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As I said there is more moving parts in our model of the cosmos.
There are more things that do not remain fixed on the celestial sphere and
just rotate with it but yet rise and sit so are off the earth.
And the next most conspicuous one after the sun is our moon.
like the sun the moon moves along the celestial sphere it's not fixed.
for the same reason, the moon
orbits the earth.
It orbits the earth in the same sense that the earth orbits the sun and the same
sense that therefore the sun appears to orbit the
earth, moving towards the east along the celestial sphere.
But the moon orbits the earth much faster.
The moon completes one orbit about the
Earth in, what is called, the sidereal month.
Which is about 27 and a third days.
This means, it's right ascension increases by 52 minutes
per day.
Now, as the moon orbits the Earth, it also rotates about its axis.
We'll discuss next week, why this is.
But this means that we are always seeing the
same side of the moon as it rotates around us.
there's the same side of the moon that faces us.
This means that, while there is no such thing as the dark side of
the moon, there certainly is such a thing as the far side of the moon.
There are parts of the moon that are never visible from Earth, and, those were first,
seen or indirectly seen by human eyes when
the Soviet Luna spacecraft first orbited the moon.
So this is why when we look up at the
moon every time we see it, it has this familiar look.
We're always seeing the same side of the moon.
The moon moves to the east along the celestial sphere
by about 52 minutes per day.
As we know the sun also moves to the east along the celestial sphere.
The sun moves much more slowly.
The sun moves by about four minutes per day.
The difference, 52 minus 4, gives us 48 minutes per day.
This is the difference between the mo, the sun's motion on
the celestial sphere and the moon's motion along the celestial sphere.
But otherwise, it is the relative motion of the moon, relative to
the sun.
The moon is 48 minutes farther east, relative to
the sun, each day, than it was the day before.
Since our clocks are attuned to the sun, this tells us that the moon
rises, on average, 48 minutes later each day than it did the day before.
And sets 48 minutes later. Then it set the day before.
The relative, the orientation of earth, sun, and moon,
therefore, repeats, but with a period longer
than a sidereal month, because that change
is not by 52 minutes per day, but only by 48 minutes per day.
Make the calculation you see that moving at a rate of 48 minutes per day,the
moon completes a full 24 hour rotation
about the celestial sphere relative to the sun.
Once every 29.5 days that is what is called a synodic month.
What happens once every synod, what repeats every synodic month therefore,
is the relative position of sun and moon on the celestial's sphere.
This controls, of course, the time of day at which the moon rises and sets.
For example, if the sun and the moon are
12 hours apart in right ascension, this means the
moon rises about 12 hours after the sun rises,
about at sunset, and sets 12 hours after the sun
sets, about at sunrise, so the moon is up at night during that time of the month.
And this repeats once every synodic month.
In addition to controlling the times, that the moon rises
and sets as we know this relative position of sun
and moon on celestial sphere, also controls the appearance of
the moon, or what we call the phases of the moon.
And the way these two are related,
is best captured by the following beautiful demonstration.
The best way to understand how the moon's position relative to the sun gives us
the phases of the moon, is to just do it. To just do it, you need a source of light.
You can use a lightbulb.
Or if you prefer, you can just use a
sun, just go outside if you can't find a lightbulb.
Anything round, I have a white styrofoam ball here to play the role of the moon.
And your head to play the role of earth.
the view from your eyes will give you the view
as seen by people on that side of earth facing the moon.
Of what the moon looks like.
And then you simply turn around to perform a complete lunation to give you
a sense of what this looks like, in case you're not going to do it yourself.
Though, I strongly recommend it.
What we have here is a, on the left side of the screen, you'll see a
setup of, in the studio, of me, holding
a moon and turning around in the relative configuration.
Whereas, on the
right side, you'll see the image of a GoPro camera that's mounted to the moon.
So, it shows you the image of the moon as I see it.
When the demonstration begins the moon is in between the sun and earth.
It's on a line between sun and earth.
And so the illuminated side of the moon as we can see in
this beautiful picture faces the sun and therefore faces away from the earth.
I see the dark side of the moon.
And the moon will be nearly invisible. As I turn to my left,
to the east.
slowly, the western side of the moon will become illuminated, and we'll see a
growing crescent, until after I've turned 90
degrees, we'll see a waxing quarter moon.
Since the moon is now six hours or 90 degrees to the
east of the sun, it will rise six hours after the sun.
In other words, the waxing quarter moon
rises about at noon, sets about at midnight.
As I continue to turn to the
east, the illuminated part of the moon will grow and become gibbous until, at
last, when I'm 12 hours away from the sun, I will see a full, round moon illuminated.
We're going to have to switch cameras at some point
to give you the full view because of studio limitations.
Don't get confused by that.
12 hours to the east of the sun, the full moon rises at sunset and
sets at sunrise, and so the full moon is the only
moon that is really up all night and only during the night.
You'll note that to give us a view of the full moon, I had to tilt the moon's orbit.
I'm holding it way above my head. We'll get back to that in a second.
As I continue to turn to the east, now
the western part of the moon is losing the sunlight.
I see the eastern part of the moon illuminated.
It is a wining gibbous moon and when I reach 90 degrees to
the sun again, I have a wining quarter moon.
The wining quarter moon, which is six hours to the west
of the sun will therefore rise six hours before the sun.
In other words, rise about at midnight, set about noon.
This is the moon we see in the morning.
Finally, as I continue turning to the east, the moon becomes closer to the
sun in the sky only the eastern edge of the moon is illuminated, I find
a wining crescent moon.
And, after a full synodic orbit is passed, the moon is back in line with the sun,
and again I see only the dark side of the moon, and we're back to new moon.
So, I, what I hope you saw, and I do encourage you to do it yourself.
It's really fun, and you can show it to your, friends and family, is that
over the course of a synodic month as the moon orbits the earth relative to
the sun the shape of the visible part of the moon changes in the sky
as well as because of the relative position of moon and sun, the rise time.
So the new moon, when the
sun and the moon are roughly at the same right ascension and the
moon is completely dark in the sky rises at sunrise and sets at sunset.
The waxing quarter moon.
When the moon is about 6 hours of right ascension to the east the sun, it rises
six hours after the sun, so the waxing quarter
moon rises at noon and sets around midnight and
is visible all afternoon.
The full moon, where the moon is 12 hours of right ascention ahead of the sun.
In other words, on the opposite side of the sky.
The full moon is the only time that the moon rises at sunset, and sets at sunrise.
And the waning quarter moon, where the moon is six hours to the west of
the sun, or 18 hours to the east, the moon being six hours to the west
of the sun, rises six hours before the sun, in other words, rises about
midnight and sets about at noon, and the waning moon is visible all morning.
So, when you see the moon the daytime, you should not be surprised, but
should you ever see a full moon at noon something has gone terribly wrong.
So both the phases and the periodic change in moon rise and set times are completely
understood in terms of this model, where the moon reflects sun light and what
we see depends on the angle between the moon, the sun, and the earth.
you can simulate this, you can go to the simulation page
of the University of Nebraska-Lincoln and get a less three dimensional version.
But I encourage you to construct a, take a light bulb
and a ball of some sort and make yourself a moon.