Let's begin by defining the sky and kind of breaking it down so that we can get
used to some terminology and directions. So here we are standing in an open field,
we have north more or less behind us and south in front of us. I guess you could
say we're facing toward the southwest if you like. The horizon is just a perfect
360 degree circle and the sky is suspended both above and below us and
the reason why I've drawn it below as well is because we know from our
experience that there are stars and planets that have yet to rise they'll
rise later this afternoon and this evening so we're going to go ahead and
just represent the entire sky even though we can't see it all. Now the
position that's directly above our heads is called the "zenith" and the point
directly beneath our feet, we call that the "nadir". And if we take a
circle and draw it from due north through the zenith to due south and back
around again, then we can define a circle that will divide the sky into eastern
and western halves this is called the "meridian" and this is going to be a
useful reference point going forward. Now while we're here I'd like to just
describe for you briefly how we can identify the position of any point in
the sky from our perspective. So let's just say we have a star and we can
describe its position above the horizon. We do that by tracing a line directly to
the horizon that's directly underneath our star and drawing an angle up to the
star. So this angle is the star's "altitude" and we define altitude as zero degrees
at the horizon and plus 90 degrees at the zenith so the zenith is directly
overhead you could just draw a 90 degree angle from the horizon to the zenith by
the way we can also describe the altitudes of objects below the horizon
by using negative values so at 0 degrees at the horizon and minus 90 degrees at
the zenith and we can also work out where along the horizon we need to look
and to do that we start at due north and we just trace a clockwise angle until we
arrive directly underneath our star on the horizon and
this angle is called the Stars azimuth so azimuth begins at 0 degrees due north
followed by 90 degrees due east 180 due south and 270 due west and for this
reason since it's aligned to the horizon it's sometimes referred to as the
horizon coordinate system or sometimes alt azimuth or alt as just for short
anyway if you could lie down it would look like you're looking up into a giant
dome and that dome would look a little bit like this
this is what we would see if we had fisheye vision or even if you could just
lie down in a field with your head oriented toward the north end your feet
toward the south if you used your peripheral vision you could kind of
build up an image like this in your brain you can sense out the horizon so
the horizon is the great circle with the grass drawn on it and anything that is
at the horizon has an altitude of 0 degrees the point directly overhead is
the zenith which we can represent like this with a green letter Z and you can
see that this is morning it's near where I live and you'll see that the Sun is up
and it is because it's the morning the Sun is somewhere in the eastern half of
the sky and it is just above the horizon the bright object to the 5 o'clock
position of the zenith that is our moon so the moon is high up but somewhere in
the southwestern sky so let me go ahead and draw the meridian this green line
arcing from due north arcing upward through the zenith to due
south and again this is a little bit tricky but you just want to remember
that you are kind of looking into a dome so the closer something is to the center
of our screen the higher in the sky or the greater its altitude above the
horizon ok well I don't want to have to make you listen to me talk for 24 hours
so what I'm going to do is just go ahead and speed up time and we're just going
to watch the Sun make its way throughout the day today
so the sun's getting a little bit higher and it's kind of moving in a south
eastern direction getting closer to the southern horizon and now you'll notice
it's about to do what we call transit the meridian so this point right here
this would represent our local noon this is not to be confused with twelve
o'clock noon because we use things like time zones and other things to define
time but this is when the Sun is at its highest position above the horizon it is
transiting the meridian so it's at its greatest altitude and I'm saying it's at
its highest or highest altitude because as the day continues the Sun now begins
to decrease its altitude as it moves toward the south western sky gradually
moving toward the due west and setting even just a little bit due north this
simulation is being run in August so we're getting a good idea of what's
going on tonight well the Sun sets and the stars come out and the thing to
notice here is that we have the Stars doing the exact same thing that the Sun
appears to do they rise in the east they cross over the meridian they transit the
meridian and then they set toward the west and the thing is is that this
pattern will repeat itself so as these stars continue to set very gradually
over time the Sun will begin to rise once again
and now it's daytime so a new day dawns the pattern repeats
and it's for this reason that many ancient civilizations and indeed
certainly many ancient Greek astronomers felt that the Stars must be some kind of
a vast sphere and not a dome at all so we have the sphere that is seems to be
rotating around us and with this regular predictive cadence they assume that this
must mean that the stars are imprinted upon a sphere well even though we know
that's not the case at all it turns out that there is something that's there's
something to this idea that's actually still useful to us in modern day
astronomy