Again, to prove that the
moon doesn't change it's size, try to do this simple experiment.

Get a ruler or pencil, stretch your arm and try to "measure" the moon
when it is near the horizon. Align the tip of the ruler or pencil to
the top edge of the moon and determine and mark the moon's lower edge
against your measureing device. Wait for two or three hours until the
moon is, as you put it, "much smaller and higher in the sky." Measure
the moon again using your marked ruler or pencil and you can clearly
see that the moon is still the same size. I expect you to ask "why" so
here is the simple answer: It is because we have no reference point to
compare with the moon when it is high in the sky. Yes, we can
sometimes see clouds but we are not used to knowing the actual size of
a cloud.

If you have any other questions about the moon, you will find the
following links very informative and helpful. Simply press the "Shift"
key before clicking on a link so it will
open a new window or right-click on the link and choose "Open in New
Window" for your convenience.

"Why is the moon?" - a child's simple question
https://answers.google.com/answers/main?cmd=threadview&id=126135

Casper Planetarium Why does the Moon orbit the earth?
http://www.trib.com/WYOMING/NCSD/PLANETARIUM/whyorbit.html

Dr. Odenwald's ASK THE ASTRONOMER page: The Moon
http://itss.raytheon.com/cafe/qadir/amoonm.html

Curious About Astronomy? The Moon
http://curious.astro.cornell.edu/moon.php

Casper Planetarium Moon QUESTIONS
http://www.trib.com/WYOMING/NCSD/PLANETARIUM/Questions.html

The moon
http://www.stemnet.nf.ca/CITE/moon.htm

If you really can't quench your thirst for knowledge, click on the
links below for more "moon links".


Search strategy:

"why does the moon move"
://www.google.com/search?num=100&hl=en&lr=&ie=ISO-8859-1&safe=off&q=%22why+does+the+moon+move%22

"questions about the moon"
://www.google.com/search?num=100&hl=en&lr=&ie=ISO-8859-1&safe=off&q=%22questions+about+the+moon%22


I hope this helps you. Should you have any comments/questions, please
feel free to post your clarification before rating this and I'll
attend to you as soon as possible. Merry Christmas and happy New Year
to you.

Feilong

---

Request for Answer Clarification by natalie5111-ga on 20 Dec 2002 10:04 PST

Dear Feilong,

Thank you for your answer -- the part about the size being different
especially makes sense.  I am still unclear though why at 7AM on one
day the moon was much higher in the sky than at 7AM the day before. 
Is it because the moon's orbit of the earth isn't on a 24-hour clock
like the earth's orbit of the sun?

Please take a moment to clarify that!  Thank you. N.

---

Clarification of Answer by feilong-ga on 20 Dec 2002 10:55 PST

Dear Natalie5111,

"Is it because the moon's orbit of the earth isn't on a 24-hour clock
like the earth's orbit of the sun?"

Careful there, the Earth has a 365-day ORBIT around the sun, and a
24-hour, 360 degrees rotation on its own axis. The moon takes 27.3
days to orbit around the Earth and less than 24 hours to rotate on its
axis. See the links below for more information:

Earth Sciences 10 Lecture 3
Origin of the Solar System Early Earth/Moon History
http://pasadena.wr.usgs.gov/office/ganderson/es10/lectures/lecture03/lecture03.html

Ritual Astronomy for Dummies
http://www.geocities.com/Athens/Oracle/6992/astro1.html

Actually, your assumption of the difference of time and speed in moon
orbit as compared to Earth orbit is partly correct. It is one factor.
Another factor that you have to consider is the Earth's tilted axis of
23.5 degrees, which you can clearly see through this link:

The University of Texas at Austin
http://www.utexas.edu/depts/grg/hudson/grg301c/hudson_grg_301c/schedule/2_sun_earth_images/3_earth_sun/3.htm

Depending on the angle you are in, the moon can be seen as higher or
lower in the sky when observed at a chosen time. In your case, it was
7AM. This is the simple answer to your clarification. To elaborate,
let's take the Sun, as our example. If you are observant, you will
notice that on certain months of the year, the sun appears to set
exactly on the west while on some months, it appears to set a little
further to the north or northwest rather than exactly on the west.
This is because of the Earth's tilt. You can find more info through
the links below aside from what I gave you earlier.

Search strategy:

"time to revolve around the earth" moon
://www.google.com/search?q=%22time+to+revolve+around+the+earth%22+moon&cat=&hl=en

"earth's tilt"
://www.google.com/search?num=100&hl=en&lr=&ie=ISO-8859-1&safe=off&q=%22earth%27s+tilt%22&sa=N&tab=dw&cat=

I hope you are more enlightened by the simple answer I gave you.
Perhaps you can try observing the night sky so you can see for
yourself what I explained to you. If you have other questions, just
let me know before you rate my answer.

Best regards,

Feilong

Thank you very much for the clarification!

I think the root of your problem is a little simpler.   The Moon rises
& sets about an hour later each day because it is traveling around the
Earth. Remember that all rising/setting for natural objects (i.e., not
satellites) is really just what seems to be happening because the
Earth is rotating on its axis and some things are dropping below your
western horizon while others are popping above your eastern horizon. 
When the Moon rises at 6 PM, for example, it's because your eastern
horizon swept past it then.  The next day, when your eastern horizon
sweeps past what is essentially the same spot, the Moon isn't there. 
It moves 1/27.3 of its orbit during that day, which is about 13
degrees (360/27.3).  Earth turns at 15 degrees per hour (360/24), so
it has to turn almost a full hour more to get your eastern horizon
past the Moon again.  This is also why, on some days, moon rise/set
tables will have "does not rise".  If the Moon comes up at 11:58 PM on
Tuesday, for example, it will come up next very early Thursday
morning, so there is no moon rise on Wed.!  You can see the same
effect, for the same reason, in the tide tables.

By the way, the Moon rotates once on its axis per revolution around
the Earth - that's why we always see pretty much the same side (again,
not exactly, but close).

Nice comment, thenextguy.

To reiterate, the statement:

"The moon takes 27.3 days to orbit around the Earth and less than 24
hours to rotate on its axis."

is false.  It seems this misinformation came from 

Ritual Astronomy for Dummies 
http://www.geocities.com/Athens/Oracle/6992/astro1.html 

which says:
 
"The Moon takes a little less than 24 hours to rotate on it’s axis and
the same amount of time to revolve around the Earth."

(An example of bad science as well as bad grammar/spelling.)