UCF Fall 2009 AST 2002 Sec 0004
Solutions to HW #3


Motion of the Sky:



Note - I took points off if you didn't give an
explanation for your answers, even if your answers
are right. Ya gotta follow directions!

Exercise #1:
Sun will appear next to the same constellation form sunrise to sunset.
Over the course of a day, the Sun doesn't really move much with
respect to the stars, so it's as if it too is stuck on the celestial
sphere.

Exercise #2:
EDCBA
Note that in this case "day" refers to the 24-hour period 
from midnight to midnight. It doesn't really refer to just
the 24-hour period starting at sunrise. E and D rise
before sunrise, and B and A rise after sunrise.

Exercise #3:
BACD
A lot of people found this one tricky. Recall what the "eastern
part of the sky" means -- it refers to half of the dome of the sky
above your head. Which half? Well the half that's on the eastern
side of your meridian. The meridian runs from the due-north horizon
up to your zenith and then back down to the due-south horizon. 
So the eastern half of the sky is the part of the sky that's
in the northeast and southeast quadrants. And more specifically,
anything on the right side of the drawing in this exercise. 
If you picture things a few hours ago, D is above the horizon,
sure, but it's not in the eastern part of the sky, it's in
the western part. C is probably below the horizon. If you
go back enough hours in tim, A will have been below the
horizon too. B is the first to appear, then A, then C, and
lastly D. 

Exercise #4:
ABCDE
Six hours from now, the Sun will be about setting. D and E
will be already below the horizon. A will be crossing the
meridian. Think about this: What would your answer have been
if the question had asked for the positions 18 hours after
the time shown? EDCBA.

Exercise #5:
D(CB)A
Many people found this tricky also. Notice that you had
to have said that C and B spend the same amount of time,
or I took off points. Note again that "day" refers to
a whole 24-hour period, so the fact that B is leading C
is immaterial. Any star on that circle would spend the
same amount of time above the horizon within a 24-hour period.
A lot of people thought A spent more time than B or C.
This is incorrect -- just because A's path around Polaris
is bigger doesn't mean it spends more time above the horizon.
Think about this: How long does it take D to go around
Polaris once? How long does it take B and C? How long does
it take A? The answer is 24 hours in all cases! What does
that mean? That A is traveling faster on its circle than
B and C are, and faster on its circle than D is. So just
because A appears to travel more distance on its bigger
circle doesn't necessarily mean it will spend more time
above the horizon --- it just means that it travels faster
doing it! So really the critical part here is to see how
much of each star's circle is below the horizon. The bigger
that part, the less time the star spends above the horizon.