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♪ [music playing throughout] ♪♪
In this lecture we're going to expand on
the earth-sun relationship by getting
to what's happening on earth.
We're going to look at the five major parallels that are on
earth, these are imaginary lines.
Then we're going to look at daylight versus darkness, the
oblique ray of the sun and the direct ray of the sun, and talk
about the solar declination.
So let's begin with the five major
parallels and draw with me.
We have the earth, and these are again imaginary lines that are
created by culture.
And so we've split the earth in two, we've got a northern
hemisphere and a southern hemisphere.
The line that separates these two hemispheres is called the
Equator and that is zero degrees north and south.
So let's go ahead and put equator here.
And then we have at 23.5 degrees north, we have another line,
imaginary line, and this is called the Tropic of Cancer.
Alright, now we'll go back to the southern hemisphere, and at
23.5 degrees south, we have the Tropic of Capricorn.
So now, before I go any further though, the 23.5 degrees north
through the 23.5 degrees south, this is the full extent of the
direct ray of the sun and we'll get to that a little bit later.
But I want to point out the reason why these lines are here
is to point out that the direct ray of the sun only goes this
far north in summer and then as it shifts, this is the furthest
it goes south, the direct ray of the sun south.
Now we bring into the last two lines, and this is at 66.5
degrees north and this is called the Arctic Circle.
Alright, and then at the 66.5 degrees south we have the
Antarctic Circle.
Now these two lines, imaginary lines, at 66.5 degrees north and
66.5 degrees south are drawn to show that this is the extent of
the oblique ray on the other side of the earth it's the
division between night and day, and I'll explain that further
when we see a cross section of what's happening on earth.
Alright, so again we have the five major parallels from the
equator north we go equator at zero degrees north and south,
23.5 degrees north is the Tropic of Cancer, 66.5
degrees north is the Arctic Circle.
On the southern hemisphere side we have the Tropic of Capricorn
at 23.5 degrees south and we have the
Arctic Circle at 66.5 degrees south.
Again, five major parallels that are drawn to show the earth-sun
relationship even further.
So we're going to go ahead and talk about these lines, more
specifically using a cross section or a side view of the
earth in describing the differences in daylight and
darkness across the earth.
So go ahead and draw with me again the earth and we're going
to identify that the sun is on this side here.
And to keep in with our previous
lecture here is the earth's axis.
[unclear dialogue]
And we'll go ahead and draw the equator and
we'll draw in the Tropic of Cancer and the Tropic of
Capricorn, 23.5 degrees north, 23.5 degrees south,
and there's our equator zero degrees north and south.
So we've got a tilt here, the North Pole, the northern
hemisphere is pointed toward the sun and so this is the point
where we have 24 hours of daylight.
So if we do this, and I should go ahead and indicate that this
light line here is 66.5 degrees north.
The reason why I want to indicate that is because that is
the extent of the shadow.
Here's the sunlight, back here is the
shadow that's created by the earth.
So we're going to go ahead and just kind
of quickly fill this in, lots of dust.
So again, here's 66.5.
So the North Pole is getting 24 hours of daylight here,
and this is what season?
It is our Summer Solstice, which we'll go ahead and label that.
Summer, Solstice.
Summer Solstice occurs on one day June 20th-23rd.
And with this I want to point out that the direct ray of the
sun is here at 23.5 degrees north, that
is it's solar declination.
The solar declination is the direct ray of the sun.
The direct ray of the sun is the solar declination.
So on June 21st, for example, the direct ray of the sun, go
ahead and put that here, direct, ray, of the
sun, is at 23.5 degrees north.
Now begin to think about what's happening during the Winter
Solstice as far as where is the direct ray of the sun.
It should be opposite.
Speaking of opposites, in the southern hemisphere, the South
Pole, it is experiencing 24 hours of darkness.
So this is the opposite of what the North Pole is experiencing,
24 hours of day, 24 hours of darkness.
So let's go ahead and start a new drawing, and I'm going to
switch this around a little bit.
Because now we've got the sun over here, and the equator here,
23.5 degrees south, 23.5 degrees north, I'm going to go ahead and
label these 23.5 degrees north, and repetition is always good,
there's the equator at zero degrees north and south, and
then we have the 23.5 degrees south.
