- [Narrator] When you look at
the moon in the night sky,
it might seem reasonably close.
But it's actually incredibly far away.
Right over here is a scaled picture
of the distance between
the earth and the moon.
Earth has a diameter
of approximately 8,000 miles.
While the moon has a diameter
of approximately 2,200 miles.
So, a little bit more
than one fourth the diameter of earth.
Now the distance between the two
is 239,000 miles.
Which you can imagine is incredibly far.
Now one thing that is very interesting,
this is why the moon looks like
it's the same size as the sun in the sky,
even though it is one four hundredth
of the diameter,
is the ratio between
the distance to the moon
and the diameter of the moon
is roughly the same as
the ratio of the distance
of the sun to the diameter of the sun.
So, for the moon,
that ratio, the distance to the moon,
239,000 miles over the radius
of the moon, 2,200.
It's approximately the same
as the ratio between
the distance from earth
to the sun which would be
93 million miles over
865,000 miles in diameter.
So this number is roughly
400 times larger than that number.
And this number is roughly
400 times larger than that number.
But what's neat is,
and this depends on where earth is
in its orbit,
but this is approximately equal to 108.
This gives me goosebumps
whenever I think about it
because it didn't have to be this way.
That the moon and the sun,
even though they have these vastly
different diameters,
that they look roughly the same size
in the sky because the ratio
of the distances is comparable
to the ratios of their diameters.
And this is why the number 108
is actually an auspicious number
in some cultures,
especially in Hinduism.
Now, when you're discussing
the moon and especially the moon
as viewed from earth,
one of the obvious questions
is why does the moon go through phases?
Why do we see different portions
of the moon lit up
from day to day?
This right over here is a diagram
from NASA's website.
And what you see here,
clearly this is not at scale.
This picture over here,
this inner picture,
the size of the moon
and the earth is roughly at scale,
but clearly the distances
between them are not.
You don't see that 239,000 miles
between them.
In this inner circle,
what you see is that the moon
and the earth is always lit
up from the right.
So, it's assuming that the sun
is 93 million miles
in that direction.
And it is lighting up
both the earth and the moon
from the right.
Now, as the moon rotates around the earth
in its 28 day cycle,
and if you're wondering,
gee a 28 day cycle
seems awfully close to a month,
that is not a coincidence.
The notion of a month comes
from the cycles of the moon.
In fact,
even the word month
and the word moon
have the exact same root
in old English.
And in proto Germanic.
They are essentially the same word.
But what you see is the moon goes
in this 28 day cycle,
when the moon is roughly between
the earth and the sun,
the lit up half of the moon
is away from what
we can see here on earth.
And so, we see the non lit up side
which would be a new moon.
Now, as the moon goes,
this is viewing from above the earth,
this would be the North Pole right over here,
as it goes counter clockwise
as viewed from above the earth,
we start to be able to see
a little bit of that half of the moon
that gets lit up.
So, when the moon is in this position,
we see, us from earth,
from this vantage point,
we're able to see a little bit
of the lit up side.
When the moon is over here,
we're able to see half
the lit up side
and half the non lit up side.
And that's called a first quarter moon.
And that keeps on going.
When we're halfway through
our cycle here,
at a full moon,
the earth is between the moon
and the sun and so from our vantage point,
we are able to see
the entire lit up side.
And that's why it is a full moon.
And then we keep going
all the way until we get back
to a new moon.
Now, one question that might be bothering you,
it bothered me for many years,
is as soon as I understood this,
the cycle of the moon,
how the moon has this 28 day cycle
as earth rotates around the sun,
I always wondered well the new moon,
it looks like the moon is between
the earth and the sun,
why doesn't it block out the sun
every time we have a new moon?
Why don't we have a solar eclipse
every 28 days?
Similarly,
for the full moons,
when we're in this position,
it looks like the earth
is between the moon and the sun.
Why doesn't earth's shadow
block out the sun
so that we have a lunar eclipse
instead of a full moon
and we would have one of those
every 28 days?
Why don't we see that?
Think about it yourself.
What's a plausible explanation?
In the next video,
we'll actually talk about why eclipses,
both solar eclipses when the moon
blocks out the sun,
and lunar eclipses when
the earth's shadow
falls on the moon,
why they're not happening every 28 days.