We call this distance along the widest point of the circle, the diameter.

Now, let's ask another question.

Actually, never!

Well, that's the same thing as g plus h.

If you were to put a tape measure to go around it and someone were to ask you what is the diameter of the circle?

OK, this is a neat property of circles.

We can multiply both sides of this by the diameter and we could say that the circumference is equal to pi times the diameter.

Now, we know how the diameter and the radius relates, but how does the circumference relate to, say, the diameter.

Well if this whole thing is your minor axis or maybe you could call your minor diameter if this whole thing is your minor diameter it's called minor, because it's the shortest of all of the diameters of this ellipse.

Diameter

Is it possible to determine the diameter from the circumference?

So we're worried about the ratio let me write it like this.

Select the segment whose length gives the diameter of the new circle...

A circle defined by its center and the length of a segment as the diameter

Construct a circle with the diameter given by the length of this segment

So maybe the circumference is always three times the diameter.

Circle by Point Radius

It has a very easy relationship with the radius.

Try to measure a circle.

Where r is the radius of the sphere.

Right there.

And so for area we need to know the radius 'cause area's pi R squared, so we can figure out the radius from the circumference by saying, well circumference is equal to two times pi times the radius of the circle, or if you say 36 pi, which we were told this is the circumference, is equal to two times pi times the radius.

And then they measure the radius of the circle right here or the diameter of that circle, and they'd say oh, the diameter

That's a, that's a ninemile radius.

So this is r squared, this is the radius squared.

What's our radius?

It is a two level style round burial mound, 23m diameter (lower level), 18m (higher).

And once again, the ratio of the circumference of the diameter was roughly 3.

So we know, or I'm telling you, that the ratio of the circumference to the diameter when I say the ratio,

Cut a semicircle with a diameter of 12 centimeters.

Vertical spacing

Circle radius

Vertical Radius

Vertical Radius

dabs per actual radius

Now what is the radius?

Now, it's obvious that a circle's circumference would get smaller or larger along with its diameter, but the relationship goes further than that.

Let's say that on this circle they measured around it and they found out that the circumference is 6 centimeters, roughly we have a bad tape measure right then.

We call that word the circumference of the circle.

The lake is three miles across.

Radiotelescope diameter

Or the horizontal radius is going to be 3.

These are just stars.

Apophis is going to hit us.

Our No.4 is Enceladus, one of Saturn's inner moons.