Alright, so I'm going to go ahead and indicate the 66 here,
66.5 degrees north, because now it's going to be different.
Here's the shading, shading goes from 66.5 degrees north all the
way to the southern hemisphere's 66.5 degrees south.
And so all of this now is in the darkness because
it is now our Winter Solstice.
Which is, on one day, December 20th-23rd.
So now we get the North Pole in total darkness, 24 hours of
darkness during the Winter Solstice, opposite of that we
get the South Pole in light, 24 hours of daylight.
The direct ray of the sun is now, DRS the direct ray of the
sun is now at 23.5 degrees south.
In summer the direct ray of the sun was 23.5 degrees north, now
because the northern hemisphere is pointed away from the sun,
the direct ray of the sun is now at 23.5 degrees south.
So those are the two extreme seasons
and their solar declination.
Now, just a simple comment that the directory of the sun in
spring and autumn is at, give you a moment to think about it,
the direct ray of the sun at equinox, Vernal Equinox, and
Autumnal Equinox is at zero degrees north and south.
So during the equinox, the solar declination, solar, declination,
is zero degrees north and south.
Those are during equinox.
Solstice, if we go Winter Solstice, the solar declination
is 23.5 degrees south.
The Summer Solstice, the solar declination is
23.5 degrees north.
And hopefully you can see that through the lens.
So this is what's happening at the
four seasons, the four seasons.
You have during the equinox, the solar declination is zero and
then the Winter Solstice it's 23.5 degrees south, Summer
Solstice it's 23.5 degrees north.
Alright, now let's talk about this,
this direct ray of the sun.
The direct ray of the sun is an intense, it is an intense beam
if you will, an intense beam of solar radiation.
Let me give you an example.
Here is light on the floor.
When we take a flashlight and we shine it on the floor directly
in front of our feet we get an intense beam, an intense beam,
and it's bright, there's a lot of energy,
it has high concentration.
So this would be the direct ray of the sun.
And it's like a flashlight beam pointed at the floor right in
front of your feet and that light is like a perfect circle,
a perfect circle on the floor.
It is bright, it is highly concentrated energy.
On the opposite side of that we get this elongated area of
energy on the floor that, it's almost like pointing the
flashlight directly in front of you, ten feet in front of you.
And so what happens is you get this lower angle and now you
have a low concentration of energy, and this is what we call
the oblique ray of the sun.
Alright, now remember those parallels that
we were talking about.
We identified that the direct ray of the sun only occurs
between 23.5 degrees north, and 23.5 degrees south.
Everything that is north or south of 23.5 degrees north and
then south of 23.5 degrees south is going to be the oblique ray.
So again, direct ray has high concentration, and it all kind
of fits if you think about it, the tropics are very warm and
the reason why they're very warm is because they have a high
concentration of energy.
There's a lot of energy gained in the tropics and that is
because it experiences the direct ray of the sun.
The poles are cold, they're cooler, the midlatitudes are
cool, we live in the midlatitude, and so we
experience less temperature, less heat throughout the year,
and that is because we experience the oblique ray, it
is low concentration.
So the further that you go toward the poles, the less
energy you're going to get, so it cools off
as you go toward the poles.
Let's go ahead and draw this out on earth.
Here's our earth, here is the equator.
I'll go ahead and draw at 23.5 degrees north and south.
So this portion here in the tropics it is referred to as the
direct ray of the sun.
So that's why it's warm, it's warm,
warm, warm throughout the year.
The further north that you go from 23.5 degrees north, and
further south that you get from 23.5 degrees south, then you get
the oblique rays.
So this is what we refer to as the oblique
ray, and this as well.
So direct rays only occur between 23.5 degrees north and
23.5 degrees south, and then everything south of 23.5 degrees
south is the oblique ray.
It gets cooler as we go toward the pole and then everything
north of 23.5 degrees north it is also
an oblique ray of the sun.
The further that we get to the pole it starts to cool off
because we start to lose the energy.
We have a lot of energy here, there's energy gained, I'll go
ahead and put plus energy.
And then I'll put minus energy here, because we have energy
loss toward the poles.
So that's why it cools off as you go toward the poles.
♪ [music playing-- no dialogue] ♪♪